Filed under: Uncategorized | Tags: biology, diversity, evolution, science, systematics, taxonomy
In the first two parts of this lecture we tackled the Origin of Life and Biological Diversity and the mechanisms of the Evolution of Biological Diversity. Now, we’ll take a look at what those mechanisms have produced so far – the current state of diversity on our planet.
The Three Domains
The organisms living on Earth today are broadly divided into three large domains: Bacteria, Archaea and Eukarya (Protista, Plants, Fungi and Animals). Our understanding of the relationship between the three domains is undergoing big changes right now. The old divisions have been based on morphological and biochemical differences, but recent genetic data are forcing us to rethink and revise the way we think about the three Domains.
It was thought before that Bacteria arose first, that Archaea evolved from a branch off of bacterial line, while the first Eukarya (protists) evolved through the process of endosymbiosis: small bacteria and archaea finding permanent homes within the cell of larger bacteria and forming organelles. It was thought that bacteria were always simple, that Archaea are somewhet more complex, and that Eukarya are the most complex.
Neither Bacteria nor Archaea possess any organelles or subcellular compartments. The chemistry of cell walls is strikingly different between the two groups. The genes of Archaea, like Eukaryia, have introns. Until recently, it was thought that bacterial genes have no introns, however remnants of bacterial introns have been recently discovered, suggesting that Bacteria used to have introns in the past but have secondarily lost them – becoming simpler over the 3.6 billions of evolution. The enzymes involved in transcription of DNA in Archaea are much more similar to the equivalent enzymes in Eukarya than those in Bacteria.
Molecular data, as well as what we know from evolutionary theory how population size affects the strength of natural selection, a new picture has emerged. The earliest Bacteria were simple, hugging the Left Wall of Complexity. While their population sizes were still small, Bacteria evolved greater and greater complexity, leaving the left wall somewhat, evolving more complex genomes, more complex mechanisms of DNA transcription (including introns), and perhaps even some organelles. Likewise, the Archaea split off of Bacteria (or perhaps they even appeared first) and evolved much greater complexity in parallel with the Bacteria. Eukarya also split off of Bacterial tree early on and evolved its own complexity. Thus there were three groups simultaneously evolving greater and greater complexity.
Then, Bacteria and Archaea grew up in population sizes. Instead of small pockets somewhere in the ocean, now bacteria and archaea occupied every spot on Earth in huge numbers. Large population size makes natural selection very strong. Greater complexity is not fit, thus it is selected against. Thus, the originally complex bacteria and archaea became simpler over time – they turned into lean, mean evolving machines that we see today – the dominant life forms on our planet throughout its history. They lost introns, they lost organelles, and lost many complicated enzymatic pathways, each species reducing its genome and strongly specializing for one particular niche.
On the other hand, Eukarya did not grow in numbers as much. The population sizes remained small, thus the selection against complexity was relaxed – the eukaryotes were free to evolve away from the Left Wall. They increased in complexity, engulfing other microorganisms that later became mitochondria and chloroplasts.
Thus, though we, for egocentric reasons, like to think of greater complexity as being better than being simple, the Big Story of the evolution of life on Earth is that of simplification. Natural selection harshly eliminated organisms that experimented with greater complexity – the Eukarya being the exception: an evolutionary accident that happened due to their existence in small, isolated populations in which selection against complexity is relaxed.
Bacteria are small, single-celled organisms with no internal structures or organelles. Bacteria may have cell walls on the surface of their cell membranes, and may have evolved cilia or flagella for locomotion. The DNA is usually organized in a single circular chromosome. Some bacteria congregate into collonies or chains, while in other species each cell lives on its own.
In the laboratory, bacteria can be easily separated into two major groups by the way their cell walls get stained by a particular stain into Gram positive (purple stain) and Gram negative (red stain) bacteria. By shape, bacteria are divided into cocci (spherical cells), bacilli (rod-like shapes) and spirilli (thread-like or worm-like cells).
Bacteria are capable of sensing their environment and responding to it – i.e., they are capable of exhibiting behavior. Bacteria are also capable of communicating with each other – for instance, they can sense how many of them are present in a particular place and they can all change their behavior once the poulation size reaches a sertain treshold – this kind of sensing is called quorum sensing.
Many bacteria are serious pathogens of plants and animals (including humans). Others are important decomposers of dead plants and animals, thus playing important roles in the ecology of the planet. Yet others are symbionts – living in mutualistic relationships with other organisms, e.g., with plants and animals.
The inside of out digestive tract provides a home for numerous microorganisms. The best way to think about out “intestinal flora” is in terms of an ecosystem. We acquire it at the moment of birth and build it up with the bacteria we get from the environment – mostly from our parents. The bacterial populations in the intestine go through stages of building an ecosystem, similarly to the secondary succession. If, due to disease or due to use of potent antibiotics, the balance of the ecosystem is disrupted, it may recover through phases akin to primary succession.
Experiments with completely internally sterile animals (mostly pigs and rabbits) demonstrated that we rely on our intestinal bacteria for some of our normal functions, e.g., digestion of some food components, including vitamins. In many ways, after millions of years of evolution, our internal bacteria have become an essential part of who we are, and there is now a push for sequencing the complete genome of our becterial flora and to include that information in the Human Genome. The composition of the bacterial ecosystem in out guts can affect the way we respond to disease, or even if we are going to get fat or not, thus there is much recent research on individual variation of the intestinal flora between human individuals, so-called “poo print” (yes, scientists do have a sense of humor).
Deinococcus radiodurans is one famous Bacterium. It thrives inside nuclear reactors. Of course, our reactors are a very recent innovations, so the scientists were puzzled for a long time as to what natural environment selected these organisms to be able to survive in such a harsh environment. It turns out that dehydration (drying-out) has the same effects on the DNA as does radioactivity – fragmenting and tearing-up of pieces of the DNA molecule. Deinococcus evolved especially fast and accurate mechanisms for DNA repair. Bioengineering projects are underway to genetically engineer these Archaea in such a way that they can be used to clean up radioactive spills and digest nuclear waste.
Archaea may have been the first life forms on the Earth. Today, they tend to occupy niches that no other organisms can. Thus, they are found living inside the rocks miles under the surface, they are found in extremely cold and extremely hot environments, in very salty, very acidic and very alkaline envrionments as well. The hot water of the Old Faithful geiser in Yellowstone national park are inhabited by a species of Archaea. They are difficult to study as they die in normal conditions in the laboratory – room temperature, neutral pH etc.
Though some Archaea have been found to live inside our bodies, not a single one has, so far, been indentified as a pathogen. Only very recently (i.e., last few weeks) has it been shown that one archaean does have an effect on our health – not as a pathogen but as an enabler. It can migrate into roots of our teeth and set up colonies there. It then changes the environment in the tooth in such a way that it becomes conducive to the immigration and reproduction of a pathogenic bacterium than can then attack the tooth.
Protists are an artificial group of organisms – every eukaryote that cannot be classified as a plant, a fungus or an animal is placed in this category. Thus, the number of species of protists is very large and the diversity of shapes, sizes and types of metabolism is enormous.
Some protists are microscopic unicellular organisms, like the Silver Slipper (Paramecium), while others are multicellular and quite large (e.g, sea kelp). Some protists, e.g., cellular slime molds, have a single-celled and a multi-celled phase of their life-cycle.
Even some of the unicellular protists can be quite large – an Acetabularia (‘mermaid’s wineglass’, see picture) cell is about 5 cm long, thus perfectly visible to the human eye. Most protists reproduce regularly by asexual processes, e.g., fission or budding, utilizing sexual reproduction (e.g., conjugation, which is gene-swapping) only in times of stress. Some protists are surrounded only by a plasma membrane, while some others form shells of silica (glass) around themselves. Some protists have flagella or cilia, while some others move by pseudopodia (false legs – ameboid movement).
Traditionally, protists have been artificially subdivided into three basic groups according to their metabolism: protists capable of photosynthesis (autotrophs) are called Algae, heterotrophs are called Protozoa, while the absorbers are Fungus-like protists. According to morphology, protists have been divided into about 15 phyla, grouped into six major groups. New molecular techniques are thoroughly changing the taxonomy and systematics of Protista. One group, the Green Algae, has recently been moved out of Protista and into the Kingdom Plantae. Another group, the Choanoflaggelata, has been moved to the Kingdom Animalia as they are most closely related to sponges.
Some protists are parasites that cause human diseases. Most well-known of those are Plasmodium (malaria), various species of Trypanosoma (sleeping sickness, leischmaniasis and Chagas Disease) and Giardia (Hiker’s Diarrhea). Dinoflagellates live on the surface of the ocean and are almost as important for absorption of CO2 and production of O2 as are forests on land.
Plants are terrestrial, multicellular organisms capable of photosynthesis (though some species have secondarily moved back into the aquatic environment or lost the ability to photosynthetize). There are about 300,000 species of plants on Earth today. They are divided into two broad categories: non-vascular and vascular plants. Mosses, liverworths and some other smaller groups are non-vascular plants. All other plants are vascular, meaning that they possess systems of tubes and canals that are used to transports water and nutrients from root to stem and leaves, and from leaves back to the root. Those tubes and canals are called phloem and xylem.
Of the vascular plants, some reproduce by forming spores, while others produce seed. Seedless vascular plants that produce spores are, among others, ferns and horsetails. Seeds are produced by two large groups: Gymnosperms (e.g., conifers) and Angiosperms (flowering plants).
An important evolutionary trend in plants was a gradual reduction of the haploid portion of the life-cycle (gametophyte) and simultaneous rise to dominance of the diploid portion – the sporophyte. In mosses, for instance, almost all of the plant is haploid, except for the diploid spores developing at the very tip of the stem. In flowering plants, e.g., trees, almost all of the plant’s cells are diploid (just like in us), while the flowers contain male and female gametes (pollen and egg).
Fungi can be unicellular (e.g., some yeasts and molds) or multicellular (e.g, mushrooms). Molecular data show that fungi are more closely related to animals than plants. Fungi are heterotrophs that obtain nutrients from the soil by secreting enzymes into the substrate and absorbing the digested materials. They cannot photosynthetize. Fungi are composed of hyphae, which are thin long filaments. A mass of hyphae is called the mycelium which can build large structures like mushrooms. Spores are the means of reproduction and are formed by sexual or asexual processes.
Fungi tend to enter into symbiotic relationships with other organisms. Some of those relationships are parasitic, as in our own fungal diseases. Other relationships are mutualistic, e.g., lychens, mycorrhizae and endophytes. Lichens are a mutualistic association between a fungus and a photosynthesizer, usually a green algae. Mycorrhizae form mutualistic associations between the fungi and plant roots (e.g., alfalfa). Endophytes are plants that have fungi living inside them in intercellular spaces and may provide protection against herbivores by producing toxins.
Animals are multicellular heterotrophs (they do not photosynthetize). They exhibit embryonic development and mostly reproduce sexually. One of the important characteristics of animals is movement. While microorganisms (bacteria, archaea and small protists) can move, large organisms (large protists, plants and fungi) cannot – they are sessile (attached to the substrate). Animals are large organisms that are capable of active movement: swimming, crawling, walking, running, jumping or flying. While some animals are also sessile, at least one phase of their life-cycle (e.g., a larva) is capable of active movement.
Some of the major transitions in the evolution of animals are evolution of tissues, evolution of symmetry (first radial, later bilateral), evolution of pseudocoelom and coelom, the difference between Protostomes and Deuterostomes, and the evolution of segmentation.
There are about 37 phyla of animals. Animals can be divided into two sub-Kingdoms: Parazoans and Eumetazoans. Parazoans are choanoflagellates and sponges. They do not have tissues – their cells are randomly organized. A sponge can be pushed through a sieve and all cells get detached from each other during the process, yet they will reconnect and form an intact sponge afterwards. Sponges move by reorganization of the whole body – cells move over each other (pulling the silicate spicules along) and can move as much as 6mm per day. All other animals are Eumetazoans – their cells are organized within proper tissues.
Parazoans also have no body symmetry. Some phyla of animals (e.g, Cnidaria) have radial symmetry – they are called Radiata. Most phyla of animals – the Bilateria – have bilateral symmetry: the left and the right side of the body are mirror images of each other. In bilaterally symmetrical animals, there is early embryonic determination not juts of up-down axis, but also of front-back axis. Bilateral symmetry gives the animal direction – it moves in one direction, the sensory organs and the mouth tend to be in front, while excretion and reproduction are relegated to the back of the animal.
Early during development, the cells of the spherical embryo (gastrula) organize into layers. Some animals (Diploblasts) have only two layers: ectoderm on the outside and endoderm on the inside. Most animals (Triploblasts) have evolved a third layer in between – the mesoderm. Ectoderm gives rise to the skin and nervous system. Endoderm gives rise to the intestine and lungs, among else. Mesoderm gives rise to muscles and many other internal organs. Usually, Radiata are Diploblasts, while Bilateria are Triploblasts.
In more primitive animals, there is no internal body cavity (e.g., flatworms). In others, a cavity forms during the development between the endoderm and mesoderm – it is called pseudocoelom (e.g., nematodes). In most animals, a proper coelom develops between two layers of mesoderm. Our abdominal and chest cavities are parts of our coelom.
In most phyla of animals, the early embryo divides by spiral cleavage. The blastopore – an opening into the cavity of the blastula- eventually becomes the mouth. These animals are called Protostomes. Protostomes are further divided into two groups: in one group animals grow by adding body mass (e.g., annelids, molluscs and flatworms), while others grow by molting (e.g., nematodes and arthoropods).
In Echinodermata and Chordata, the embryo divides by radial cleavage. The blastopore becomes the anus. These animals are Deuterostomes.
Three large phyla of animals – Annelida, Arthropoda and Chordata evolved segmentation, using Hox genes to drive the development of each segment.
You will HAVE to read the three relevant animal chapters in the textbook to learn more about the following phyla: sponges, cnidarians, annelids, molluscs, arthropods and chordates.
Phylum Chordata is the one we are most interested in for egocentric reasons – because we are chordates. The phylum consists of some invertebrate groups and the Vertebrata (all other animal phyla are also Invertebrata). The invertebrate chordates are hemichordates (acorn worms), tunicates (Urochordata – sea squirts) and cephalochordates (e.g., the lancelet – Amphioxus, see picture). The larvae of invertebrate chordates are very similar to the larvae of echinoderms, both groups are also Deuterostomes, and recent molecular data confirm close relationship between chordates and echinoderms as well.
All chordates have, at least at some point during the development, a notochord. The early chordates were aquatic animals. Hagfish and lampreys are two of the most primitive groups of vertebrates. Before the molecular analysis was performed, these two groups were clumped into a single group of Jawless Fish (Agnatha), but have since been split into two separate classes.
‘Fish’ is the lay term for several different groups of aquatic vertebrates. The most important classes are cartilagenous fish (Chondrichthyes, e.g., sharks, rays and sturgeons), lobe-finned fish (Sarcopterygii, e.g., gars) and ray-finned fish (Actinopterygii – most fish that you can think of). The latter two of those are also sometimes lumped together and called the bony fish (Teleostei). Chrossopterygii, a once-prominent group of lobe-finned fish that survives today with only one living species (Coelacanth, or Latimeria), is the group that gave rise to ancient amphibians – the first vertebrates to invade the land (check out the Tiktaalik website for more information).
Amphibians are frogs, toads, salamanders and cecilians. At least one portion of the life-cycle – reproduction and early development – is dependent on water. They have legs for locomotion and lungs for respiration on land.
Reptilia are a large and diverse class of vertebrates. They include lizards, snakes, tuataras, turtles, tortoises and crocodilians. They have scaly skins that allows them to survive in arid environments. They have evolved an amniotic egg – an egg that contains nutrien-rich yolk and is contained within a leathery shell. Thus, reproduction and development are not dependent on water. Many reptiles live in deserts.
A now-extinct group of ancient reptiles (therapsyds) gave rise to mammals (class Mammalia) about 220 million years ago. The early mammals were quite large carnivores. However, during the 150 million year reign of the Dinosaurs (another extinct group of reptiles) mammals were constrained to a very small niche – that of nocturnal burrowing insectivores. Only after the demise of Dinosaurs (65 million years ago) could mammals embark on a fast evolutionary radiation that produced groups we know now.
Birds and mammals are endotherms – they can control (and keep constant) their body temperature by producing the heat in organs like muscles and liver. This is a metabolically expensive strategy that requires these animals to eat very frequently, but gives them speed and stamina and allows these animals to live in every part of the Earth, incuding polar regions. Other vertebrate classes are ectotherms – they gain their heat from the environment and, if they are cold, they are slow and sluggish.
As it is very difficult for large bodies to lose heat, large reptiles (like dinosaurs), once heated, can retain their body temperature for long periods of time – they are effectively warm-blooded. Some reptiles, notably pythons and iguanas, are capable of producing some of the heat internally. While they cannot keep a constant body temperature, they are capable of some degree of thermoregulation (e.g., becoming somewhat warmer than the external environment). By shivering their muscles, pythons raise their body temperature above ambient and use this heat to incubate their eggs.
There are about 4500 species of mammals, organized into 19 orders. The defining characteristics of mammals are milk producing glands and hair.
Monotremes (platypus and echidna) are egg-laying mammals. Their mammary glands are not completely evolved yet – the young lick the milk of off mothers hair.
Marsupials are the pouched mammals (e.g., kangaroo, koala, opossum). The immature newborn offspring crawls up into the pouch and lives inside it until they are large enough to fend for themselves.
Placental mammals (the remaining 17 orders) have a placenta that nourishes their embryos during development. The new molecular data, coupled with a number of exciting newly-discovered fossils, are changing our understanding of genealogical relationships between different orders of mammals, including our new knowledge about the evolution of whales, the relationship between elephants and hyraxes, between Carnivores and Pinnipedieans (seals, etc.) and between rodents and rabbits.
The most recent vertebrate class – the birds (Aves) – evolved out of a branch of Dinosaurs. There are 28 orders of bird in 166 families. Two primary characteristics distinguish birds from reptiles: feathers and flight skeleton. Their feathers are modified reptile scales. Feathers are obviously important for flight, but also insulate as birds are endotherms.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapters 19, 20, 21, 22, 23 and 24.
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In the previous segment of the lecture, we looked at the Origin of Life and the beginnings of the evolution of biological diversity. Now we move to explanations of the mechanisms by which diversity arises.
Although traits can be inherited by non-DNA ways, and DNA sequence does not neccessarily translate directly onto the traits, in the long term the differences between species tend to be recorded in the genome. Thus, differences between genomes of different species are most important differences between them. How do differences between genomes arise? There are six major (and some minor) ways this happens:
Mutations are small changes in the sequences of DNA. Some of the changes are just substitutions of one nucleotide with another, others are deletions, insertions and duplications of single nucleotides or small strings of nucleotides within a gene, or within a non-coding regulatory sequence. Such small changes may alter the function of the gene-product (protein) which may translate into changes in traits which may be selected for by natural or sexual selection.
Gene duplication occurs quite often due to errors in DNA replication during cell division, or due to errors in ‘crossing-over’ phase of meiosis. Instead of a single copy of a gene, the offspring have two copies of that same gene. As long as one copy remains unaltered and functions properly, the other gene is free to mutate (i.e., there will stabilizing selection on the first copy, and no selection for the preservation of the sequence of the second copy). The second gene may transiently become non-functional, but as it keep mutating it may beging coding for a completely novel protein which will start interacting with other molecules in the cell. If this new interaction confers increased fitness on the organism, this new gene sequence will become selected for and fine-tuned by natural (or sexual) selection for its new function.
Chromosome duplication may also occur due to errors in DNA replication during cell division. Instead of just one gene being duplicated, a large number of genes now exist in two copies, each pair of copies consisting of one copy that is preserved by stabilizing selection and another copy that is free to mutate and thus potentially evolve novel traits.
Genome duplication has occured many times, especially in plants. The whole genome doubles, i.e., all of the chromosomes are duplicated. The resulting state is called polyploidy. This provides a very large amount of genetic material for natural selection to tinker with and, over time, produce novel traits.
Rearrangement of segments of the DNA along the same chromosome, or between chromosomes, places different genes that were once far from each other into closer proximity. Thus, genes that were previously quite independent from each other may now be expressed together or may start influencing each other’s expression. Thus, the genes become linked together (or unlinked from each other), restructuring the batteries of genes that work together in a common function. This may free some genes to evolve independently, while tying some genes together and thus constraining the direction in which development of traits may evolve.
Lateral transfer (sometimes called ‘horizontal transfer’) is an exchange of DNA sequences between individuals of the same species or of different species. While vertical transfer moves genes from parents to offspring, lateral transfer moves genes between unrelated individuals. Such transfer is very common in microorganisms. Some species of Bacteria, Archaea and Protista routinely engage in gene swapping, which results in increase of genetic diversity of the species and thus provides raw material for evolution to build new traits. Gene swapping between organisms of different species may transfer a complete function from one species to another. Sometimes viruses act as carriers of genes from one species to another. For instance, a virus may take a piece of a bacterial genome and later insert it into a genome of a plant or a mammal. Some key genes involved in the development of the placenta originated as bacterial genes inserted into early mammalian genomes via viruses.
One important thing to bear in mind is that evolution has to ensure the survival of the individual at all stages of its life-cycle, not just the adult. Thus, evolution of new traits can occur only if it does not disrupt the viability of eggs, larvae, immature adults and mature adults.
Another important thing to keep in mind is that traits arise through embryonic and post-embryonic development. Thus, evolution of traits is really evolution of development. Evolution of genomes, thus, is not evolution of random grab-bags of many genes, but evolution of complexes of genes involved in development of particular traits.
A product of a gene is a protein. A protein that is capable of binding to DNA and thus regulating the expression of other genes is called a transcription factor. When bound to a gene, a transcription factor may induce its expression, block its expression, or increase or decrease the rate of its expression. The patterns of gene expression are key to embryonic development and cell differentiation, so it is not surprising that transcription factors play a large role in evolution of new traits via development.
A novel pattern of gene expression may arise in two ways. First, by mutation of a transcription factor (so-called trans-factors), it changes which genes it affects and the way it affects them. Second, by mutations in regulatory regions (so-called cis-factors) of the target genes, the transcription factors may or may not bind to them, or a different transcription factor may bind to them, or the effect of the binding on transcription of the gene may change.
Most important genes in evolution of development are transcription factors. Often, they work in batteries (or complexes or toolkits), where one gene induces transcription of the second gene which in turn induces transcription of the third gene, and so on. Such batteries tend to be strongly preserved in many species of living organisms, though the genes that act as final targets of action of such complexes differ between species. Such complexes may determine what is up and what is down in an early embryo, or what is forward and what is bakward in an embryo. Such complexes are used over and over in evolution to produce protruding structures, like limbs. Another such complex has been used in 40 different groups of animals for the construction of 40 quite different types of eyes.
Possibly the most important such complex in animals is the complex of Hox genes that regulates segmentation. Most animals are segmented. While this is obvious in earthworms where all segments look alike, in many other animals segments are formed in the early embryo and each segment then develops unique structures on it. Thus, an insect will develop jaws and antennae on its head segment, wings and legs on its thoracic segment, and reproductive structures and stings on its abdominal segment. You will need to carefully read the handout “A Brief Overview of Hox Genes” (http://scienceblogs.com/pharyngula/2006/04/a_brief_overview_of_hox_genes.php) and be able to define Homeotic genes, Homeobox (DNA sequence), Homeodomain (protein structure) and Hox genes. Interestingly, non-segmented Cnidarians (corals and jellyfish) do not have true Hox genes, though they do have scatterings of Hox-like genes, which may be evolutionary precusors of true Hox genes.
Thus, evolution of diversity can be thought of in terms of changes in the way developmental toolkits are applied in each species. The same toolkits are used over and over for development of similar traits. The sequences of the genes within the toolkits will vary somewhat between species, and the sequences of genes that are final targets of action of toolkits will vary much more.
Thus, with quite a limited number of genetic toolkits, nature can develop a myriad different forms, from cabbages and sponges to honeybees and humans. This also explains why we do not need more than 30,000 genes to develop a human, as well as why our genome is about 99% identital to the chimpanzee genome. It is not the sequence of genes, but the combinatorics of the way the genes are turned on and off during the development that results in the final phenotype.
The common theme, then, is that evolution keeps tinkering with the same genetic toolkits over and over again. It is not neccessary to evolve thousands of completely new genes in order to have a new species spring up out of its ancestral species. A little tweak in developmental patterns of gene expression is all that is needed. The same genes may be expressed at a different place in the embryo in two different species (heterotopy), or may be expressed at a different time during development (heterochrony), or may result in expression of other final-target genes (heterotypy). Such changes account for most of the evolution of diversity of life on Earth.
Of course, such changes take a long time. It took about 3.6 billion years for life to evolve from the first primitive bacteria-like cells to the current diversity of millions of species of Bacteria, Archaea, Protista, Fungi, Plants and Animals. Our brains have never before needed to be able to comprehend such vastness of time. We do qute well with durations of seconds, minutes, hours and days. We are pretty good at mentally picturing the duration of weeks, months and years. A decade is probably the longest duration of time that our brains can correctly imagine. Already our perception of a century is distorted. Perception of a thousand years is impossible for human brains. Now try to imagine how long 10,000 years is? Any luck? Now try 100,000. How about 1.000,000 years? Add another zero and try comprehending 10.000,000 years. Multiply by ten again and try 100.000,000 years. Now try 1,000.000,000 years. Now try four times more – 4 billion years.
It is not surprising that some people, unable to comprehend 4 billion years, just plainly refuse to acknowledge that this amount of time actually passed and stick to a shorter, emotionally more pleasing yet incorrect number of about 6,000 years for the age of the Universe. Such people, of course, cannot believe that evolution actually happened, although mountains of evidence show us not just that it happened, but exactly how it happened. You can see exactly what happened when if you take your time and do this animation. You’ll notice how the whole of human history is too short to be visible on a line representing billions of years. Given such enormous amount of time, the evolution of amazing diversity of life is not surprising. Actually, if such diversity did not arise – that would be a surprise.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 18
Filed under: Uncategorized | Tags: biology, evolution, origin of life, science
Adaptation vs. Diversity
Biology is concerned with answering two Big Questions: how to explain the adaptation of organisms to their environments and how to explain the diversity of life on Earth.
Much of the course content so far engaged the question of the origin and evolution of adaptation, and much of the remainder of the course will also look at particular adaptations of humans and other vertebrates. This is the only lecture specifically targeting the question of diversity.
The way this material is usually taught is to go over long lists of organisms and tabulate their characters, how the members of one group are similar to each other and different from members of other groups. We, in our course, will try a different approach, i.e., not just describing, but also explaining diversity – how it comes about.
If you think about it, knowing what we learned so far about the way evolution works, the origin of adaptation and the origin of diversity are deeply intertwined: as local populations evolve adaptations to their current local environments, they become more and more different from each other until the species splits into two or more new species. Thus, evolution of adaptations to local conditions leads to proliferation of new species, thus to the increase in overall diversity of life on the planet.
Origin of Life
One can postulate four ways the life on Earth came about: a) it was created – poof! – out of nothing by an intelligent being, e.g., God; b) it was created – poof! – out of nothing by an intelligent being, e.g., space aliens, either on Earth or elsewhere, then brought to Earth; c) it spontaneously arose elsewhere in the Universe and was brought to Earth by comets and meteors; and d) it spontaneously arose out of chemical reactions in the ancient seas in the presence of the ancient atmosphere.
Science is incapable of addressing the first notion – being untestable and unfalsifiable (impossible to prove that it is wrong), it is properly outside of the realm of science and within the domain of religion.
The first three notions also just move the goalposts one step further – how did life (including God and/or Aliens) arise elsewhere in the Universe? Thus, scientists focus only on the one remaining testable hypothesis – the one about spontaneous and gradual generation of life out of non-life, a process called abiogenesis. The scientific study of abiogenesis cannot say and does not attempt to say, anything about existence of God or Aliens. It only attempts to figure out how life could have arisen on its own, sometime between 3 and 4 billion years ago.
All of life on Earth descends from a single common ancestor. It is quite possible that life initially arose multiple times, but as soon as one life form became established and competitive enough, all the other instances of spontaneous generation of life were outcompeted and did not leave progeny.
It is difficult to study the origin of life as molecules do not leave fossils. They do leave chemical traces, though, so we know a lot about the chemistry of the ancient oceans, soil and atmosphere. Thus, we know under what conditions and what available materials (and energy) life first arose. By replicating such conditions in the laboratory, we can study the details of how life might have evolved out of non-life.
The study of the origin of life is a lively and exciting area of biology, perhaps because so little has yet been settled with great certainty. There are a number of competing hypotheses promoted by various research groups. Those hypotheses can be classified into groups: RNA First, Protein First, RNA-Protein First and Bubbles First.
RNA is a molecule that can be replicated and thus can serve as the original hereditary material (DNA is too large and complex even for some of today’s viruses, let alone for the first simple organisms). RNA is also capable of catalytic activity – promoting and speeding up reactions between other molecules, as well as replicating itself. Thus, RNA is the best candidate for the first molecule of life. Still, it is not capable of everything that life needs, so a few simple polypeptides (and those are really easy to synthetize in a flask mimicking the original Earth conditions) were probably involved from the very beginning. For those reactions to occur, they had to be separated from the remaining ocean – thus some kind of “cell membrane”, like a soap bubble, was also neccessary for the origin and early evolution of life.
Those early “cells” competed against each other. Those that, through chemical evolution, managed to become good enough at remaining stable for a decent amount of time, capable of acquiring the energy from the environment, and capable of dividing into two “daughter cells” outcompeted the others – chemical evolution turned into biological evolution. As they changed through trial and error, some cells gradually got better at “living” and outcompeted all others. One best competitor of the early living world is the common ancestor of all of the subsequent life on Earth, including us.
Directionality of Evolution
There are two common misconceptions about evolution. First is the idea that evolution tends towards perfection. But, always remember that evolution favors individuals who are slightly better optimized to current local conditions than other individuals of the same species, i.e., what wins is the relative fitness, not absolute fitness (i.e., perfection). In other words, you have to be capable of surviving and reproducing in your current environment and be just a tad little bit better at it than your conspecifics – there is no need to be perfectly adapted.
The second common misconception about evolution is that it has a tendency to generate greater complexity. Originally, right after the initial origin of life on Earth, evolution did produce greater complexity, but only because there was no way to become any more simple than the first organisms already were. There is a “left wall” of complexity in the living world, i.e. there is a minimum complexity that is neccessary for something to be deemed alive.
Thus, initially, the only direction evolution could take was away from the left wall (red dot), i.e., becoming more complex. But once reasonable complex organisms evolved, they were not snuggled against the left wall any more (yellow dot). Adaptation to current local conditions can equally promote simplification as it does complexification of the organism in question. In other words, as populations evolve, the members of the populations are equally likely to become simpler than they are to become more complex.
Actually, as we know from the world of man-made machines, there is such a thing as being too complex (blue dot). Over-complicated machines break down much more easily and are more difficult to maintain and repair. Likewise, organisms of great complexity are often not as fit as their simpler relatives – their genomes are so large that the error rate is greater and cell division is more difficult. Cells can “go wild” and turn into cancer. Also, with so many interacting parts, it is more difficult for complicated organisms to evolve new adaptations as the development of the whole complex system has to change and adapt to such changes.
Thus, simplification is as often seen in evolution as is acquisition of greater complexity. Just think of parasites – they are all simplified versions of their free-living relatives – no need for eyes, other sensory organs or means of locomotion if one spends one’s life attached to the lining of the host’s intestine, sucking in nutrients and growing billions of eggs.
Measuring Diversity – Taxonomy and Systematics
People have always tried to classify living beings around them, by grouping them according to some man-made criteria, usually by the way they look, where they live, and how useful they may be to us. Only for the past 150 years we have understood that all organisms on our planet are geneologically related to each other, so we started classifying them according to the degree of relatedness – drawing family trees of Life.
Initially, classification was done according to anatomy and embryology of organisms. Such methods brought about the division of Life on Earth into six great Kingdoms: Bacteria, Archaea, Protista, Plants, Fungi and Animals. The first two are Prokaryotes (no nucleus), the latter four are Eukaryotes (cells have a nucleus).
The Kingdoms were, like Russian dolls, further subdividied into nested hierachies: each Kingdom was composed of a number of Phyla (Phylum = type). Each Phylum consists of Classes, those are made of Orders that are further subdivided into Families. Each family consists of Genera and each Genus is composed of the most closely related Species.
The proper name of each living organism on Earth is its binomial Latin name – capitalized name of the Genus and lower-case name of the species, both italicized, e.g., Homo sapiens, Canis familiaris, Equus caballus, Bos taurus (human, dog, horse and cow, respectively).
Lately, modern molecular genetic techniques have been applied to testing relationships between species, resulting in many changes in classification at lower levels of systematics (e.g,. species, genus, family, etc).
The knowledge gained from this approach also resulted in some big changes in the way we classify living organisms. Instead of six Kingdoms, we now divide life on Earth into three Domains: Bacteria, Archaea and Eukarya.
We are now aware that endosymbiosis (intercellular parasites, originally small bacterial cells entering and living inside larger bacterial cells) gave rise to organelles, like mitochondria and chloroplasts. We are now aware how much lateral (or horizontal) tranfer of genetic material is going on between species, i.e., the branching tree of life has many traversing connections between branches as well.
Cladistics is a relatively new method of classifying organisms, using multiple (often many) different genetic, morphological and other traits and building “trees” by calculating (using computer software) the probabilities of each two of the species being related to each other. Thus, “most likely” trees are plotted as cladograms which can further be tested and refined.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 17
Filed under: Uncategorized
Imagine a small meadow. And imagine in that meadow ten insects. Also imagine that the ten insects are quite large and that the meadow has only so much flowers, food and space to sustain these ten individuals and not any more. Also imagine that the genomes of those ten insects are identical, except for one individual: that one has a mutation in one gene (due to an error in DNA replication, or due to crossing-over during meiosis). That mutation, during development led to the induction of the production of more mitochondria in each muscle cell.
Normally, that mutation is not obvious – the insect flitters from flower to flower just like anyone else. However, if the situation arises, the mutant individual is just a tiny little bit faster because the additional mitochondria in muscles allow it to switch from aerobic to anaerobic sources of energy later than in other individuals. Thus, the “normal” individuals can fly one yard in one second, while the mutant can fly one yard plus one inch in one second.
Now imagine that, over some time period, a bird comes by the meadow four times. Each time, the bird chases the insects and catches the one that is the closest to her. Which individual is, statistically speaking, least likely to get caught and eaten? The mutant, as the little extra speed may give it just enough edge in comparison to other individuals. This comparative “extra edge” is called increased fitness.
After four insects have been eaten, six remain – three males and three females. They pair up, mate, lay eggs and die. Each pair lays, let’s say eight eggs, which all hatch, proceed normally through the larval development and become adults. This makes a total of 24 insects in a meadow that can support only ten individuals. At the same time, the bird has laid eggs, the eggs hatched and the hatchlings sometimes come to the meadow to hunt.
Let’s look at the genetics of this population for a moment. Two pairs of “normal” insects produced a total of 16 offspring, all of them “normal”. The offspring of one “normal” and one “mutant” each got one of the chromosomes from the mother, the other one from the father. All of them will have the mutation on one, and not on the other chromosome. Let’s say that having a mutation on only one chromosome adds a half-inch to the yard-per-second flaying speed. The full mutant is homozygous for this mutation. The half mutant is heterozygous for this mutation. The heterozygous individuals are still relatively more fit than the “normals”. As the hatchling birds hunt down the insects and cut down the population to ten individuals, the half-mutants are more likely to be present in the remaining population than the non-mutants.
Let’s call the “normal” variant of the gene A and the “mutant” variant of the same gene a. A and a are alleles of the same gene.
In the next generation, some normals will breed with normals, producing normal offspring. Some half-mutants will mate with normals and produce a mix of normals and half-mutants. Some half-mutants will mate with some half-mutants and the resulting eight offspring will consist of 2 normals (AA), two mutants (aa), and four semi-mutants (Aa).
As the a allele confers relative fitness to its carriers, this allele will spread through the population over several generations and either completely eliminate allele A, or attain some stable balanced ratio in the population.
When one compares the genetic composition of this population over generations, one notices that it changes over time, from preponderance of A in the first generation, through a series of intermediate stages, to the preponderance of a in the last generation.
The change of genetic composition of a population over multiple generations is called evolution. That sentence is the most commonly used definition of evolution.
The process that favored one allele over the other, resulting in evolution of flight speed in these insects, is called natural selection.
The environment – the carrying capacity of the meadow plus the bird predators – was the selecting agent. The process that turns a genetic change (mutation) into a trait that can affect fitness of the whole organism is development. Thus, one can also define evolution as “change of development by ecology”.
For evolution to proceed, the trait must vary in a population, one of the variants has to confer greater fitness than the other variants, there has to be a limit on the fecundity (how many offspring can survive in each generation) leading to differential rate of reproduction, and the trait has to be heritable, i.e., the offspring have to be more like parents in respect to that trait than like other individuals in the population. The inheritance is usually, though not always, conferred by the genome (the DNA sequence).
The example we used is quite unrealistic. Populations are much more likely to number in thousands or millions than just ten individuals. Thus, instead of a few generations, it may take thousands or millions of generations for a new allele to sweep through the population. In annually breeding organisms, this means thousands to millions of years. In slow-breeding animals, like elephants, it will take even longer. In fast reproducers, like bacteria, this may only take several months or years, as in evolution of antibiotic resistance in bacteria or evolution of pesticide resistance in agricultural pests.
Another way that the example was unrealistic was the assumption that all the individuals were genetically identical to each other except for that one mutation in that one gene. In reality, there will be variation (two or more alleles) in every gene, and new mutations show up all the time. Some mutations decrease fitness, some are neutral and some increase fitness. Some alleles affect fitness depending on which other alleles of other genes are present in the same individuals, or depending on the environment it finds itself in at a particular time, as in the norm of reaction phenomenon. Due to this, some combinations of alleles may tend to move from one generation to the next together.
Finally, in many organisms, genes can be transmitted horizontally – not from parent to offpspring but directly from one individual to another. This most often happens in bacteria, where individual bacteria may excahge bits and pieces of their DNA. Likewise, viruses are carriers of DNA sequences from one organism to another as well. Some of the sequences in our genome are of bacterial origin, transmitted some time in the past by viruses, and now fully integrated into our genome and even assuming an indispensible function. For instance, HERV genes are originally viral genes that are now parts of our genome and are neccessary for the development of the placenta.
Thus, in the real world, the situation is more complicated than in our example. Still, the proportions of various alleles of many genes are constantly changing – evolution occurs all the time.
Let’s now assume that our insects live in a much larger area and that there are millions of them. The frequences of various alleles fluctuate all the time, and there is quite a lot of genetic variation contained in the population. Natural selection may work on preserving the average phenotype as its fitness is high and outliers at each end have lower fitness. This is called stabilizing selection.
As the climate slowly changes, or other aspects of the environment change, the relative frequences of alleles of various genes will track those changes. New conditions may, for instance select for larger body size. The largest individuals tend to leave most offspring, while the smallest individuals, on average, put the least of their genes into the next generation. The selection for large body size is an example of directed selection.
In some cases, selection may favor the extremes, but not the middle. Fast fliers may be selected for because they can escape the birds. The slowest fliers may be selected because they mostly walk or crawl and are thus not easily spotted by birds. They are also fit, but via a different strategy. The medium-speed fliers are selected against. This is an example of discruptive selection, forming two different morphs of the same species.
If those two morphs tend to, on average, be more likely to find each other and mate with each other within a morph than between two morphs, this may lead to splitting the species into two species – this is called sympatric speciation. As the gene flow between the two groups declines, more and more mutations/alleles will be found only in one morph and not the other. Those genes will also be under the influence of selection, and the selecting environment is different between crawlers and fliers. Soon enough, the individuals belonging to the two groups will not even recognize each other as belonging to the same species. Even if they recognize each other, they may not like each other (“mate-choice”) enough to mate. Even if they mate, their eggs may not be fertile. Even if their eggs are fertile, the resulting offspring may not be fertile (hybrids, like mules for instance). If, for whatever reason, two related populations do not, will not or cannot interbreed, they have became separate species – speciation occured.
Imagine now that a small cohort of about ten individuals got blown away by wind from the mainland to a nearby island. The mainland population is huge. The island population is tiny. The ability of any mutation or any allele to spread fast through the population is much greater in a small group. The selective pressures are also different.
It may be better for the island insects to be small and for the mainland insects to be large, perhaps due to the types of flowers or kinds of predators that are present. The mainland insects may be selected for high flying speed because of bird predation. The island insects may not have any bird predators, but, those individuals who are the best fliers are most likely to be swept off the island by wind and drown in the ocean, never placing their genes into the next generation. Thus, they are selected not to fly, even to lose their wings.
If, after a number of generations, those two populations again get into contact – e.g., a land bridge gradually arises, or another cohort of mainland insects floats on a log onto the island, the two populations will not recognize each other as the same species (or not like each other enough to mate, or not having fertile eggs or offspring). Thus, they have also become reproductively isolated, thus, by definition, they have become two separate species. Speciation occured. This type of speciation, where a geographic barrier separates two parts of a population preventing gene flow between them is called allopatric speciation, and is much better documented and much less controversial than sympatric speciation.
Billions of such speciation events, meaning branching of species into two or more species, resulted in the evolution of all species of organisms on Earth from a single common ancestor (a very primitive bacterium) over a period of more than 3.5 billion years.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapters 14, 15 and 16
Filed under: Uncategorized | Tags: biology, genetics, genotype, phenotype, science, science education
One often hears news reports about discoveries of a “gene for X”, e.g., gene for alcoholism, gene for homosexuality, gene for breast cancer, etc. This is an incorrect way of thinking about genes, as it implies a one-to-one mapping between genes and traits.
This misunderstanding stems from historical precedents. The very first genes were discovered decades ago with quite primitive technology. Thus, the only genes that could be discovered were those with large, dramatic effects on the traits. For instance, a small mutation (change in the sequence of nucleotides) in the gene that codes for RNA that codes for one of the four elements of the hemoglobin protein results in sickle-cell anemia. The red blood cells are, as a result, mishapen and the ability of red blood cells to carry sufficient oxygen to the cells is diminished.
Due to such dramatic effects of small mutations, it was believed at the time that each gene codes for a particular trait. Today, it is possible to measure miniscule effects of multiple genes and it is well understood that the “one gene/one trait” paradigm is largely incorrect. Most traits are affected by many genes, and most genes are involved in the development of multiple traits.
A genome is all the genetic information of an individual. Each cell in the body contains the complete genome. Genomes (i.e., DNA sequences) differ slightly between individuals of the same species, and a little bit more between genomes of closely related species, yet even more between distantly related species.
Exact DNA sequence of an individual is its genotype. The collection of all observable and measurable traits of that individual is phenotype.
If every position and every function of every cell in our bodies was genetically determined, we would need trillions of genes to specify all that information. Yet, we have only about 30,000 genes. All of our genes are very similar to the equivalent genes of chimpanzees, yet we are obviously very different in anatomy, physiology and behavior from chimpanzees. Furthermore, we share many of the same genes with fish, insects and even plants, yet the differences in phenotypes are enormous.
Thus, it follows logically that the metaphor of the genome as a blueprint for building a body is wrong. It is not which genes you have, but how those genes interact with each other during development that makes you different from another individual of the same species, or from a salmon or a cabbage.
But, how do genes interact with each other? Genes code for proteins. Some proteins interact with other proteins. Some proteins regulate the transcription or replication of DNA. Other proteins are enzymes that modify other chemicals. Yet other proteins are structural, i.e., become parts of membranes and other structures.
A slight difference in the DNA sequence will have an effect on the sequence of RNA and the sequence of the resulting protein, affecting the primary, secondary and tertiary structure of that protein. The changes in 3D shape of the protein will affect its efficiency in performing its function.
For instance, if two proteins interact with each other, and in order to do so need to bind each other, and they bind because their shapes fit into each other like lock and key, then change of shape of one protein is going to alter the efficiency of binding of the two. Changes in shapes of both proteins can either slow down or speed up the reaction. Change of rate of that one reaction in the cell will have effects on some other reaction in the cell, including the way the cell reacts to the signals from the outside.
Thus genes, proteins, other chemicals inside the cell, intercellular interactions and the external environment ALL affect the trait. Most importantly, as the traits are built during development, it is the interactions between all these players at all levels of organizations during development that determine the final phenotype of the organism.
The importance of the environment can be seen from the phenomenon of the norm of reaction. The same genotype, when raised in different environments results in different phenotypes. Furthermore, different genotypes respond to the same environmental changes differently from each other. One genotype may produce a taller plant at higher elevation while a slightly different genotype may respond quite the opposite: producing a shorter plant at higher elevations.
So, if genes do not code for traits, and the genome is not a blueprint, what is the best way to think about the genome and the genotype/phenotype mapping? I have given you handouts (see below) with four different alternative metaphors, at least one of which, I hope, will feel clear and memorable to each student. I will now give you a fifth such metaphor, one of my own:
Imagine that a cell is an airplane factory. It buys raw materials and sells finished airplains. How does it do so? The proteins are the factory workers. Some of them import the materials, others are involved in the sale of airplanes. Some guard the factory from thieves, while others cook and serve food in the factory cafeteria.
But the most important proteins of this cell are those that assemble the parts of airplanes. When they need a part, e.g., a propeller, they go to the storeroom (nucleus) and check the Catalogue Of Parts (the DNA), and press the button to place an order for a particular part. Other proteins (storeroom managers) go inside and find the correct part and send it to the assembly floor (endoplasmatic reticulum).
But, protein workers are themselves robots assembled out of parts right there in the same factory, and the instructions for their assembly are also in the Catalogue of Parts (DNA) in the nucleus.
How do you wear your genes? by Richard Dawkins.
An analogy for the genome by Richard Harter.
It’s not just the genes, it’s the links between them by Paul Myers
PZ Myers’ Own Original, Cosmic, and Eccentric Analogy for How the Genome Works -OR- High Geekology by Paul Myers
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 12
Filed under: Uncategorized | Tags: biology, cell, cell differentiation, embryonic development, science
There are about 210 types of human cells, e.g., nerve cells, muscle cells, skin cells, blood cells, etc. Wikipedia has a nice comprehensive listing of all the types of human cells.
What makes one cell type different from the other cell types? After all, each cell in the body has exactly the same genome (the entire DNA sequence). How do different cells grow to look so different and to perform such different functions? And how do they get to be that way, out of homogenous (single cell type) early embryonic cells that are produced by cell division of the zygote (the fertilized egg)?
The difference between cell types is in the pattern of gene expression, i.e., which genes are turned on and which genes are turned off. Genes that code for enzymes involved in detoxification are transribed in lver cells, but there is not need for them to be expressed in muscle cells or neurons. Genes that code for proteins that are involved in muscle contraction need not be transcribed in white blood cells. The patterns of gene expression are specific to cell types and are directly resposible for the differences between morphologies and functions of different cells.
How do different cell types decide which genes to turn on or off? This is the result of processes occuring during embryonic development.
The zygote (fertilized egg) appears to be a sphere. It may look homogenous, i.e., with no up and down, left or right. However, this is not so. The point of entry of the sperm cell into the egg may provide polarity for the cell in some organisms. In others, mother may deposit mRNAs or proteins in one particular part of the egg cell. In yet others, the immediate environment of the egg (e.g., the uterine lining, or the surface of the soil) may define polarity of the cell.
When the zygote divides, first into 2, then 4, 8, 16 and more cells, some of those daughter cells are on one pole (e.g., containing maternal chemicals) and the others on the other pole (e.g., not containing maternal chemicals). Presence of chemicals (or other influences) starts altering the decisions as to which genes will be turned on or off.
As some of the genes in some of the cells turn on, they may code for proteins that slowly diffuse through the developing early embryo. Low, medium and high concentrations of those chemicals are found in diferent areas of the embryo depending on the distance from the cell that produces that chemical.
Other cells respond to the concentration of that chemical by turning particular genes on or off (in a manner similar to the effects of steroid hormones acting via nuclear receptors, described last week). Thus the position (location) of a cell in the early embryo largely determines what cell type it will become in the end of the process of the embryonic development.
The process of altering the pattern of gene expression and thus becoming a cell of a particular type is called cell differentiation.
The zygote is a totipotent cell – its daughter cells can become any cell type. As the development proceeds, some of the cells become pluripotent – they can become many, but not all cell types. Later on, the specificity narrows down further and a particular stem cell can turn into only a very limited number of cell types, e.g., a few types of blood cells, but not bone or brain cells or anything else. That is why embryonic stem cell research is much more promising than the adult stem cell research.
The mechanism by which diffusible chemicals synthesized by one embryonic cell induces differentiation of other cells in the embryo is called induction. Turning genes on and off allows the cells to produce proteins that are neccessary for the changes in the way those cells look and function. For instance, development of the retina induces the development of the lens and cornea of the eye. The substance secreted by the developing retina can only diffuse a short distance and affect the neighboring cells, which become other parts of the eye.
During embryonic development, some cells migrate. For instance, cells of the neural crest migrate throughout the embryo and, depending on their new “neighborhood” differentiate into pigment cells, cells of the adrenal medula, etc.
Finally, many aspects of the embryo are shaped by programmed cell death – apoptosis. For instance, early on in development our hands look like paddles or flippers. But, the cells of our fingers induce the cell death of the cells between the fingers. Similarly, we initially develop more brain cells than we need. Those brain cells that establish connections with other nerve cells, muscles, or glands, survive. Other brain cells die.
Sometimes just parts of cells die off. For instance, many more synapses are formed than needed between neurons and other neurons, muscles and glands. Those synapses that are used remain and get stronger, the other synapses detach, and the axons shrivel and die. Which brain cells and which of their synapses survive depends on their activity. Those that are involved in correct processing of sensory information or in coordinated motor activity are retained. Thus, both sensory and motor aspects of the nervous system need to be practiced and tested early on. That is why embryos move, for instance – testing their motor coordination. That is why sensory deprivation in the early childhood is detrimental to the proper development of the child.
The details of embryonic development and mechanisms of cell differentiation differ between plants, fungi, protists, and various invertebrate and vertebrate animals. We will look at some examples of those, as well as some important developmental genes (e.g., homeotic genes) in future handouts/discussions, and will revisit the human development later in the course.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 40
Filed under: Uncategorized | Tags: biology, cell division, DNA, DNA replication, meiosis, mitosis, science
In the first lecture, we covered the way science works and especially how the scientific method applies to biology. Then, we looked at the structure of the cell, building a map of the cell – knowing what processes happen where in the cell, e.g., the production of energy-rich ATP molecules in the mitochondria.
In the third part of the lecture, we took a closer look at the way DNA code gets transcribed into RNA in the nucleus, and the RNA code translated into protein structure in the rough endoplasmatic reticulum. Finally, we looked at several different ways that cells communicate with each other and with the environment, thus modifying cell function.
All of that information will be important in this lecture, as we cover the ways cells divide, how cell-division, starting with a fertilized cell, builds an embryo, how genetic code (genotype) influences the observable and measurable traits (phenotype) and, finally, how do these processes affect the genetic composition of the populations of organisms of the same species – the process of evolution.
The only way to build a cell is by dividing an existing cell into two. As the genome (the complete sequence of the DNA) is an essential part of a cell, it is neccessary for the DNA to be duplicated prior to cell division.
In Eukaryotic cells, chromosomes are structures composed mostly of DNA and protein. DNA is a long double-stranded chain-like molecule. Some portions of the DNA are permanently coiled and covered with protective proteins to prevent DNA expression (transcription). Other parts can be unraveled so transcription can occur.
The number of chromosomes is different in different species. Human cells possess 23 pairs of chromosomes. Prior to cell division each chromosome replicates producing two identical sister chromosomes – each eventually landing in one of the daughter cells.
The process of DNA replication – the way all of the DNA code of the mother cell duplicates and one copy goes into each daughter cell – is the most important aspect of cell division. It is wonderfully described in your handout and depicted in the animation. Other cell organelles also divide and split into two daughter cells. Once the process of DNA replication is over, the new portion of the cell membrane gets built transecting the cell and dividing all the genetic material into two cellular compartments, leading the cell to split into two cells.
Meiosis is a special case of cell division. While mitosis results in division of all types of cells in the body, meiosis results in the formation of sex cells – the gametes: eggs and sperm. Mitosis is a one-step process: one cell divides into two. Meiosis is a two-step process: one cell divides into two, then each daughter immediately divides again into two, resulting in four grand-daughter cells.
Each cell in the body has two copies of the entire DNA – one copy received from the mother, the other from the father. Fertilization (fusion of an egg and a sperm) would double the chromosome number in each generation if the egg and sperm cells had the duplicate copy. Meiosis ensures that gametes have only one copy of the genome – a mix of maternal and paternal sequences. Such a cell is called a haploid cell.
Once the egg and a sperm fuse, the resulting zygote (fertilized egg) again contains double dose of the DNA and is called a diploid cell. Thus the resultant zygote inherits genetic material from both its father and its mother. All the cells in the body except for the gametes are diploid. Sexual reproduction produces offspring that are genetically different from either parent.
DNA replication is a complex process of duplication of the DNA involving many enzymes. It is the first and the most important process in cell division. Please read the handout (BREAKFAST OF CHAMPIONS DOES REPLICATION by David Ng) to appreciate the complexity of the process, but you do not need to memorize any of the enzymes for the exams. Also, it will help your understanding of the process if you watch this animation.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 11
THE CELL CYCLE: A UNIVERSAL CELLULAR DIVISION PROGRAM By David Secko
Mitosis – Neurotopia
Filed under: Uncategorized | Tags: biology, cell, cell-cell interactions, hormones, science
Cells do not exist in complete isolation. For a coordinated function of cells in a tissue, tissues in an organ, organs in a system and systems in the body, cells need to be able to communicate with each other. Each cell should be capable of sending chemical signals to other cells and of receiving chemical signals from oter cells, as well as signals (chemical or other) from its immediate environment.
Cell membrane is a double layer of molecules of fat. Some small chemical messengers are capable of passing through the membrane. Most ions and most molecules cannot pass through the membrane, thus the information between the inside and the outside of the cell is mediated by proteins embedded in the membrane.
Membrane proteins serve various functions. For instance, such proteins form tight junctions that serve to glue neighboring cells together and prevent passage of substances between the two cells. Other surface proteins are involved in cell-cell recognition, which is important for the immune response. Other membrane proteins serve functions in communication between the inside of the cell and the cell’s immediate environment.
How does a cell send a signal?
A cell can communicate signals to other cells in various ways. Autocrine signaling is a way for a cell to alter its own extracellular environment, which in turn affects the way the cell functions. The cell secretes chemicals outside of its membrane and the presence of those chemicals on the outside modifies the behavior of that same cell. This process is important for growth.
Paracrine signaling is a way for a cell to affect the behavior of neighboring cells by secreting chemicals into the common intercellular space. This is an important process during embryonic development.
Endocrine signaling utilizes hormones. A cell secretes chemicals into the bloodstream. Those chemicals affect the behavior of distant target cells. We will go into more details of autocrine, paracrine and endocrine signaling later on, when we tackle the human endocrine system.
Direct signaling is a transfer of ions or small molecules from one cell to its neighbor through pores in the membrane. Those pores are built out of membrane proteins and are called gap junctions. This is the fastest mode of cell-cell communication and is found in places where extremely fast and well-coordinated activity of cells in needed. An example of this process can be found in the heart. The muscle cells in the heart communicate with each other via gap junctions which allows all heart cells to contract almost simultaneously.
Finally, synaptic signaling is found in the nervous system. It is a highly specific and localized type of paracrine signalling between two nerve cells or between a nerve cell and a muscle cell. We will go into details of synaptic signaling when we cover the human nervous system.
How does a cell receive a signal?
Some small molecules are capable of entering the cell through the plasma membrane. Nitrous oxide is one example. Upon entering the cell, it activates an enzyme.
Some small hormones also enter the cell directly, by passing through the membrane. Examples are steroid hormones, thyroid hormones and melatonin. Once inside the cell, they bind cytoplasmic or nuclear receptors. The hormone-receptors complex enters the nucleus and binds to a particular sequence on the DNA. Binding dislodges a protein that inhibits the expression of the gene at that segment, so the gene begins to be transcribed and translated. Thus, a new protein appears in the cell and assumes its normal function within it (or gets secreted). The action of nuclear receptors is slow, as it takes some hours for the whole process to occur. The effect is long-lasting (or even permanent) and changes the properties of the cell. This type of process is important in development, differentiation and maturation of cells, e.g., gametes (eggs and sperm cells).
There are three types of cell surface receptors: membrane enzymes, ion channels, and transmembrane receptors.
When a signaling chemical binds to the membrane enzyme protein on the outside of the cell, this triggers a change in the 3D conformation of that protein, which, in turn, triggers a chemical reaction on the inside of the cell.
When a signaling molecule binds to an ion channel on the outside of the cell, this triggers the change of the 3D conformation of the protein and the channel opens, allowing the ions to move in or out of the cell following their electrical gradients and thus altering the polarization of the cell membrane. Some ion channels respond to non-chemical stimuli in the same way, including changes in electrical charge or mechanical disturbance of the membrane.
G protein-linked receptors are seven-pass transmembrane proteins. This means that the polypeptide chain traverses the membrane seven times. When a chemical – a hormone or a pharmaceutical agent – binds to the receptor on the outside of the cell, this triggers a series of chemical reactions, including the movement and binding of the G-protein, transformation of GTP into GDP and activation of second messengers. Second messengers (e.g., cyclic AMP) start a cascade of enzymatic reactions leading to the cellular response. This signaling method is quite fast and, more importantly, it amplifies the signal. Binding of a single hormone molecule quickly results in thousands of molecules of second messengers acting on even more molecules of enzymes and so on. Thus, the response to a small stimulus can be very large. We will go into details of G-protein-mediated signaling when we tackle the endocrine and the sensory systems.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 5
Filed under: Uncategorized | Tags: biology, cell, DNA, protein, RNA, science, transcription, translation
The DNA code
DNA is a long double-stranded molecule residing inside the nucleus of every cell. It is usually tightly coiled forming chromosomes in which it is protected by proteins.
Each of the two strands of the DNA molecule is a chain of smaller molecules. Each link in the chain is composed of one sugar molecule, one phosphate molecule and one nucleotide molecule. There are four types of nucleotides (or ‘bases’) in the DNA: adenine (A), thymine (T), guanine (G) and cytosine (C). The two strands of DNA are structured in such a way that an adenine on one strand is always attached to a thymine on the other strand, and the guanine of one strand is always bound to cytosine on the other strand. Thus, the two strands of the DNA molecule are mirror-images of each other.
The exact sequence of nucleotides on a DNA strand is the genetic code. The total genetic code of all of the DNA on all the chromosomes is the genome. Each cell in the body has exactly the same chromosomes and exactly the same genome (with some exceptions we will cover later).
A gene is a small portion of the genome – a sequence of nucleotides that is expressed together and codes for a single protein (polypeptide) molecule.
Cell uses the genes to synthetise proteins. This is a two-step process. The first step is transcription in which the sequence of one gene is replicated in an RNA molecule. The second step is translation in which the RNA molecule serves as a code for the formation of an amino-acid chain (a polypeptide).
For a gene to be expressed, i.e., translated into RNA, that portion of the DNA has to be uncoiled and freed of the protective proteins. An enzyme, called DNA polymerase, reads the DNA code (the sequence of bases on one of the two strands of the DNA molecule) and builds a single-stranded chain of the RNA molecule. Again, where there is a G in DNA, there will be C in the RNA and vice versa. Instead of thymine, RNA has uracil (U). Wherever in the DNA strand there is an A, there will be a U in the RNA, and wherever there is a T on the DNA molecule, there will be an A in the RNA.
Once the whole gene (100s to 10,000s of bases in a row) is transcribed, the RNA molecule detaches. The RNA (called messenger RNA or mRNA) may be further modified by addition of more A bases at its tail, by addition of other small molecules to some of the nucleotides and by excision of some portions (introns) out of the chain. The removal of introns (the non-coding regions) and putting together the remaining segments – exons – into a single chain again, is called RNA splicing. RNA splicing allows for one gene to code for multiple related kinds of proteins, as alternative patterns of splicing may be controlled by various factors in the cell.
Unlike DNA, the mRNA molecule is capable of exiting the nucleus through the pores in the nuclear membrane. It enters the endoplasmatic reticulum and attaches itself to one of the membranes in the rough ER.
Three types of RNA are involved in the translation process: mRNA which carries the code for the gene, rRNA which aids in the formation of the ribosome, and tRNA which brings individual amino-acids to the ribosome. Translation is controlled by various enzymes that recognize specific nucleotide sequences.
The genetic code (nucleotide sequence of a gene) translates into a polypeptide (amino-acid sequence of a protein) in a 3-to-1 fashion. Three nuclotides in a row code for one amino-acid. There are a total of 20 amino-acids used to build all proteins in our bodies. Some amino-acids are coded by a single triplet code, or codon. Other amino-acids may be coded by several different RNA sequences. There is also a START sequence (coding for fMet) and a STOP sequence that does not code for any amino-acid. The genetic code is (almost) universal. Except for a few microorganisms, all of life uses the same genetic code.
When the ribosome is assembled around a molecule of mRNA, the translation begins with the reading of the first triplet. Small tRNA molecules bring in the individual amino-acids and attach them to the mRNA, as well as to each other, forming a chain of amino-acids. When a stop signal is reached, the entire complex disassociates. The ribosome, the mRNA, the tRNAs and the enzymes are then either degraded or re-used for another translational event.
Protein synthesis – post-translational modifications
Translation of the DNA/RNA code into a sequence of amino-acids is just the beginning of the process of protein synthesis.
The exact sequence of amino-acids in a polypeptide chain is the primary structure of the protein.
As different amino-acids are molecules of somewhat different shapes, sizes and electrical polarities, they react with each other. The attractive and repulsive forces between amino-acids cause the chain to fold in various ways. The three-dimensional shape of the polypeptide chain due to the chemical properties of its component amino-acids is called the secondary structure of the protein.
Enzymes called chaperonins further modify the three-dimensional structure of the protein by folding it in particular ways. The 3D structure of a protein is its most important property as the functionality of a protein depends on its shape – it can react with other molecules only if the two molecules fit into each other like a key and a lock. The 3D structure of the fully folded protein is its tertiary structure.
Prions, the causes of such diseases as Mad Cow Disease, Scabies and Kreutzfeld-Jacob disease, are proteins. The primary and secondary structure of the prion is almost identical to the normally expressed proteins in our brain cells, but the tertiary structure is different – they are folded into different shapes. When a prion enters a healthy brain cell, it is capable of denaturing (unwinding) the native protein and then reshaping it in the same shape as the prion. Thus one prion molecule makes two – those two go on and make four, those four make eight, and so on, until the whole brain is just one liquifiied spongy mass.
Another aspect of the tertiary structure of the protein is addition of small molecules to the chain. For instance, phosphate groups may be attached to the protein (giving it additional energy). Also, short chains of sugars are usually bound to the tail-end of the protein. These sugar chains serve as “ZIP-code tags” for the protein, informing carrier molecules exactly where in the cell this protein needs to be carried to (usually within vesicles that bud off the RER or the Golgi apparatus). The elements of the cytoskeleton are used as conduits (“elevators and escalators”) to shuttle proteins to where in the cell they are needed.
Many proteins are composed of more than one polypeptide chain. For instance, hemoglobin is formed by binding together four subunits. Each subunit also has a heme molecule attached to it, and an ion of iron attached to the heme (this iron is where oxygen binds to hemogolobin). This larger, more complex structure of the protein is its quaternary structure.
Filed under: Uncategorized | Tags: biology, cell structure, organelles, science
All living organisms are composed of one or more cells – the cell is the unit of organization of Life.
Most cells are very small. Exceptions? Ostrich egg is the largest cell. Nerve cell in a leg of a giraffe may be as long as 3m, but is very thin.
Basic Structure of the Cell
A cell is a small packet or bag of liquid. The liquid is cytoplasm (or cytosol), which is essentially salty water with various organic molecules suspended in it.
The cytoplasm is contained within a cell membrane. Cell membrane is a phospholypid bilayer – this means that it is composed of two layers of tighly packed molecules of fat. Within the membrane, proteins are embedded into the bilipid layer and are more or less free to move around within the membrane. These proteins are important for the communication between the inside and outside of the cell.
You can see a good image here.
On the outside of the membrane, some cells may have additional structures. For instance, many bacterial and plant cells have thick cell walls that confer more rigidity to the cell as well as better defense against mechanical, chemical or biological insults.
Some cells also have hair-like cilia on the surface (e.g., a protist called Silver Slipper), or long whip-like flagella at one end (e.g., sperm cells). Both of these structures allow the cell to move utilizing its own energy.
Inside every cell, there is hereditary material – DNA. Exceptions? Red blood cells which have a membrane and cytoplasm, but no hereditary material.
Differences between prokaryotes and eukaryotes:
Prokaryotes (bacteria) have a cell membrane and cytoplasm and no other organelles.
Eukaryotes (plants, animals, fungi, protista) have a number of different cell organelles.
The nuclear material in Prokaryotes is a single, circular strand of DNA.
The nuclear material in Eukaryotes is organized in multiple chromosomes contained with a nucleus.
Eukaryotic cells have organelles. Organelles are subcellular structures that provide internal compartmentalization and other functions.
Nuclues is a large membrane-bound organelle. Its function is to sequester the DNA from the rest of the cell. The nuclear membrane (or nuclear envelope), which is also a phospholipid bilayer, selectively allows molecules to pass between the nucleus and cytoplasm. Inside the nucleus, DNA is organized in chromosomes. A chromosome is a tighly coiled and wound strand of DNA packaged with various proteins (e.g,. histones).
Smooth endoplasmic reticulum is a system of membranes and is involved in carbohydrate and lipid synthesis.
Rough endoplasmic reticulum is a system of membranes that possesses ribosomes. Proteins are synthesized in the rough ER.
Golgi apparatus stores and packages various molecules. When a molecule is needed elsewhere in the cell, a portion of the Golgi membrane closes off and forms a vesicle that can be transported around the cell.
Some eukaryotic organelles contain a little bit of their own DNA: the mitochondria and the chloroplasts. These two organelles used to be intercellular parasites, i.e., different species of bacteria that, over time, became an integral part of a cell.
Chloroplasts are found in plant cells. Photosynthesis is the process that occurs in them.
Mitochondria are found in all Eukaryotic cells. Breakdown of glucose begins in the cytoplasm and ends in the mitochondria, where the final products of the breakdown are ATP, water, CO2 and heat. This process requires oxygen – that is why we breath: to provide the oxygen for the mitochodria and to get rid of carbon dioxide produced in the mitochondria.
ATP (adenosine triphosphate) is the energy currency of the living world. Every cellular process that requires energy gets it from ATP. Thus, mitochondria are sometimes refered to as “factories of the cell”.
The final portion of the process of glucose digestion (the Krebs cycle) is, like any process, not 100% efficient. Errors happen and not every atom of every glucose molecule ends up where it should: in ATP, water or CO2. The result of this inefficiency is production of heat and production of highly reactive small molecules called free radicals (e.g., hydrogen peroxyde, H2O2). Free radicals tend to quickly react with whatever molecule they first encounter upon leaving the mitochondria. Such reactions damage those molecules, be they proteins, lipids, sugars or nucleic acids. The intercellular damage caused by free radicals is one aspect of the process of aging.
Some animals – birds and mammals – have harnessed the heat production by the mitochondria to keep a stable internal temperature. The efficiency of the mitochondrial “machine” is held low under the control of hormones like thyroid hormones. As a result, there is a greater production of free radicals, so warm-blooded animals evolved particularly good mechanisms for neutralizing free radicals and for repairing the damage. If a person keeps a constant low temperature or constant low-grade fever, the first thing the physician will check is the function of the thyroid gland.
The cytoskeleton, composed of filaments and microtubules, anchors the organelles and gives a cell its shape. Microtubules move organelles, including vesicles, within a cell. They also move the membrane-embedded proteins around where they are needed.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 4
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Filed under: Uncategorized | Tags: biology, philosophy of science, science, scientific method
Introduction to Biology and the Scientific Method
A. Biology and Life
Biology is the science that studies life. What is life?
Unlike non-living matter, living things exhibit the following properties:
Order: a hierarchical organization (a ‘nested hierarchy’, like Russian dolls). This means that organisms are composed of organs that work together in a systematic manner, the organs are composed of tissues, tissues of cells, cells of organelles, organelles of molecules and molecules of atoms, with the entire organization built in a way that maximizes the internal order, survival and reproduction of the organism.
Crystals exhibit order, but it is not hierarchical, and does not give the crystal a maximal chance of survival and reproduction. In living organisms, the properties of higher levels of organization cannot, unlike in crystals, be explained by the elements at the lower level of organization. Interactions between lower-level elements result in emergent properties at higher levels. For instance, from the order of nucleotides in the DNA we cannot infer how the whole organism looks like or behaves because the sequence does not specify the rules of interactions between the genes, gene-products (proteins), cells during development, and organisms inside their environments.
Sensitivity: response to stimuli in the environment. Even the simplest organisms, like bacteria, are capable of sensing changes in the environment and responding to such changes – they may swim away from or towards areas with higher concentrations of nutrients, salt, oxygen, or levels of illumination. Such responses (e.g., swimming) are active. A seed or a spore, seemingly “dead”, will actively respond to good growing conditions by germinating. A piece of dead matter may expand or even melt at high temperature, but that response is passive – due purely to the laws of physics.
Growth, Development and Reproduction: having a life-cycle. Crystals may grow, but the growth does not change the basic organization of the crystal. On the other hand, growth of an organism is accompanied by reorganization, cell division and cell differentiation. Each organism, at least during some parts of its life cycle, undergoes growth, developmental changes, and production of offspring. The results of reproduction – the offspring – are similar to the parent(s) due to the code inherited via a molecule, either DNA or RNA.
Regulation: All organisms have evolved well-orchestrated biochemical, physiological and behavioral mechanisms that regulate all the organism’s functions, which include finding and ingesting nutrients, processing nutrients and supplying all cells with the end-products of such processing, sequestering and eliminating the by-products of nutrient use. Likewise, every organism has evolved elaborate mechanisms for absorbing, storing, converting, using and dissipating energy – this last criterion may be the most important criterion for testing if something is alive or not, e.g., if one discovers a potentially living form on another planet.
Homeostasis: maintaining relatively constant internal conditions. We will cover this in much detail when we start the unit on human anatomy and physiology.
We will study the details of all five of the above criteria in this course. During the first three lectures, we will look at general properties of living organisms at all levels, from molecules, organelles and cells, through tissues, organs, systems and organisms, to populations, species, communities and ecosystems. During the remainder of the course we will take a look at specific cases: bacteria, protista, fungi, plants and animals, as well as details of the functioning of the human body.
A. Scientific Method and Process
Deductive reasoning applies general principles to predict specific results. Inductive reasoning uses specific observations to construct general principles. Here is a brief description of the steps in the hypothetico-deductive method:
Scientists make observations of processes and events found in nature.
The observations lead to questions: what is this, how does it work, why does it work the way it does? This may necessitate further observations to be made.
The questions are then asked in a form that suggests a possible explanation (hypothesis) for the observations. Scientists try to come up with all possible explanations and pit them against each others as alternative hypotheses.
Using the available knowledge and understanding of the related phenomena, the scientist makes a best guess at which of the alternative hypotheses is most likely to be correct.
Experiments are designed in such a way that one or more hypotheses are tested. This means that the experiment is geared specifically towards rejecting one’s favored hypothesis: it is directly testing if that hypothesis is wrong. If the results are positive, the favored hypothesis is not rejected, but the alternative hypotheses may be rejected. If the results are negative, the favored hypothesis is rejected and one or more of the alternative hypotheses are accepted and further directly tested.
Often, two experiments are conducted at the same time. In one experiment, all the variables are kept constant except one, while the other experiment is called the control experiment, and in that experiment, that variable is left unaltered. The results of the two experiments are compared to each other using statistical methods to determine if the tested variable (the one not kept constant) indeed has an effect on the outcome.
After performing a series of experiments, a paper is written that provides some background information, describes the experimental methods and results, provides the statistical analysis, and draws conclusions from the results. The paper is then submitted for peer review and published in a scientific journal. We will take a look at some real scientific papers later on in the course, so you can see the structure and form of it and be able to find and read such primary literature.
Once all but one alternative hypothesis has been rejected over a series of experiments, the one remaining hypothesis is further tested. The hypothesis, if correct, can be used to make predictions which can be directly tested in subsequent experiments. Predictions provide a way to test the validity of a hypothesis.
As more and more studies are done and the hypothesis gets stronger and stronger (as all possible alternatives get rejected), it grows in its predictive power and it may also grow in its ability to explain a broader range of phenomena. Once a hypothesis reaches the stage at which it is supported with large amounts of evidence after repeated testing, it becomes a theory.
A theory is a body of interconnected concepts most strongly supported by scientific reasoning and experimental evidence. It is a scientific term that is used to denote the scientific concepts that have stood the test of time and are best supported by experimental evidence.
This sense of the word “theory” – the scientific ideas with the greatest certainty that they are correct – is in contrast to the colloquial use of the term, which means almost opposite – lack of certainty (as in “it’s my theory that Secretariat was the greatest American athlete of all times”, or “it’s just a theory – nothing you should trust on its face”). Purveyors of pseudoscience (for financial, religious or political reasons) like to utilize the difference between the two senses of the word, dishonestly implying that a scientific theory they don’t like is uncertain when just the opposite is true.
The strongest theories are those that are supported by a wide variety of kinds of evidence. Theory of evolution is one of the best supported theories of all science not only because it is backed up by mountains of evidence (and no evidence against it), but also because the evidence comes from many different areas of science: paleontology (fossils), biogeography, ecology, mathematical modeling, population and quantitative genetics, comparative genomics, medicine, agricultural breeding, study of animal behavior, comparative anatomy, comparative physiology and comparative embryology.
The way disparate data from quite different areas of science, when put together, all strengthen a single theory, is called consilience. Recently, this word has been misused in popular literature (including a book of the same name) and press to mean quite the opposite – taking the methodology or findings from one discipline and applying it to a variety of other disciplines, e.g., taking the logic of evolution by natural selection and applying it to chemistry, pharmacology, psychology or computer science. That is a worthy endeavor, but it not a correct meaning of the term ‘consilience’.
Sometimes you will see (as opposed to the image on p.5 of your textbook) scientific method schematically depicted like this:
There are two reasons why the Biology textbook does not show a graph like this: a) it is not applicable to biology, and b) it is wrong.
It is wrong because it places “law” above the theory. Actually, the opposite is true – many laws (in physics, for instance) are elements of a greater theory and are parts of the evidence that the theory is correct. Laws are usually mathematical depictions of regular behavior of some aspect of nature. In other words, laws describe nature but do not explain it. Theories explain nature and are thus on the top of the hierarchy of scientific knowledge.
The model above is inapplicable to biology (it was probably drawn by a physicist) because there are no laws in biology. There are rules (like Bergmann-Allen Rule in ecology or Cope’s Rule in evolutionary biology), there are generalizations (e.g., Scaling), there are mathematical models (e.g., in population genetics) and there are Principles (e.g., the Principle of Natural Selection), but there are no laws. Biology deals with processes at much higher levels than does physics, where emergent properties of complex systems introduce a dose of unpredictability. All potential “laws” in biology have many exceptions, or have to be limited to a very small subset of processes, or to a small subset of organisms – they are not exception-less as laws of physics are.
Hypothetico-deductive method described above, while arguably the most powerful part of the scientific method, is not the only one. There is a continuum of scientific “methods” as depicted here (from Brandon 1996):
Collecting the information about all the species of birds and salamanders in the mountains of North Carolina is not a test of hypothesis and is not manipulative (and is not experimental) – yet it is certainly science (place a dot in the bottom right corner of the graph) – it provides important information about the natural world. If patterns emerge from such a survey and prompt new ideas about species distribution, this can then be tested in a more experimental fashion.
Human Genome Project is highly manipulative (and expensive!), yet it is not hypothesis-testing (place a dot in the bottom left corner). Nobody predicted that we would find anything but the four nucleotides known to make up DNA. We had no predictions as what the sequence will be and what would it all mean. Once the work was done, we could use the HGP as a tool for testing new hypotheses, e.g., how many genes do we have, how they are related to the genes of chimps, how diverse are particular gene sequences in human population as a whole, etc.
Paleontology is somewhere in the middle. It is somewhat manipulative (it takes hard work and a lot of people to do it) and it is somewhat hypothesis-testing (place a dot smack in the middle of the graph). Paleontologists do not dig randomly – they dig in particular places on the planet in particular layers of the sediment, looking for fossils of particular kinds of organisms. For instance, a group recently did an excavation in a particular bed of Late Silurian layer, looking specifically for a fossil of an early tetrapod, i.e., a transitional organism between fully aquatic and fully terrestrial mode of life. They discovered exactly that – a fossil named Tiktaalik whose fins were better suited for walking on land than that of fishes (like mudskippers, catfish and lungsfish), yet not completely evolved for land use as in amphibians.
Sometimes nature provides an experiment that tests a hypothesis (a dot in the top right corner). For instance, a biogeographical model of island succession was tested when the volcano Krakatoa erupted and eliminated all life from the island. The scientists went there and observed which organisms flew in from the mainland, in which order, and how the ecosystem passed through several stages until it reached its mature stage, thus confirming (and somewhat modifying) their hypotheses.
No matter how strongly a theory is supported by empirical evidence, it is always theoretically conceivable that one day, some data will come in that will force the scientists to modify or even eliminate the theory. Even if the scientists are 99.999999999999999999999999999999999% certain that the theory is true, it is philosophically incorrect to say that it is 100% true and to call it the Truth with the capital T. That is why scientists, when interviewed in the media, often sound uncertain and wishy-washy, while some quack or pseudoscientist pronounces his absolute certainty. Audience not educated in the scientific method is likely to swallow the pseudoscience bait, hook and sinker because we, as humans, crave certainty. It takes some scientific training to be able to fully embrace and even love uncertainty. That is why it is difficult for scientific knowledge to counteract financially, religiously and politically motivated assaults on it. However, nature does not care about what we like and wish for: the apples will continue to fall down, the continents will continue to move around the globe (causing earthquakes and volcanic eruptions) and the organisms will continue to evolve whether we like it or not, whether we believe in it or not.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 1
Figure 2 from:
Brandon, RN, Does biology have laws? The experimental evidence. PSA 1996, vol. 2, 444–457.
# 1. Biology and the Scientific Method, Chapter 1
# 2. Cell Structure, Chapter 4
# 3. Protein Synthesis: Transcription and Translation, Chapters 9 and 10
# 4. Cell-Cell Interactions, Chapter 5
# 5. From One Cell To Two: Cell Division and DNA Replication, Chapter 11
# 6. From Two Cells To Many: Cell Differentiation and Embryonic Development, Chapter 40
# 7. From Genes To Traits: How Genotype Affects Phenotype, Chapters 12
# 8. From Genes To Species: A Primer on Evolution, Chapters 14, 15 and 16
# 9. Origin of Biological Diversity, Chapter 17
# 10. Evolution of Biological Diversity, Chapter 18
Exam I, Plant/Animal presentations
# 11. Current Biological Diversity, Chapters 19, 20, 21, 22, 23 and 24
# 12. Organisms In Time and Space: Ecology, Chapters 26, 27, 28 and 29
# 13. What Creatures Do: Animal Behavior, Chapter 25
# 14. Introduction to Anatomy and Physiology, Chapter 31
# 15. Physiology: Regulation and Control, Chapters 37, 38 and 39
# 16. Physiology: Coordinated Response, Chapters 32, 33, 34, 35 and 40
Exam II and Organ System presentations