From: L.A. Moran To: All Msg #72, Jan-23-93 02:19PM Subject: DEFINITION OF EVOLUTION Organization: UTCS Public Access From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran) Message-ID: Newsgroups: talk.origins WHAT IS EVOLUTION? version 2, January 22, 1993 Most non-scientists seem to be quite confused about precise definitions of biological evolution. Such confusion is due in large part to the inability of scientists to communicate effectively to the general public and also to confusion among scientists themselves about how to define such an important term. When discussing evolution it is important to distinguish between the existence of evolution and various theories about the mechanism of evolution. And when referring to the existence of evolution it is important to have a clear definition in mind. What, exactly, do biologist mean when they say that they have observed evolution or that humans and chimps have evolved from a common ancestor. One of the most respected evolutionary biologists has defined biological evolution as follows, "In the broadest sense, evolution is merely change, and so is all-pervasive; galaxies, languages, and political systems all evolve. Biological evolution ... is change in the properties of populations of organisms that transend the lifetime of a single individual. The ontogeny of an individual is not considered evolution; individual organisms do not evolve. The changes in populations that are considered evolutionary are those that are inheritable via the genetic material from one generation to the next. Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population (such as those determining blood types) to the successive alterations that led from the earliest protoorganism to snails, bees, giraffes, and dandelions." Douglas J. Futuyma in Evolutionary Biology, Sinauer Associates 1986 It is important to note that biological evolution refers to populations and not to individuals and that the changes must be passed on to the next generation. In practice this means that, Evolution is a process that results in heritable changes in a population spread over many generations This is a good working scientific definition of evolution; one that can be used to distinguish between evolution and similar changes that are not evolution. Another common short definition of evolution can be found in many textbooks, "In fact, evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next." Helena Curtis and N. Sue Barnes BIOLOGY 5th ed. 1989 Worth Publishers, p.974 One can quibble about the accuracy of such a definition (and we have often quibbled on these newsgroups) but it also conveys the essence of what evolution really is. When biologists say that they have observed evolution they mean that they have detected a change in the frequency of genes in a population. (Often the genetic change is inferred from phenotypic changes that are heritable.) When biologists say that humans and chimps have evolved from a common ancestor they mean that there have been successive heritable changes in the two separated populations since they became isolated. Unfortunately the common definitions of evolution outside of the scientific community are different. For example, in the Oxford Concise Science Dictionary we find the following definition; "evolution: The gradual process by which the present diversity of plant and animal life arose from the earliest and most primitive organisms, which is believed to have been continuing for the past 3000 million years." This is inexcusable for a dictionary of science. Not only does this definition exclude prokaryotes, protozoa, and fungi, but it specifically includes a term "gradual process" which should not be part of the definition. More importantly the definition seems to refer more to the history of evolution than to evolution itself. Using this definition it is possible to debate whether evolution is still occurring but the definition provides no easy way of distinguishing evolution from other processes. For example, is the increase in height among Caucasians over the past several hundred years an example of evolution? Are the color changes in the peppered moth population examples of evolution? This is not a scientific definition. Standard dictionaries are even worse. "evolution: ...the doctrine according to which higher forms of life have gradually arisen out of lower.." Chambers "evolution: ...the development of a species, organsim, or organ from its original or primitive state to its present or specialized state; phylogeny or ontogeny: Webster's These definitions are simply wrong. Unfortunately it is common for non- scientists to enter into a discussion about evolution with such a definition in mind. This often leads to fruitless debate since the experts are thinking about evolution from a different perspective. When someone claims that they don't believe in evolution they cannot be referring to an acceptable scientific definition of evolution because that would be denying something which is easy to demonstrate. It would be like saying that they don't believe in gravity! Recently I read a statement from a creationist who claimed that scientists are being dishonest when they talk about evolution. This person believed that evolution was being misrepresented to the public. The real problem is that the public, and creationists, do not understand what evolution is all about. This person's definition of evolution was very different from the common scientific definition and as a consequence he was unable to understand what evolutionary biology really meant. This is the same person who claimed that one could not "believe" in evolution and still be religious! But once we realize that evolution is simply "a process that results in heritable changes in a population spread over many generations" it seems a little silly to pretend that this excludes religion! Scientists such as myself, must share the blame for the lack of public understanding of science. We need to work harder to convey the correct information. Sometimes we don't succeed very well but that does not mean that we are dishonest. On the other hand, the general public, and creationists in particular, need to also work a little harder in order to understand science. Reading a textbook would help. Laurence A. Moran (Larry) --- Squish v1.01 * Origin: Universal Electronics Inc [714 939-6401] HST/DS (1:103/208) From: L.A. Moran To: All Msg #73, Jan-23-93 02:23PM Subject: THE MODERN SYNTHESIS Organization: UTCS Public Access From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran) Message-ID: Newsgroups: talk.origins THE MODERN SYNTHESIS A Theory of the Mechanism of Evolution (version 1.5, January 22, 1993) Many people do not understand current ideas about evolution. The following is a brief summary of the modern consensus among evolutionary biologists. The idea that life on Earth has evolved was widely discussed in Europe in the late 1700's and the early part of the last century. In 1859 Charles Darwin supplied a mechanism, namely natural selection, that could explain how evolution occurs. Darwin's theory of natural selection helped to convince most people that life has evolved and this point has not been seriously challenged in the past one hundred and thirty years. It is important to note that Darwin's book "The Origin of Species by Means of Natural Selection" did two things. It summarized all of the evidence in favor of the idea that all organisms have descended with modification from a common ancestor, and thus built a strong case for evolution. In addition Darwin advocated natural selection as a mechanism of evolution. Biologists no longer question whether evolution has occurred or is occurring. That part of Darwin's book is now considered to be so overwhelmingly demonstrated that is is often referred to as the FACT of evolution. However, the MECHANISM of evolution is still debated. We have learned much since Darwin's time and it is no longer appropriate to claim that evolutionary biologists beleive that Darwin's theory of Natural Selection is the best theory of the mechanism of evolution. I can understand why this point may not be appreciated by the average non-scientist because natural selection is easy to understand at a superficial level. It has been widely promoted in the popular press and the image of "survival of the fittest" is too powerful and too convenient. During the first part of this century the incorporation of genetics and population biology into studies of evolution led to a Neo-Darwinian theory of evolution that recognized the importance of mutation and variation within a population. Natural selection then became a process that altered the frequency of genes in a population and this defined evolution. This point of view held sway for many decades but more recently the classic Neo-Darwinian view has been replaced by a new concept which includes several other mechanisms in addition to natural selection. Current ideas on evolution are usually referred to as the Modern Synthesis which is described by Futuyma; "The major tenets of the evolutionary synthesis, then, were that populations contain genetic variation that arises by random (ie. not adaptively directed) mutation and recombination; that populations evolve by changes in gene frequency brought about by random genetic drift, gene flow, and especially natural selection; that most adaptive genetic variants have individually slight phenotypic effects so that phenotypic changes are gradual (although some alleles with discrete effects may be advantageous, as in certain color polymorphisms); that diversification comes about by speciation, which normally entails the gradual evolution of reproductive isolation among populations; and that these processes, continued for sufficiently long, give rise to changes of such great magnitude as to warrent the designation of higher taxonomic levels (genera, families, and so forth)." Futuyma, D.J. in EVOLUTIONARY BIOLOGY Sinauer Associates, 1986; p.12 This description would be incomprehensible to Darwin since he was unaware of genes and genetic drift. The modern theory of the mechanism of evolution differs from Darwinism in three important respects: 1) It recognizes several mechanisms of evolution in addition to natural selection. One of these, random genetic drift, may be as important as natural selection. 2) It recognizes that characteristics are inherited as discrete entities called genes. Variation within a population is due to the presence of multiple alleles of a gene. 3) It postulates that speciation is (usually) due to the gradual accumulation of small genetic changes. This is equivalent to saying that macroevolution is simply a lot of microevolution. In other words, the Modern Synthesis is a theory about how evolution works at the level of genes, phenotypes, and populations whereas Darwinism was concerned mainly with organisms, speciation and individuals. This is a major paradigm shift and those who fail to appreciate it find themselves out of step with the thinking of evolutionary biologists. Many instances of such confusion can be seen here in the newsgroups, in the popular press, and in the writings of anti-evolutionists. The major controversy among evolutionists today concerns the validity of point #3 (above). The are many who believe that the fossil record at any one site does not show gradual change but instead long periods of stasis followed by rapid speciation. This model is referred to as Punctuated Equilibrium and it is widely accepted as true, at least in some cases. The debate is over the relative contributions of gradual versus punctuated change, the average size of the punctuations, and the mechanism. To a large extent the debate is over the use of terms and definitions, not over fundamentals. No new mechanisms of evolution are needed to explain the model. Some scientists continue to refer to modern thought in evolution as Neo-Darwinian. In some cases these scientists do not understand that the field has changed but in other cases they are referring to what I have called the Modern Synthesis, only they have retained the old name. Laurence A. Moran (Larry) --- Squish v1.01 * Origin: Universal Electronics Inc [714 939-6401] HST/DS (1:103/208) From: L.A. Moran To: All Msg #74, Jan-23-93 02:25PM Subject: GENETIC DRIFT Organization: UTCS Public Access From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran) Message-ID: Newsgroups: talk.origins RANDOM GENETIC DRIFT version 1, January 22, 1993 The two most important mechanisms of evolution are natural selection and genetic drift. Most people have a reasonable understanding of natural selection but they don't realize that drift is also important. The anti- evolutionists, in particular, concentrate their attack on natural selection not realizing that there is much more to evolution. Darwin didn't know about genetic drift, this is one of the reasons why modern evolutionary biologists are no longer "Darwinists". (When anti-evolutionists equate evolution with Darwinism you know that they have not done their homework!) Random genetic drift is a stochastic process (by definition). One aspect of genetic drift is the random nature of transmitting alleles from one generation to the next given that only a fraction of all possible zygotes become mature adults. The easiest case to visualize is the one which involves binomial sampling error. If a pair of diploid sexually reproducing parents (such as humans) have only a small number of offspring then not all of the parent's alleles will be passed on to their progeny due to chance assortment of chromosomes at meiosis. In a large population this will not have much effect in each generation because the random nature of the process will tend to average out. But in a small population the effect could be rapid and significant. Suzuki et al. explain it as well as anyone I've seen; "If a population is finite in size (as all populations are) and if a given pair of parents have only a small number of offspring, then even in the absence of all selective forces, the frequency of a gene will not be exactly reproduced in the next generation because of sampling error. If in a population of 1000 individuals the frequency of "a" is 0.5 in one generation, then it may by chance be 0.493 or 0.0505 in the next generation because of the chance production of a few more or less progeny of each genotype. In the second generation, there is another sampling error based on the new gene frequency, so the frequency of "a" may go from 0.0505 to 0.501 or back to 0.498. This process of random fluctuation continues generation after generation, with no force pushing the frequency back to its initial state because the population has no "genetic memory" of its state many generations ago. Each generation is an independent event. The final result of this random change in allele frequency is that the population eventually drifts to p=1 or p=0. After this point, no further change is possible; the population has become homozygous. A different population, isolated from the first, also undergoes this random genetic drift, but it may become homozygous for allele "A", whereas the first population has become homozygous for allele "a". As time goes on, isolated populations diverge from each other, each losing heterozygosity. The variation originally present within populations now appears as variation between populations." Suzuki, D.T., Griffiths, A.J.F., Miller, J.H. and Lewontin, R.C. in An Introduction to Genetic Analysis 4th ed. W.H. Freeman 1989 p.704 Of course random genetic drift is not limited to species that have few offspring, such as humans. In the case of flowering plants, for example, the stochastic element is the probabilty of a given seed falling on fertile ground while in the case of some fish and frogs it is the result of chance events which determine whether a newly hatched individual will survive. Drift is also not confined to diploid genetics; it can explain why we all have mitochondria that are descended from those of a single women who lived hundreds of thousands of years ago. "This does not mean that there was a single female from whom we are all descended, but rather that out of a population numbering perhaps several thousand, by chance, only one set of mitochondrial genes was passed on. (This finding, perhaps the most surprising to us, is the least disputed by population geneticists and others familiar with genetic drift and other manifestations of the laws of probability.)" Curtis, H. and Barnes, N.S. in Biology 5th ed. Worth Publishers 1989 p. 1050. But random genetic drift is even more that this. It also refers to accidental random events that influence allele frequency. For example, "Chance events can cause the frequencies of alleles in a small population to drift randomly from generation to generation. For example, consider what would happen if [a]... wildflower population ... consisted of only 25 plants. Assume that 16 of the plants have the genotype AA for flower color, 8 are Aa, and only 1 is aa. Now imagine that three of the plants are accidently destroyed by a rock slide before they have a chance to reproduce. By chance, all three plants lost from the population could be AA individuals. The event would alter the relative frequency of the two alleles for flower color in subsequent generations. This is a case of microevolution caused by genetic drift... Disasters such as earthquakes, floods, or fires may reduce the size of a population drastically, killing victims unselectively. The result is that the small surviving population is unlikely to be representative of the original population in its genetic makeup - a situation known as the bottleneck effect.... Genetic drift caused by bottlenecking may have been important in the early evolution of human populations when calamities decimated tribes. The gene pool of each surviving population may have been, just by chance, quite different from that of the larger population that predated the catastrophe." Campbell, N.A. in Biology 2nd ed. Benjamin/Cummings 1990 p.443 Several examples of bottlenecks have been inferred from genetic data. For example, there is very little genetic variation in the cheetah population. This is consistant with a reduction in the size of the population to only a few individuals - an event that probably occurred several thousand years ago. An observed example is the northern elephant seal which was hunted almost to extinction. By 1890 there were fewer than 20 animals but the population now numbers more than 30,000. As predicted there is very little genetic variation in the elephant seal population and it is likely that the twenty animals that survived the slaughter were more "lucky" than "fit". Another example of genetic drift is known as the founder effect. In this case a small group breaks off from a larger population and forms a new population. This effect is well known in human populations; "The founder effect is probably responsible for the virtually complete lact of blood group B in American Indians, whose ancestors arrived in very small numbers across the Bering Strait during the end of the last Ice Age, about 10,000 years ago. More recent examples are seen in religious isolates like the Dunkers and Old Order Amish of North America. These sects were founded by small numbers of migrants from their much larger congregations in central Europe. They have since remained nearly completely closed to immigration from the surrounding American population. As a result, their blood group gene frequencies are quite different from those in the surrounding populations, both in Europe and in North America. The process of genetic drift should sound familiar. It is, in fact, another way of looking at the inbreeding effect in small populations ... Whether regarded as inbreeding or as random sampling of genes, the effect is the same. Populations do not exactly reproduce their genetic constitutions; there is a random component of gene-frequency change." Suzuki et al. op. cit. There are many well studied examples of the founder effect. All of the cattle on iceland, for example, are descended from a small group that were brought to the island more than one thousand years ago. The genetic make-up of the icelandic cattle is now different from that of their cousins in Norway but the differences agree well with those predicted by genetic drift. Similarly, there are many pacific islands that have been colonized by small numbers of fruit flies (perhaps one female) and the genetics of these populations is consistant with drift models. Thus, it is wrong to consider natural selection as the ONLY mechanism of evolution and it is also wrong to claim that natural selection is the predominant mechanism. This point is made in many genetics and evolution textbooks, for example; "In any population, some proportion of loci are fixed at a selectively unfavorable allele because the intensity of selection is insufficient to overcome the random drift to fixation. Very great skepticism should be maintained toward naive theories about evolution that assume that populations always or nearly always reach an optimal constitution under selection. The existence of multiple adaptive peaks and the random fixation of less fit alleles are integral features of the evolutionary process. Natural selection cannot be relied on to produce the best of all possible worlds." Suzuki, D.T., Griffiths, A.J.F., Miller, J.H. and Lewontin, R.C. in An Introduction to Genetic Analysis 4th ed., W.H. Freeman, New York 1989 "One of the most important and controversial issues in population genetics is concerned with the relative importance of genetic drift and natural selection in determining evolutionary change. The key question at stake is whether the immense genetic variety which is observable in populations of all species is inconsequential to survival and reproduction (ie. is neutral), in which case drift will be the main determinant, or whether most gene substitutions do affect fitness, in which case natural selection is the main driving force. The arguments over this issue have been intense during the past half- century and are little nearer resolution though some would say that the drift case has become progressively stronger. Drift by its very nature cannot be positively demonstrated. To do this it would be necessary to show that selection has definitely NOT operated, which is impossible. Much indirect evidence has been obtained, however, which purports to favour the drift position. Firstly, and in many ways most persuasively is the molecular and biochemical evidence..." Harrison, G.A., Tanner, J.M., Pilbeam, D.R. and Baker, P.T. in Human Biology 3rd ed. Oxford University Press 1988 pp 214-215 The book by Harrison et al. is quite interesting because it goes on for several pages discussing the controversy. The authors point out that it is very difficult to find clear evidence of selection in humans (the sickle cell allele is a notable exception). In fact, it is difficult to find good evidence for selection in most organisms - most of the arguments are after the fact (but probably correct)! The relative importance of drift and selection depends, in part, on estimated population sizes. Drift is much more important in small populations. It is important to remember that most species consist of numerous smaller inbreeding populations called "demes". It is these demes that evolve. Studies of evolution at the molecular level have provided strong support for drift as a major mechanism of evolution. Observed mutations at the level of gene are mostly neutral and not subject to selection. One of the major controversies in evolutionary biology is the neutralist-selectionist debate over the importance of neutral mutations. Since the only way for neutral mutations to become fixed in a population is through genetic drift this controversy is actually over the relative importance of drift and natural selection. Laurence A. Moran (Larry)