Genetic drift

Genetic drift is a change in allele and genotype frequencies, which is simply the effect of chance. By chance, some individuals will have more offspring than others, not due to an advantage conferred by some genetically-encoded trait, but simply because one male happened to be in the right place at the right time (when the receptive female walked by) or because the other one happened to be in the wrong place at the wrong time (when a fox was hunting).

Rabbits that are homozygous for B gene with two capital B alleles or heterozygous for B gene with one capital B allele and one lowercase b allele are both brown. Rabbits that are homozygous for little b alleles are white. In the first generation, there are 2 homozygous brown rabbits, 6 heterozygous brown rabbits and 2 white rabbits. The capital B gene frequency is represented by p=0.5 and lowercase b gene frequency is represented by q=0.5. Red line indicating reproducing rabbits encircles both homozygous brown rabbits and 3 heterozygous brown rabbits. In the second generation, there are 4 homozygous for capital B allele brown rabbits and 5 heterozygous brown rabbits and 1 homozygous for lowercase b allele white rabbit. For this generation, p=0.7 and q=0.3. Red line encircling two homozygous brown rabbits indicates reproducing rabbits. In the third generation, all rabbits are brown and homozygous for the capital B allele. p=1 and q=0.

Figure 1: Three generations of rabbit B gene allelic frequencies.

Genetic drift in a population can lead to the elimination of an allele from a population by chance. In this example, rabbits with the brown coat color allele (capital B) are dominant over rabbits with the white coat color allele (lower case b). In the first generation, the two alleles occur with equal frequency in the population, resulting in p and q values of 0.5. Only half of the individuals reproduce, resulting in a second generation with p and q values of 0.7 and 0.3, respectively. Only two individuals in the second generation reproduce, and by chance these individuals are homozygously dominant for brown coat color. As a result, the recessive b allele is lost in the third generation.

Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. If one individual in a population of 10 individuals happens to die at a young age and before it leaves any offspring to the next generation, all of its genes, one tenth of the population’s gene pool, will be suddenly lost. In a population of 100, that’s only 1 percent of the overall gene pool; therefore, it has a much smaller impact on the population’s genetic structure.