Figure 1.

Species networks used for simulations. a) Gene flow between sister lineages: species A and B diverged at time τ_t, and introgression occurred from B into A at time τ_h. b) Gene flow between non-sister lineages (from species D into C at time τ_h. c) Gene flow from an unsampled ghost lineage O (shown in gray) into species A. Divergence time ${\displaystyle (\tau )}$ is given in units of population size ${\displaystyle (\theta )}$. Population size is constant among all branches. Node names are shown with lower-case letters. The direction of introgression is from node g to h, indicated by the arrow. Simulations under the IM model use the same species trees, but with migration occurring after species divergence at the constant rate of M = Nm migrants per generation.