From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran)
Subject: GENETIC DRIFT
Organization: UTCS Public Access
Date: Sat, 23 Jan 1993 22:25:24 GMT


                         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)