Random Genetic Drift
Copyright © 1993-1997 by Laurence Moran
[Last Update: January 22, 1993]
he 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...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".
"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)
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.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.
"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.)
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)And:
"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.
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