Figure 1.

The phenotypic pattern of reinforcement. (a) A schematic of the relationship between two species each with sympatric and allopatric populations. Secondary contact between the lineages in sympatry results in costly hybridization, causing reinforcement in Species 1. (b) Reinforcement selects for a new reproductive trait value in Species 1 (solid line) to prevent mating with Species 2 (dashed line) in sympatry (purple). Allopatric populations of Species 1 (red) retain the ancestral phenotype. In this simplified scenario, Species 2 does not change trait value in sympatry or allopatry (blue), but in some cases Species 2 may also diverge in trait value in sympatry. (c) For Species 1, reinforcement increases reproductive isolation (solid line) with Species 2 in sympatry relative to allopatry.