Genetic drift (or allelic drift) is the change in the relative frequency with which a gene variant (allele) occurs in a population. A population's allele frequency is the fraction of the gene copies that share a particular form.
In contrast to natural selection, which makes gene variants more common or less common due to their causal effects on reproductive success, the changes due to genetic drift are not driven by environmental or adaptive pressures, and may be beneficial, neutral, or detrimental to reproductive success.
The effect of genetic drift is larger in small populations, and smaller in large populations. Vigorous debates wage among scientists over the relative importance of genetic drift compared with natural selection.
Genetic Catastrophes and Population Bottlenecks
Suppose fishermen looking for minnows (any small, shiny fish) to use as bait begin netting the mixed population of small fish in a lake. They keep (remove) the bright colored ones but toss back the dark colored ones. Following this “catastrophe,” genetic drift for at least one to possibly several generations could possibly result in the removal of the unique genes carried by the bright colored fish.
Genetic drift acts more quickly to reduce variation in small populations than it does in large populations. This can create a population bottleneck. Suppose the fisherman seeking bait decided to keep all the small fish they netted from the lake, regardless of color. If a substantial number of fish were removed from what was already a small population to begin with, a population bottleneck for that species in that lake would result, a catastrophe from which they would only slowly, if ever, recover.
The Hardy-Weinburg Equilibrium Model
In 1908, an English mathematician, Godfrey Hardy, and a German physician, Wilhelm Weinberg, independently proposed a mathematical model that states that both allele and genotype (the genome of an individual or group) frequencies in a population remain constant – that is, they are in equilibrium – from generation to generation unless specific disturbing influences are introduced.
The Hardy-Weinberg Equilibrium Model as it has come to be known is based on probability and concludes that gene frequencies are inherently stable but that evolution should be expected in all populations virtually all of the time.
Evolutionary biologists and population geneticists came to understand that evolution will not occur in a population if seven conditions are met:
- mutation is not occurring
- natural selection is not occurring
- the population is infinitely large
- all members of the population breed
- all mating is totally random
- all matings produce the same number of offspring
- there is no migration in or out of the population
In other words, if no mechanisms of evolution are acting on a population, evolution will not occur and the gene pool frequencies will remain unchanged. However, in the real world few if any of the above conditions exist. Hence, evolution is the inevitable result.
If a population is small, Hardy-Weinberg may be violated. Chance alone may eliminate certain members out of the proportion to their numbers in the population. In such cases, the frequency of a gene may begin to drift toward higher or lower values. Genetic drift may ultimately cause the frequency of the gene to represent 100% of the gene pool or, depending on the initial frequency of the alleles, completely disappear from the gene pool altogether.
By the simple vagaries of chance, in each generation some individuals may leave behind a few more descendants that other individuals. These individuals passed on their genes due to lucky accident, not because they were naturally selected. Unlike natural selection, genetic drift is a totally random process that doesn’t work to produce new adaptations.
Genetic drift produces evolutionary change, but there is no guarantee that the new population will be more fit than the original one. Evolution by drift is aimless, not adaptive.
Mechanisms of Evolution
Evolution encompasses changes on two vastly different scales – from an increase in the frequency of a gene for colored spots on the feathers of a bird (microevolution) to something as grand in scale as the evolution of the entire bird lineage (macroevolution). Despite the scale on which it happens, evolution at both levels is driven by the primary mechanisms of natural selection, mutation, genetic drift, and gene flow.
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