HSC · Topic explainer
Why genetic drift is stronger in small populations
Genetic drift is a chance change in allele frequency between generations. The same change in allele count has a larger effect on frequency in a small gene pool. Gene flow changes a population’s allele composition when individuals or gametes move between populations and contribute to reproduction.
Worked example
Compare diploid populations of 10 and 1,000 individuals. At an autosomal locus, they have 20 and 2,000 copies respectively. A chance difference of two copies corresponds to 10 percentage points in the smaller pool and 0.1 percentage points in the larger pool.
When interpreting a graph, compare the scale of fluctuations and the population sizes. A random change can remove a rare allele from a small population. To identify gene flow, look for evidence of alleles arriving or leaving through migration and reproduction.
Official exam practice
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