This section covers genetic drift as chance-driven change in allele frequency and how alleles can become more or less common without natural selection.
Allele frequencies can change by chance
Genetic drift: a change in allele frequency caused by chance, with the effect being particularly important in small populations.
Not every individual in a population contributes alleles to the next generation. Sometimes the individuals that survive, reproduce or successfully produce offspring are determined partly by random events rather than by a phenotype giving them a selective advantage.
As a result, one allele may become more common while another becomes less common even though neither change is being driven by an environmental selection pressure.
Alleles can increase in frequency purely by chance.
Alleles can decrease in frequency even when they are not disadvantageous.
An allele may eventually disappear from a small population following repeated chance effects.
Consider a small population of plants in which alleles associated with blue and pink flowers initially occur in equal numbers.
Seeds are dispersed near a playground containing both fertile soil and a rubber surface. By chance, many seeds carrying the pink-flower allele land on the rubber and fail to germinate, whereas more seeds carrying the blue-flower allele reach soil and grow.
The difference does not occur because blue flowers are better adapted. It results from where the seeds happened to land.
If this chance imbalance continues through generations, the pink-flower allele can become progressively rarer and may eventually disappear from the population.
Exam Tip: When explaining genetic drift, make clear that the change is due to chance rather than a selective advantage. You should then state that the random event changes the frequency of alleles in the population.