Figure 1.

Tests for the effect of selection in divergence among populations (modified from refs. 24 and 25). (a) Four regions are defined. Shading indicates different habitats, and the bold horizontal line indicates a partial or complete barrier to gene flow (e.g., geographic distance or a physical barrier). Populations A and A′ occupy one habitat, and populations B and B′ occupy a different habitat. (b) By comparing morphological divergence in fitness-related traits or reproductive divergence among populations, relative roles of drift and selection in divergence can be evaluated. If selection is driving population divergence, then, for a given level of genetic divergence, greater reproductive isolation (or morphological divergence) is expected among populations from different habitats than among populations occupying similar habitats. If the degree of reproductive divergence is similar within and among habitats, then factors acting independent of the environment (e.g., drift) are indicated. (c) Historical relationships among populations provide an additional test of the hypothesis that selection is important in speciation. Populations A and B (also A′ and B′) are sister groups that occupy different habitats whereas populations A and A′ occupy similar habitats but are not sister groups. In parallel speciation (24), similar adaptive divergence of populations occurs independently two or more times. In the single habitat shift scenario, A is the ancestral habitat, and a single shift to habitat B occurs. If reproductive divergence is greater between A and B (and A′ and B′) than between A and A′ (and B and B′), then selection is implicated in divergence.