Simply put, genetic drift is the change in allelic frequencies due to chance. This phenomenon is particularly important in small populations as a smaller population size increases the importance of chance effects.
An general illustration of the principle will certainly elucidate this principle. Suppose there is a population of organisms, and in this population, a new allele arises with good potential of having a profound effect on the allelic frequencies in this population. But, some catastrophic event happens (a flood, a fire, a disease, …) and all the organisms carrying the new allele are wiped out. If this is the case, the allele frequencies of that population will look different than when natural selection was allowed to run its course.
Vice versa, a new allele can spread quickly if most of the organisms are pushed into extinction, aznd the few that remain just happen to have a certain allele. This allele will thus have a much higher frequency then before the catastrophic event.
These examples of course are oversimplifications, but nevertheless, they illustrate that chance can have a profound effect on the allelic frequencies in a population.
Effects of Genetic Drift
In this section, effects on genetic variation due to genetic drift will be discussed. In general, there are three ways genetic drift can influence genetic variation:
- Firstly, genetic drift leads to a change in allelic frequencies in a population. Since this drift is random, the allelic frequency of the allele under consideration is just as likely to increase as it is to decrease. This results in the allelic frequency ‘wandering’ as time passes (which is the reason why it is called genetic ‘drift’).
- Secondly, genetic drift tends to lead to a reduction in genetic variation within populations. Through random change, an allele may reach a frequency of 0 or 1, at which point all individuals in the population are homozygous for the allele. When an allele reaches a frequency of 1, it has reached fixation, which reduces genetic variation. Given enough time, any small population will reach fixation for an allele through genetic drift.
- Thirdly, genetic drift results in a divergence of different populations over time. Because of the random nature of genetic drift, allelic frequencies will change in a different way in different populations. These populations will thus acquire genetic differences. Eventually, all the populations involved will reach fixation for certain alleles, which do not need to be the same alleles in the different populations.
These three effects on genetic variation due to genetic drift occur simultaneously. The first two effects take place within populations, and the third one between populations.
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- Mettler, L.E.; Gregg, T.G. & Schaffer, H.S. (1998). Population Genetics and Evolution. Englewood Cliffs. NJ: Prentice Hall.
- Pierce, B.A. (2002) Genetics: A Conceptual Approach. W.H. Freeman.