N

must also be continually increasing as we move back in time toward the MRCA, since N is shrinking. The result is that genealogies in growing populations do not follow the rule that the final coalescence time from two lineages to the MRCA is the longest on average in a genealogy established in the last section for populations of constant size. Instead, genealogies in growing populations tend to have longer times between coalescent events toward the present and shorter times between coalescent events in the past (Fig. 3.29). Genealogies from rapidly growing populations therefore tend to have longer branches toward the present and shorter branches deep in the tree.

In a population shrinking in size over time, the probability of a coalescent event is greatest near the present because the population size is at its smallest. The effect on genealogies is that coalescent waiting

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