Genetic drift is a change in allele frequencies in a population from one generation to the next that happens by chance, not because an allele gives any advantage.It happens because not every individual reproduces, and each offspring receives a random half of its parents' alleles. By chance, some alleles are passed on more often than others, so their frequency rises, while others fall.Genetic drift happens in all populations, but its effect is much greater in small populations. In a large population, random changes tend to cancel out. In a small population, a few chance events can change allele frequencies a lot.
Over time, drift can lead to an allele being:- lost: its frequency falls to 0;
- fixed: its frequency rises to 1 (100%), so every individual is homozygous for it.Either way, genetic diversity falls. Bottlenecks and the founder effect are special cases in which drift has a very large effect.Worked example. A population of 5 plants has 10 copies of a gene: 3 are allele A and 7 are allele a, so the frequency of A is 3 ÷ 10 = 0.3. By chance, the plants carrying A produce more seeds, and the next generation has 5 copies of A out of 10. The frequency of A is now 5 ÷ 10 = 0.5. This change is not due to any advantage of A.Unlike natural selection, drift is random: it can increase the frequency of a neutral or even a harmful allele. In small populations, drift can outweigh selection.
Over time, drift can lead to an allele being:- lost: its frequency falls to 0;- fixed: its frequency rises to 1 (100%), so every individual is homozygous for it.Either way, genetic diversity falls. Bottlenecks and the founder effect are special cases in which drift has a very large effect.Worked example. A population of 5 plants has 10 copies of a gene: 3 are allele A and 7 are allele a, so the frequency of A is 3 ÷ 10 = 0.3. By chance, the plants carrying A produce more seeds, and the next generation has 5 copies of A out of 10. The frequency of A is now 5 ÷ 10 = 0.5. This change is not due to any advantage of A.Unlike natural selection, drift is random: it can increase the frequency of a neutral or even a harmful allele. In small populations, drift can outweigh selection.