Quick questions on Genetic drift, gene flow and the Hardy-Weinberg principle: WACE Year 12 Human Biology Unit 4

Genetic drift is a change in allele frequencies due to random chance rather than to differences in fitness. In every generation, which individuals happen to survive and reproduce, and which alleles happen to be passed on, involves an element of luck. In a large population these chance effects average out, but in a small population they can swing allele frequencies sharply from one generation to the next, even eliminating or fixing an allele regardless of whether it is beneficial.

The founder effect occurs when a small group breaks away from a larger population to start a new one. The founders carry only a chance sample of the original population's alleles, so the new gene pool may differ markedly from the original, and some alleles may be over-represented while others are absent. This is why some isolated human populations have unusually high frequencies of particular alleles or genetic conditions.

The bottleneck effect occurs when a population is drastically reduced in size by a disaster, disease or hunting, so that only a few individuals survive. The survivors carry only a chance sample of the original alleles, so genetic variation is reduced and the surviving allele frequencies may differ from the original. Even if the population later recovers in number, its genetic diversity stays low, which can make it more vulnerable.

Gene flow (migration) is the movement of alleles between populations when individuals or their gametes move and breed in a new population. Immigration adds alleles to a gene pool and emigration removes them. Gene flow tends to make separate populations more genetically similar and can introduce new alleles to a population. When gene flow is blocked (by isolation), populations are free to diverge, which links to speciation.

The Hardy-Weinberg principle describes a theoretical population in which allele frequencies do not change from generation to generation, meaning no evolution is occurring. This genetic equilibrium holds only if five conditions are met: a very large population (no drift), no mutation, no gene flow (no migration), random mating, and no natural selection. Because real populations rarely meet all five, the principle is used as a baseline: if observed allele frequencies differ from the Hardy-Weinberg expectation, one of the conditions is broken and the population is evolving.