N

where t indicates the total number of generations (all other assumptions of Wright-Fisher populations are met). The harmonic mean gives more weight

120 r

120 r

Generation

Figure 3.16 Schematic representation of a genetic bottleneck where census population size fluctuates across generations. The harmonic mean of census population size (N) is 25 and provides an estimate of the effects of genetic drift over three generations, or the effective population size (Ne). In other words, this population of fluctuating size would experience as much genetic drift as an ideal Wright-Fisher population with a constant population size of 25. The squares represent alleles present in each generation. In the first generation the alleles are equally frequent, but they end up at frequencies of 25 and 75%. Such an allelic state transition would be extremely unlikely if 200 gametes were sampled to found generations two and three. However, the observed allelic transition is expected 25% of the time in a sample of four gametes.

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