Imagine that you are a zebra, grazing in the savannah. Suddenly, you smell a lion. A moment later, you hear a lion approaching and, out of the corner of your eye, you see the lion running towards you.
What happens next? You start running away, of course. How does that happen? Your brain receieved information from your sensory organs, processed that information and made a decision to puruse a particular action. That decision is relayed to the muscles that do the actual running.
In short, that is behavior and it can be schematically depicted like this:
Environment———> Sensor ———-> Integrator———> Effector
Here, the change in the environment (appearance of a lion) is perceived by the sensors (eyes, nose, ears), processed by the integrator (the brain) and results in the activity of the effectors (muscles).
But, it is usually not that simple. The flow chart, as depicted, may be accurate when describing behavior of a bacterium, a protist, a fungus or a plant. A molecule in the cell membrane of a bacterium may sense nutrients, toxins or light. This information is processed by the cell as a whole, and as a result, the cilia or flagella move the bacterium in an appropriate direction.
Specialized cells in the shoot-tips or root-tips may detect up and down, or the position of the Sun, and guide growth in an appropriate direction (shoots up, roots down). Sunflowers and some other plants track the position of the Sun throughout the day. Many plants open and close their flowers or leaves at particular times of day. Some flowers, e.g, Venus flytrap and some orchids, can move even faster in order to capture insects.
Pilobolus, a fungus (seen as fine white fuzz on manure), shoots its spores towards the Sun at a particular angle at a particular time of day. Those are all simple behaviors involving a single sensor, a single integrator and a single effector in a simple unidirectional flow of information.
Once we get to animals with central nervous systems, things get a little bit more complicated. There are often multiple sensors. In the zebra example, the changes in environment are detected by three separate sensors: for vision, audition and olfaction. Effectors are many muscles, working in a highly coordinated manner.
Sensors located in the muscles feed the information about their activity back to the integrator. Integrator feeds back to the sensors as well – raising the sensitivity of the sensory organs, including vision, hearing, smell and the tactile sense (touch), while reducing the sensitivity of other sensors, e.g., for pain. The subjective perception of the rate of passage of time slows down, allowing for more fine-grained sensation and faster decision-making by the integrator.
Furthermore, the integrator will stimulate secretion of the hormones which, in turn, may increase the ability of effectors (muscles) to do their work. Integrator will also raise the activity of other organ systems that are important in allowing muscles to perform at their maximal level, e.g., circulatory and respiratory systems that bring oxygen and energy to the muscles.
At the same time, the brain temporarily shuts down the activity of organ systems not neccessary for short-term survival, but which may take the valuable energy away from the muscles. Thus, the digestive, immune, excretory and reproductive systems are inhibited.
As the zebra runs away, the act of running results in subsequent changes in the environment, which are again detected by the sensors. The integrator makes decisions to suddenly swerve if the lion gets closer, or to buck and kick if the lion gets very close, or to stop and find the safest route back to the herd if the lion has abandoned the chase.
All the changes described in the zebra example above are elements of the stress response, which is an excellent example of a complex behavior. There are multiple sensors, multiple effectors, various modifications of the body’s physiology, and several kinds of information feedbacks involved. Behavioral biology studies all aspects of it.
In addition, it is not just the activity itself, but also the propensity for such activity that is studied by behavioral biology. Probability of a behavior happening depends on the motivation, or the state of the integrator. The state can be modified by hormones, hunger, tiredness, libido, general energy levels, etc. The integrator (e.g, the brain) also possesses timing mechanims (clocks and celandars) which make some behaviors much more likely during the day or during the night, some more likely during spring or summer, others more likely during fall or winter.
What Is Behavior?
It is difficult to define behavior without resorting to just listing examples of various kinds of behaviors, but let’s try to define it anyway: Behavior is a change in body’s position, shape or color, or a change in potential for such change, in response to changes in the external or internal environment. Behavior is endogenously generated (i.e., if I move your arm – that is not your behavior, it’s mine), purposive (meant to achieve a goal), and is an evolved adaptation that contributes to survival or reproduction, thus increases one’s fitness (which is obvious in the case of the fleeing zebra).
How to study behavior?
The most informative and profitable way to study behavior is an integrative approach. This means that the behavior under study is approached at all levels of organization (from molecules to ecosystems) and from four different angles. The first angle is Mechanism, which denotes study of the physiology underlying behavior. Most of the analysis of the zebra’s behavior described above focused on this aspect – the physiology of the sensory, neural, muscular and other systems and the way they work together to produce the behavior.
The second one is Ontogeny, the study of embryonic and post-embryonic development of the behavior – how does an individual acquire the behavior, how much is the behavior inherited vs. learned, at what time in one’s life cycle can the behavior be learned or expressed, at what times of day or year are the behaviors most likely to be expressed, etc.
These first two angles – mechanism and ontogeny – are sometimes called Proximate Causes of behavior and are designed to ask and answer the “How” questions of behavior (how does it work, how does it develop). The next two are called Ultimate Causes of behavior and are designed to ask and answer the “Why” questions (why behave in such way).
History is the third approach. It studies the evolutionary history of a behavioral trait, usually by employing the comparative method, i.e., comparison of a number of related species, trying to discover if the behavior is common in all of them, in which case it is present due to the deep phylogenetic history, or of it most reliably varies with the type of environment the species lives in, suggesting that the behavior is a recent adaptation for a particular way of life. Finally, the fourth approach is Function. It tests the hypothesis that the behavior in question increases the animal’s fitness, aids in survival and/or reproduction, and has evolved for that function – is it an adaptation.
Recently a fifth question has been added to this list. Animal cognition asks “Can animals think?” Here, careful use of some unusual (and quite controversial) methods, including anecdotes, introspection and anthropomorphism, aids in the development of testable hypotheses about the inner worlds of animals.
No other area of biology is as integrative as behavioral biology. It is possible for a biochemist to ignore ecology or for an ecologist to ignore biochemistry (though at the risk of performing irrelevant research), but a behavioral biologist cannot ignore any aspect of the biology of the species under study. This makes the study of behavior the glue that holds all of biology together. This makes behavioral biology difficult to do, as one needs to have strong background in many areas of biology, technical expertise in a broad range of laboratory and field techniques, and lots of time to follow up on the literature in a number of related fields.
Only a few – the best – behavioral biologists are capable of exploring every aspect of a behavior at all levels. Mostly, the problem is divided among a number of laboratories around the world, each researcher using a slightly different approach and different techniques. The laboratories then communicate with each other via formal channels – the publications in scientific journals – and via informal channels – conferences and personal communication. Thus, a big picture is slowly being built out of its smaller parts, each piece of research being informed by all other pieces of research.
Types of behaviors
Foraging behavior involves finding, catching, handling and ingesting food. It includes the formation and use of feeding territories, learning the hunting techniques, the physiology of hunger, as well as behavioral strategies for avoiding becoming prey.
Animal movement includes, most prominently, long-distance migration including the neural mechanisms of spatial orientation and navigation.
Communication is the ability of animals to communicate information to each other (within and betwen species) via several sensory channels (or modalities). Those modalities include vision (including infrared, ultraviolet and polarized light, as well as thermoreception), sound (including ultrasound, infrasound and substrate vibrations), chemical signals (smells, pheromones, taste), touch and electrical signals (as in electrical fish).
Reproductive behaviors encompass a broad range of behaviors. Mate-finding, male-male competition, mate-choice and courtship are behaviors involved in securing a mate. Mating behavior ensures fertilization. Nesting and parenting behaviors are meant to ensure the survival of the offspring.
Reproductive behaviors are important elements of evolutionary change. Many phenotypic traits are a result not of natural selection, but of sexual selection, where a trait is selected not by the physical environment but by potential mates. Traits favored by the individuals of the opposite sex tend to be more likely to be passed on to the next generation in that population. This leads to the evolution of exaggerated traits (e.g., the peacock’s tail) and to differences between sexes (e.g., in many bird species the male is brightly colored while the female looks drab).
Mate choice can, potentially, be involved in sympatric speciation, if different individuals in the population favor different traits in their mates, so the gene flow between the two groups gets progressively smaller with each generation. This kind of mating is called assortative mating (as opposed to random mating, where each individual is equally likely to mate with each individual of the opposite sex).
The most common types of mating systems are monogamy, polygyny, and polyandry. A good example of polygyny is the elephant seal in which only one male (after defeating all the other males in one-on-one fights) mates with all the females in his territory.
Polyandry is found only a little less often – one female mates with multiple males over the course of a breeding season, resulting in her offspring being of mixed paternity (i.e., different eggs were fertilized by different males). This has been studied mostly in frogs.
Monogamy is the rarest form of mating strategy in the animal kingdom. A distinction is made between social monogamy and sexual monogamy. Many animals that form breeding pairs, including most species of birds, are engaged in social monogamy – the male and the female build the nest together, mate and raise the chicks together. However, DNA fingerprinting has shown that a small proportion of the eggs is invariably fertilized by a different male – a fleshy neighbor who may not be a good “husband” and “father”, but whose size, bright colors or powerful song indicate other genetic qualities. Thus, some of the progeny of the same female will be fleshy sons, some will be “good husband” sons and some will be daughters – the female is hedging her bets about the production of grandoffspring.
Humans are not officially classified as monogamous animals – though human polygamy (both polygyny and polyandry) tends to be in the form of serial monogamy, i.e., sticking monogamously with one partner for a particular length of time, then changing the partner. Social norms have strongly opposed, but did not eradicate human non-monogamy. Increased life-span, invention of reliable contraception, and economic independence of women are making it more and more difficult to supress the non-monogamous tendencies in humans, as seen from statistics for divorce (around 50%), re-marrying, and cheating (around 60% of both men and women) that have held quite steady over the past 50 years or so.
Social behaviors involve relationships between individuals of the same species. Some animals tend to live alone, each individual defending a territory, and a male and a female meeting only briefly during the mating season. Other animals tend to live in smaller or larger groups. Some animals change their social structure seasonally – for instance, European quail live in coveys (10-12 birds) during the winter), in huge flocks during spring and fall migrations, and in breeding pairs during summer.
Within groups, there is often a hierarchy of individuals – the so-called “pecking order”. The social hirearchy is established through aggression, often in form of ritualized displays. In many species, the ritualized aggressive behaviors are so-called “fixed-action patterns“, i.e., a strongly heritable order of particular movements. Mating behaviors are also often fixed-action patterns.
In some species, the mating fixed-action patterns are also used for aggressive encounters. In some cases, when a male mounts another male utlizing a typical mating pattern, this is actually a display of social dominance. However, in other species, a male mounting a male is actually homosexual behavior, evolved not to determine social hirearchy, but quite the opposite, to increase social coherence within the group (“making friends”). In pygmy chimps (bonobos), everyone in a troup mates with everyone else in the troup, regardles of gender. This makes the troup socially cohesive (which helps in group’s defense if attacked by another troup).
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapter 25
Ecology is the study of relationships of organisms with one another and their environment. Organisms are organized in populations, communities, ecosystems, biomes and the biosphere.
A population of organisms is a sum of all individuals of a single species living in one area at one time.
Individuals in a population can occupy space in three basic patterns: clumped spacing, random spacing and uniform spacing.
Metapopulations are collections of populations of the same species spread over a greater geographic area. There is some migration (ths gene-flow) between populations. Larger populations are sources and smaller populations are sinks of individuals within a metapopulation.
Population size is determined by four general factors: natality, mortality, immigration and emigration.
Natality depends on a number of factors: the proportion of the population that are at a reproductive age (as opposed to pre-reproductive and post-reproductive), proportion of the reproductively mature individuals that get to reproduce, sex-ratio of the reproductives, the mating system, the fertility of individuals (sometimes affected by parasites), the fecundity (number of offspring per female), the maturation rate (the amount of time needed for an individual to attaint sexual maturity), and longevity (amount of time an individual can live after reproducing).
Mortality is affected by bad weather, predation, parasitism and infectious diseases. It depends on the mortality of pre-reproductive stages (from eggs and embryos, through larva and juveniles), mortality of reproductive stages, and mortality of post-reproductive stages (often from disease or aging).
A population can, theoretically, grow exponentially indefinitely. However, in the real world, the growth is limited by the amount of space, food (energy) and predators. Thus, the population size often plateaus at an optimal number – the carrying capacity of that population.
Some organisms produce a large number of progeny, most of which do not make it to maturity. This is r-strategy. The population size of such species often fluctuates in boom-and-bust patterns.
Other organisms produce a small number of progeny and make a heavy investment into parenting and protecting each offspring, This is K-strategy. The population size of such species grows more slowly and tends to stabilize around the carrying capacity.
All populations show small year-to-year fluctuations of population sizes around the optimum number. Some species, however, exhibit regular oscillations in population sizes. Such oscillations often involve populations of two different species, usually a predator and its prey, the most famous example being that of the snowshoe hare and the lynx.
Correct prediction of future changes in a population size is essential for the assessment of the populations viability and for its protection.
A biological community is a collection of all individuals of all species in a particular area. Those species interact with each other in various ways, and have evolved adaptations to life in each others’ presence.
Niche is a term that describes a life-role, or job-description, or one species’ position in the community. An example may be a large herbivore, a nocturnal burrowing seed-eater, a seasonal fruit-eater, etc.
Within one community only one species can occupy any particular niche. If two species share some of their niche, they are in competition with each other. If two species occupy an identical niche, they cannot coexist – one of the species will be forced to move out or go extinct.
If two species compete for the same resource (food, territory, etc.), one will utilize the resource better than the other. Competitive exclusion is a process in which one species drives another species out of the community.
Complete exclusion is not inevitable. The competition between two species can be reduced by natural selection, i.e., one of the species will be forced to assume a slightly different niche. For instant, two species can geographically partition the territory, e.g., one living at higher altitude than the other on the same mountain-side. Two species can also temporally partition the niches, for instance one remaining active at night and the other becoming active during the day.
Predation is one of the most important interaction between species in a community. Predation often causes evolutionary arms-races between predators and prey. For instance, by killing the slowest zebras, lions select for greater speed in zebras. Greater speed in zebras selects for greater speed in lions.
The most interesting examples of evolutionary arms-races between pairs of enemies are those in which the prey is dangerous to the predator, often by being toxic or venomous. For example, garter snakes and tiger salamanders on the West coast are involved in one such arms-race. Prey – the salamander – secrete tetrodotoxin from its skin. This toxin paralyzes the snake. Locally, some snakes have evolved an ability to tolerate the toxin, but the side-effect of such evolution is that these snakes are slow and sluggish – themselves more vulnerable to predation by birds.
Ground squirrels (prey) in the Western deserts have evolved immunity to rattlesnake venom, so the rattlesnakes (predators) are becoming more venomous. Similarly, and in the same area, desert mice have evolved immunity to the toxin of their prey – the scorpions, resulting in increasing toxicity of the scorpion venom in that region (but not in areas where these two species do not overlap). A Death’s-head sphynx moth steals honey from beehives and has evolved partial immunity to honey-bee venom.
Many plants have evolved thorns or toxic chemicals to ward off their enemies – the herbivores. Monarch butterflies are capable of feeding on milkweed despite this plant’s toxic content. Moreover, the Monarchs store the noxious chemical they extracted from milkweed and that chemical makes the butterflies distasteful to their own predators.
The shape and color of the prey often evolves to protect from predation. Warning coloration, usually in very bright colors, informs the predators that the prey is dangerous. Aposomatic coloration is one commonly found kind of warning coloration – the black and yellow stripes on the bodies of many bees and wasps are almost a universal code for dangerous venomous stings.
Cryptic coloration, or camouflage, on the other hand, allows an animal to blend in with its surroundings. Many insect look like twigs, leaves or flowers, effectively hiding them from the eyes of predators. Some animals have evolved behavioral color-change, e.g., chameleons, some species of cuttlefish and the flounder.
Batesian mimicry is a phenomenon in which non-toxic species evolve to resemble a toxic species. Thus, some butterflies look very similar to Monarch butterflies and some defenseless flies and ants have aposomatic coloration.
Mullerian mimicry is a phenomenon in which two or more dangerous species evolve to look alike. This is “safety in numbers” strategy as a predator who tastes and spits out one of them, will learn to avoid all of them in the future.
Co-evolution does not occur only between enemies. It can also occur between species that positively affect each other. The best example is co-evolution of flowers and insect pollinators.
Symbiosis is a relationship between organisms that are not direct enemies (e.g,. predator and prey) to each other. Commensalism, mutualism and parasitism are forms of symbiosis.
In commensalism, one partner benefits, while the other one is not affected at all. For instance, birds building nests in a tree do not in any way affect the fitness of the tree.
Mutualism benefits both partners. The best known examples are lichens, mycorrhizae, and legumes. Birds that clean the skin or teeth of crocodiles, hippos or rhinos are protected by their hosts.
Parasitism is detrimental to one of the partners. Parasites that are too dangerous, i.e., those that kill their host, are not successful since they also die without leaving offspring. Thus, parasites evolve to be minimally harmful to their hosts. The same logic goes for infectious agents – the disease should help propagate the microorganism (e.g, by causing sneezing, diarrhea, etc.) without killing the host.
The organisms that make up ecosystems change over time as the physical and biological structure of the ecosystem changes. Right now, one of the effects of global warming is that some species migrate and others do not. Thus, old ecosystems break down and new ones are formed. The ecosystems are in a process of remodelling. During that process, many species are expected to go extinct.
When an ecosystem is disturbed to some extent, but not completely eradicated, the remodelling process that follows is called primary succession.
When an ecosystem is completely wiped out (e.g,. a volcanic eruption on an island), secondary succession occurs, with a predictable order in which species can recolonize the space. One species prepares the ground (quite literally) for the next one. The process may start with bacteria, lichens and molds, continuing with mosses, fungi, ferns and some insects, etc, finally ending with trees, birds and large mammals. The final structure of the ecosystem is quite stable over time – this is a mature ecosystem.
Audesirk, Audesirk and Byers, Biology, 8th edition., Chapters 26, 27, 28 and 29
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.
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 | 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