Subject: The theory of evolution INTRODUCTION Evolution is one of the most powerful theori

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From: Chris Colby Subject: The theory of evolution INTRODUCTION Evolution is one of the most powerful theories science has ever known. For a variety of reasons, however, it is also one of the most misunderstood. One common misunderstanding is that the phrase "survival of the fittest" summarizes evolutionary theory. In fact, it does not. The phrase is both incomplete and misleading. The notions that evolution represents progress and, that organisms can be arranged on an evolutionary ladder from bacteria to man, are two other common misunderstandings. This post is an outline of the basics of evolutionary theory. It is intended to be a brief overview of the concepts and mechanisms of evolution. Creationist arguments are not addressed directly here; nor is a "laundry list" of reasons to believe in evolution provided. (I'm still not done with this post -- insert standard excuse. As always, questions/comments are appreciated. Be aware that there are typos. All the info here is the standard dogma in the field, exceptions exist for some of my statements. Also, some of the points made here are not universally agreed upon by biologists. Evolution wouldn't be an exciting and in- teresting field of science if they were. The majority of the post is, I believe, noncontroversial to biologists in the field.) WHAT IS EVOLUTION? Evolution is a change in the gene pool of a population over time. The gene pool is the set of all genes in a species or population. The English moth, _Biston__Bistularia_, is a frequently cited example of observed evolution. In this moth, rare black variants spread through the population as a result of their habitat becoming darkened by soot from factories. Birds could see the lighter colored moths more readily and ate more of them. Thus, the moth population changed from mostly light colored moths to mostly dark colored moths. Since their color was determined by a single gene, the change in moth color represented a change in the gene pool. This change, by definition, was evolution. Many creationists, when confronted with this example, say "You started with moths and ended with moths... where's the evolution?" The kind of evolution documented above is termed by some "microevolution", while larger changes (taking more time) are termed "macroevolution". Some biologists feel the mechanisms of macroevolution are different from those of microevolutionary change. Others, including myself, feel the distiction between the two is arbitrary. Macroevolution is cummulative microevolution. In any case, evolution is defined as a change in the gene pool. Later in this post I will discuss macroevolution as well as microevolution. For the sake of brevity I will use the terms as if it is useful to draw a distiction between them. WHAT ISN'T EVOLUTION? For many people evolution is equated with morphological change, i.e. organisms changing shape or size over time. An example would be a dinosaur species slowly turning into a bird species. It is important to note that evolution is often accompanied by morphological change, but this need not be the case. Evolution can occur without morphological change; and morphological change can occur without evolution. For instance, humans are larger now than in the past few hundred years, but this is not an evolutionary change. Better diet and medicine brought about this change, so it is not an example of evolution. The gene pool did not change -- only it's manifestation did. An organism's morphology is determined by both its genes and its environment. Morphological changes induced solely by changes in environment do not count as evolution, because this change is not heritable. In other words, the change is not passed on to the organisms offspring. Most changes due to environment are fairly subtle (e.g. size differences). Large scale morphological changes (such as dinosaur to bird) are obviously due to genetic changes, and therefore are evolution. HOW DOES EVOLUTION WORK? If evolution is a change in the gene pool; what causes the gene pool to change? Several mechanisms can bring about a change in the gene pool, among them: natural selection, genetic drift, gene flow, mutation and recombination. I will discuss these in more detail later. It is important to understand the difference between evolution (change in the gene pool) and the mechanisms that bring about this change. Bringing about a change in the gene pool assumes that there is genetic variation in the population to begin with. Genetic variation is "grist for the evolutionary mill". For example, if there were no dark moths, the population could not have evolved from mostly light colored to mostly dark colored. In order for continuing evolution there must be mechanisms to both increase genetic variation (e.g. mutation) and decrease it (e.g. natural selection, genetic drift). HOW IS GENETIC VARIATION DESCRIBED? Genetic variation has two components: allelic diversity and non-random associations of alleles. Alleles are different versions of the same gene at a given locus. For example, at one eye color locus (locus means location) humans can have the blue allele or the brown allele (or perhaps a green allele). Most organisms, including humans, are diploid. This means they contain two alleles for every gene at every locus. If the two alleles are the same type (for instance both blue eye alleles) the individual would be termed "homozygous" for that locus. An individual with two different alleles is called "heterozygous". Allelic diversity is simply the number of alleles at each locus scaled by their frequency in the gene pool. At any given locus there can be many different alleles in the gene pool. It is important to realize that there can be more alleles in the gene pool (at a given locus) than any single organism can possess. Linkage disequilibrium is a measure of association of alleles in the gene pool. If each gene assorted entirely independently, the gene pool would be at linkage equilibrium. However, if some alleles were often found togethor in organisms (ie. did not assort randomly) these genes would be in linkage disequilibrium. Linkage disequilibrium can be a result of physical proximity of the genes or maintained by natural selection if some combinations of alleles work better as a team. HOW MUCH GENETIC VARIATION IS THERE? Considerable variation has been detected in natural populations. At about 70% of gene loci, there is more than one allele present in the gene pool. Any given individual is likely to be heterozygous at 30% of it's loci. Most loci have been found to be assorting independently (i.e. they are at linkage equilibrium). In most populations, there are enough loci and enough different alleles that every individual (barring monozygotic twins) has a unique combination of alleles. MECHANISMS THAT DECREASE GENETIC DIVERSITY MECHANISMS OF EVOLUTION: NATURAL SELECTION Natural selection is held to be the most important mechanism as far as adaptive evolution is concerned; it is defined as differential reproductive success. Selection is not a force in the sense that gravity or magnetism is. However, biologists often, for the sake of brevity, refer to it that way. Selection is not a guided or cognizant entity; it is simply an effect. Some organisms have genes that enable them to reproduce more efficiently than others of their species. Organisms with these genes, therefore eventually replace the others of their species without these genes. If environmental conditions change, new traits (new combinations of alleles) will be selected for. Natural selection is a mechanism that allows organisms to adapt to their current environment only; it does not have any foresight. Traits or structures do not evolve for future utility. The organism must be, to some degree, adapted to it's environment at each stage of it's evolution. Of course, this raises the question; how do complex traits evolve? If half a wing is no good for flying, how did wings evolve? Half a wing may be no good for flying, but it may be useful in other ways. Feathers are thought to have evolved as insulation (ever worn a down jacket?) and/or as a way to trap insects. Later, proto-birds may have learned to glide when leaping from tree to tree. Eventually, the feathers that originally served as insulation now became co-opted for use in flight. A traits current utility is not always indicative of it's past utility. It can evolve for one purpose, and be used later for another. A trait evolved for it's current utility is called an adaptation; a trait evolved for another utility than it's current use is termed an exaptation. Natural selection works at the level of the individual. The example I gave earlier was an example of evolution via natural selection. Dark colored moths had higher reproductive success because light colored moths suffered a higher predation rate. The decline of light colored moths was caused by light colored individuals being removed from the gene pool (selected against). It is the individual organism that either reproduces or fails to reproduce. Individual genes are not the unit of selection (because their success depends on the organisms other genes as well); niether are groups of organisms a unit of selection. The individual organism is what reproduces or fails to reproduce. It competes primarily with others of it own species for it's reproductive success. For this reason, organisms do not perform any behaviours that are for the good of the species. Natural selection favors selfish behavior because any truly altruistic act increases the recipient's reproductive success while lowering the donors. Altruists would quickly disappear from a population as the non-altruists would get the benefits, but not pay the cost, of being an altruist. Of course, many observable behaviors appear, at first glance, to be altruistic in nature. Biologists, however, can demonstrate (in the cases they have studied) that these behaviors are only apparently altruistic. Cooperating with or helping other organisms is often the most selfish strategy for an animal. Of all the mechanisms of evolution, natural selection has the potential to change gene frequencies the fastest. It usually acts to keep gene frequncies constant, however. This led a famous evolutionist, George Williams, to say "Evolution proceeds in spite of natural selection". MECHANISMS OF EVOLUTION: GENETIC DRIFT Another important mechanism of evolution is genetic drift. Drift is a binomial sampling error of the gene pool. What this means is, the genes that form the next generation are a sample of the genes in the current generation. Organisms produce more gametes than are needed. Females produce many more eggs than are ever fertilized and males produce billions of sperm that never fertilize an egg. The genes in this sample of gametes are likely to be slightly different than the genes in the parental gene pool due solely to chance. Drift is a rather abstract concept to some; I will try to explain it via a somewhat simple analogy. Imagine you had a swimming pool full of one million marbles (this will represent the parental gene pool), half are red and half are blue. If you repeatedly picked ten marbles out, do you think you would get five red and five blue every time (assume you replaced your sample to the pool each time)? If you picked one hundred marbles out, do you think you would get fifty red and fifty blue out every time? In both cases the answer is no, some times the frequency of red marbles in the sample would deviate from 0.50. In the case of the 100 marble sample, the frequency of red marbles would deviate much less, however. If, after picking out ten or one hundred marbles, you refilled the pool with marbles at the frequency of that sample and repeated the process over and over; what do you think would happen? What would happen is that the frequency of red to blue would fluctuate over time. Eventually, there would be only one color marble left in the pool. This is roughly analogous to how genetic drift works. Both natural selection and genetic drift decrease genetic variation. If they were the only mechanisms of evolution, populations would eventually become genetically homogenous and further evolution would be impossible. There are, however, mechanisms that replace variation depleted by selection and drift. These are discussed below. MECHANISMS THAT INCREASE GENETIC DIVERSITY MECHANISMS OF EVOLUTION: MUTATION A mutation is a change in a gene. There are many kinds of mutations. A point mutation is a mutation in which one "letter" of DNA is changed to another. Lengths of DNA can also be deleted or inserted in a gene; these are also mutations. Finally, genes or parts of genes can become inverted or duplicated. Mutation is a mechanism of evolution because it changes allele frequencies very slightly. If an allele "A" mutates to another allele "a", the frequency of "a" has increased from zero to some small number (1/2N in a diploid population where N is the effective population size). The allele "A" will also decrease slightly in frequency. Evolution via mutation alone is very slow; for the most part, mutation just supplies the raw material for evolution -- genetic variation. Most, but not all, mutations are neutral or slightly deleterious. Because the genetic code is redundant and genes contain sequences (introns) that do not code for anything, any change in single nucleotide of DNA is not likely to have much of an effect. Most mutations that produce any noticable phenotypic effect are substantially deleterious. Natural selection quickly "weeds out" these mutations from the gene pool. Rarely, though, mutations are beneficial. A great example of this occurred recently in the mosquito _Culex pipiens_. A gene that helped the organism degrade insecticide became duplicated. Progeny from the mosquito this mutation occurred in quickly swept over vast geographic areas, because their increased tolerance to insecticides made them able to leave more progeny, on average, than non-mutated mosquitoes. [expand - examples] Mutations are random with respect to their adaptive significance. [expand - preadaptive mutations - "directed" mutagenesis - Lamarckian evolution] MECHANISMS OF EVOLUTION: RECOMBINATION recombination inter- and intra- genic recomb exon shuffling MECHANISMS OF EVOLUTION: GENE FLOW gene flow/ horizontal transfer SPECIATION Speciation is the process of a single species becoming two or more distinct species. Many biologists feel speciation is key to understanding evolution. These biologists believe certain evolutionary phenomena apply only at speciation and macroevolutionary change cannot occur without speciation. Other biologists think major evolutionary change can occur without speciation. Changes between lineages are only an extension of the changes within each lineage. In general, paleontologists fall into the former category and geneticists in the latter. MODES OF SPECIATION Biologists recognise two types of speciation: allopatric and sympatric speciation. The two differ in geological distribution of the populations in question. Allopatric speciation is thought to be the most common form of speciation. It occurs when a population is split into two (or more) subdivisions that organisms cannot bridge. The two populations are geographically isolated; organisms from subdivision A can only breed with organisms from subdivision A and B organisms can only breed with B organisms. Eventually, the two populations gene pools change (both independently) until they could not interbreed even if they were brought back togethor. In other words they have speciated. Sympatric speciation occurs when two subpopulations become reproductively isolated without first becoming geographically isolated. Monophytophagous insects (insects that live on a single host plant) provide a model for sympatric speciation. If a group of insects switched host plants they would not breed with other members of their species still living on their former host plant. The two subpopulations could diverge and speciate. Some biologists call sympatric speciation microallopatric speciation to emphasize that the subpopulations are still physically separate not at a geographic level, but on an ecological level. Biologists know little about the genetic mechanisms of speciation. Some think series of small changes in each subdivision gradually lead to speciation; others think there may be a few key genes that could change and confer reproductive isolation. (One famous biologist thinks most speciation events are caused by changes in internal symbionts. Most doubt this, however.) Populations of organisms are very complicated. It is likely that there are many ways speciation can occur. Thus, all of the above ideas may be correct, each in different circumstances. OBSERVED SPECIATIONS It comes as a surprize to some to hear that speciation has been observed. [give plant and fly examples] ARE WE STILL EVOLVING? Yes, evolution is still occurring; all organisms continue to adapt to their surroundings and "invent" new ways of better competing with members of their own species. In addition, allele frequencies are being changed by drift, mutation and gene flow constantly. MACROEVOLUTION VS. MICROEVOLUTION evolution not linear progress/scale of nature punctuated equilibrium WHO STUDIES EVOLUTION AND HOW IS IT STUDIED? [mention rough number of ev biologist, journals, techniques used/different fields etc.] SOME BOOKS ABOUT EVOLUTION Evolutionary Biology, by Douglas Futuyma, 1986, Sinauer, Sunderland, Mass


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