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Compare the probability of coalescence PC = 1 - 0.995 = 0.005 for a population of 100 and Pc = 1 - 0.95 = 0.05 for a population of 10. During the bottleneck there is a 10-fold greater chance each generation that two lineages find their ancestor or coalesce than before or after the bottleneck, leading to shorter branch lengths (times to coalescence). Thus, the bottleneck causes increased sampling of lineages and a greater chance that ancestral lineages are lost to sampling. After the bottleneck, the probability of coalescence returns to the same probability as in the first generation. However, the lineages present after the bottleneck are now much more likely to be recently related so the overall probability of coalescence is like that in a smaller population of constant size.

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