Star-Shaped Genealogy

In a large population that has grown rapidly from a small size, genetic lineages sampled in the present likely have few shared recent ancestors in the most generations. Rather, the lineages are likely to reach a common ancestor back during the period when the population was small. In that period, the lineages quickly find common ancestors, producing a “star-shaped genealogy” in which they all appear to emanate from a shared ancestral node.

The shadows of these palm fronds represent lineages. The width of the stone region between the arches depicts a population that has grown in time upward from a small size, represented by the top of the post. The lineages find their common ancestor in the period during which the population was smallest.