Models of population structure reflecting the (A) island, (B) splitting and (C) isolation-with-migration (IM) models. The island model assumes equilibrium gene flow (m) between subpopulations that have no shared ancestry. The divergence model describes an ancestral population, which splits at time t into two daughter populations that do not exchange genes in subsequent generations. The isolation-with-migration model describes a constant-sized ancestral population that splits into two daughter populations that can exchange genes and change in size. There are seven parameters in the isolation-with-migration model: effective population size of the ancestral deme (NA), effective population sizes of the two descendent demes (N1 and N2), unidirectional migration between the descendent populations (m1 and m2), proportion of the ancestral population founding deme 1 (S), and population divergence time (t).