Figure 1. Isolation with Migration Models.

(A) The basic IM model. The demographic terms are effective population sizes (N₁, N₂, and N_A), gene flow rates (m₁ and m₂), and population splitting time (t). Also shown are parameters scaled by the neutral mutation rate (u), as they are actually used in the model fitting. Terms for basic demographic parameters, including N, m, t, and u, are not italicized. Note that the migration parameters are identified by the source of migrants as time goes backward in the coalescent. In other words, the migration rate from population 1 to population 2 (i.e., m₁) actually corresponds to the movement of genes from population 2 to population 1 as time moves forward.

(B) The IM model with changing population size. An additional parameter, s, is the fraction of N_A that forms N₁ (i.e., the fraction 1 − s gives rise to N₂)