Relationship between time and number of lineages under the coalescent. (A) Expected coalescent times for 2–5 lineages in units of N generations for a haploid locus (2 N generations for autosomal loci). (B) Representative coalescent genealogy. Note that many lineages exist to record events in the recent past, while few lineages remain to represent older time points. Only recombination involving an extant lineage (shaded points) can be observed today. The probability that recombination involves an extant lineage is high in the present, where many shaded points exist, but declines exponentially into the past, where shaded points are scarce.
Cox et al. BMC Genetics 2013 14:11 doi:10.1186/1471-2156-14-11