INTRODUCTION Evolution is one of the most powerful theories science has ever known. For a

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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. Many interesting topics in evolutionary biology are not covered here (symbiosis and endosymbiosis, origins of life, punctuated equilibrium, sexual selection, human evolution and much more) because I can't include everything and keep this down to a readable length. Also, what I know the most about is the process of evolution as it occurs within a lineage (anagenesis), so that is the focus of this post. 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 of 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. I have defined "evolution", here, as a process and that is how I will use the term in this essay. Keep in mind, however, that in everyday use "evolution" often refers to a variety of things. The fact that all organisms are linked via descent to a common ancestor is often called evolution. The theory that life arose solely via natural processes is often called evolution (instead of abiogenesis). Often, people use the word "evolution" when they really mean natural selection -- one of the many mechanisms of evolution. 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. WHAT EVOLUTION ISN'T Evolution is not progress. Organisms simply adapt to their current surroundings and do not neccessarily become "better" over time. A trait or strategy that is successful for an organism at one time may be deleterious at another. Studies in yeast have shown that "more evolved" strains of yeast can sometimes be competitively inferior to "less evolved" strains. An organisms success or failure depends to a great deal on the behavior of it's contemporaries; for most traits or behaviors there is likely no optimal design or strategy, only contingent designs/strategies. 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, or a way to generate it. 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 to mostly dark. In order for continuing evolution there must be mechanisms to both increase genetic variation, or create it, (e.g. mutation) and decrease variation (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 (there are other alleles also). 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 two blue eye alleles) the individual would be termed "homozygous" for that locus. An individual with two different alleles at a locus 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 VARIATION ------------------------------------------ MECHANISMS OF EVOLUTION: NATURAL SELECTION Natural selection is held to be the only 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. This illustrates the point that 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. An example of an exaptation would be a penguins wing. Penguins evolved from flying ancestors. Now, however, they are flightless and use their wings for swimming. 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. There are some exceptions to this "rule". 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 their 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. Often, this is called by the oxymoronic name "reciprocal altruism". A good example of this is blood sharing in vampire bats. In these bats, those lucky enough to find a meal will often share part of it with an unsuccessful bat by regurgitating the blood into the others mouth. Biologists have found that these bats form bonds with other bats and help each other out when the other is needy. If a bat is found to be a "cheater", (ie. he accepts blood when starving, but does not donate when his partner is) the partner will abandon the cheater. Keep in mind that the words "selfish" and "altruistic" have connotations in everyday usage that biologists do not intend. "Selfish" simply means behaving so that one's own best interest comes first; "altruistic" means behaving so that anothers best interest comes first. Natural selection does not induce genetic variation to occur, it only distinquishes between existing variation. 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 alleles that form the next generation are a sample of the alleles 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 alleles in this sample of gametes are likely to be slightly different than the alleles 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 VARIATION ------------------------------------------ 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 the genetic code 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 slightly deleterious or neutral. The genome of most organisms (certainly all eukaryotes) contains enormous amounts of junk sequences. In addition, even in coding regions, many sites can undergo mutation and still maintain there original meaning. In other words, the genetic code is redundant. So, most mutations are neutral or nearly so; but, the overwhelming majority of mutations that produce any detectable phenotypic effect are deleterious. "Good" mutations, however, do occur. One example of a beneficial mutation comes from the mosquitoe _Culex_ _pipiens_. In this organism, a gene that was involved with breaking down organophophates - common insecticide ingrediants - became duplicated. Progeny of the organism with this mutation quickly swept across the worldwide mosquitoe population. There are numerous examples of insects developing resistance to chemicals, especially DDT - which was once heavily used in this country. Mutations occur at random with respect to their adaptive significance. Organisms cannot "decide" that they need a mutation and have it occur. The frequency of a mutation occuring is independent of the potential effect it would have. Recently, certain exceptions have been found to the above "rule" in some bacteria (E. Coli). It appears that these organisms can undergoe directed mutagenesis to repair "broken genes". The reversion mutation, that restores the gene to normal functioning, occurs at a several orders of magnitude more frequently when the gene is needed than when it isn't. It is unlikely, however, that this could occur in multi-cellular organisms. MECHANISMS OF EVOLUTION: RECOMBINATION Recombination can be loosely thought of as gene shuffling. Most organisms have linear chromosomes and their genes lie at specific locations (loci) along them (bacteria have circular chromosomes). In most sexually reproducing organisms, there are two of each chromosome type in every cell. For instance in humans, there are two chromosomes number one (through 22 and two sex chromosomes), one inherited from the mother, the other inherited from the father. When an organism produces gametes, the gametes end up with only of each chromosome per cell. Haploid gametes are produced from diploid cells by a process called meiosis. In meiosis, homologous chromosomes line up. The DNA of the chromosome is broken on both chromosomes in several places and rejoined with the other strand. Later in meiosis, the two homologous chromosomes are split in to two separate cells (gametes). But, because of recombination, both of the chromosomes are a mix of alleles from the mother and father. For example, lets say an organism has a chromosome with three genes, (A,B and C -- in that order). Assume that at each of these three loci there are at least two alleles. From the father, the organism inherited a chrosome with the alleles A1, B1 and C1. From the mother the organism inherited A2,B2 and C2 alleles. In meiosis the two chrosomes would line up and the two A alleles would line up, as would the B and C alleles. If recombination occured between locus A and locus B, the resulting chrosomes in the two gametes would be; one chromosome carrying A1, B2 and C2 alleles and one chromosome carrying A2, B1 and C1 alleles. Real chromosomes carry many more than three genes and recombination occurs at many locations along the chromosome. The end result is that the two homologous chromosomes have "shuffled" alleles. Recombination can occur not only between genes, but within genes as well. Recombination within a gene can form a new allele. Recombination is a mechanism of evolution because it adds new alleles and combinations of alleles to the gene pool. A beneficial aspect of recombination is that beneficial new alleles can be brought togethor onto the same chromosome, even if the mutations originally occured in separate organisms. MECHANISMS OF EVOLUTION: GENE FLOW Gene flow simply means new genes added to a population by migration from another population. In some closely related species, fertile hybrids can result from interspecific matings. These hybrids can vector genes from species to species. 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. One of the commonly cited examples involves three new species in the plant genus _Tragopogon_. 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 is not linear progress. The incorrect idea that evolution can be represented as progress from simple cells through complex life forms to humans (the pinnacle of evolution), can be traced to Linneaus' Scale of Nature. This view is, as I mentioned, wrong. Evolution is better viewed as a branching tree or bush. Populations or species of organisms split and become two or more species as time goes on. No living organisms today are our ancestors. Every species we see today is as fully modern as we are; each has it's own unique evolutionary history. Other species are not "lower life forms", atavistic stepping stones paving the road to humanity. If you view evolution as a branching tree, it's best to view it as one that has been severely pruned a few times in it's life. The history of life on this earth includes many episodes of mass extinction in which many taxa (groups of organisms) were wiped off the face of the planet. Mass extinctions are followed by periods of radiation where new species evolve to fill the empty niches left behind. The most famous extinction occured at the boundry between the Cretaceous and Tertiary Periods (the K/T Boundry). This extinction eradicated the dinosaurs. Following this extinction the mammalian radiation occured. SOME GOOD EVOLUTION TEXTS (IMHO) A good introductory text in evolutionary biology is: Evolutionary Biology, by Douglas Futuyma, 1986, Sinauer, Sunderland, Mass The text assumes some previous knowledge of biology, but reviews most critical background material. It contains numerous references to the primary literature. A good introductory text into population genetics, the field that mathematically describes changes in the gene pool is: Principles of Population Genetics, by Hartl and Clark , 1989, Sinauer, Sunderland, Mass None of the math is very daunting (it's just an intro text after all) but it's really critical (IMHO) to really understanding what evolution is all about. And again, lots of refs. A text that deals with the interface of molecular biology and evolution is: Fundamentals of Molecular Evolution, Li and Graur, 1991, Sinauer, Sunderland, Mass A very concise introduction to this field. Chris Colby --- email: colby@bu-bio.bu.edu --- "'My boy,' he said, 'you are descended from a long line of determined, resourceful, microscopic tadpoles--champions every one.'" --Kurt Vonnegut from "Galapagos"

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