Why does rapid evolution occur in small populations?

Population size, technically the effective population size, is related to the strength of drift and the likelihood of inbreeding in the population. Small populations tend to lose genetic diversity more quickly than large populations due to stochastic sampling error (i.e., genetic drift). This is because some versions of a gene can be lost due to random chance, and this is more likely to occur when populations are small. Additionally, smaller population size means that individuals are more likely to breed with close relatives. In closed populations, individuals will be more closely related to each other compared to individuals in the previous generation. For example, in a hypothetical population consisting of only four individuals, if two pairs each produced two offspring (meaning that four new individuals are present in the next generation), the offspring must either mate with a sibling, a parent, or an individual from the other pair. Assuming they choose the non-sibling/non-parent option, all of the offspring in the third generation must mate with individuals that have the same grandparents or choose to forgo reproduction. Although this example is extreme due to the very small hypothetical population, the same patterns and forces are present in larger – albeit still small – populations.

Although the mechanism of the loss of genetic diversity due to inbreeding and drift is different, the effects on populations are the same. Both inbreeding and drift reduce genetic diversity, which has been associated with an increased risk of population extinction, reduced population growth rate, reduced potential for response to environmental change, and decreased disease resistance, which impacts the ability of released individuals to survive and reproduce in the wild.

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1. Values above the diagonal are mean pairwise FST values across all 5.4 k SNPs. Values along the diagonal (bold) are mean nucleotide diversity (π) within populations. Values below the diagonal are mean proportion of pairwise differences (π). Population abbreviations: Oyster Creek Triad (Southern Reference – Rutgers Basin (RB), TE Population – Oyster Creek Generating Station (OC), Northern Reference – Mantoloking, New Jersey (Mg)) Brayton Point Triad (Southern Reference – Succotash Marsh, Matunuck, Rhode Island (SR), TE Population – Brayton Point Generating Station (BP), Northern Reference – Horseneck Beach, Massachusetts (HB)). N: number of individuals in final SNP dataset