ABOUT THIS IMAGE. This image shows two random genealogical trees simulated under the coalescent, conditional on having specific numbers of ancestors at a fixed point in time.

The fixed time point is represented by the white horizontal line. A genealogy forced to have a small number of ancestors far back in the past (left) appears similar to typical genealogies simulated unconditionally under the coalescent. However, a genealogy forced to have many ancestors contains several long external branches (right). The figure is based on the work of Blum & Rosenberg (Estimating the number of ancestral lineages using a maximum-likelihood method based on rejection sampling; Genetics 176: 1741-1757, 2006).

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