Phenotypic divergence during speciation is inversely associated with differences in seasonal migration

Abstract

Differences in seasonal migration might promote reproductive isolation and differentiation by causing populations in migratory divides to arrive on the breeding grounds at different times and/or produce hybrids that take inferior migratory routes. We examined this question by quantifying divergence in song, colour, and morphology between sister pairs of North American migratory birds. We predicted that apparent rates of phenotypic differentiation would differ between pairs that do and do not form migratory divides. Consistent with this prediction, results from mixed effects models and Ornstein-Uhlenbeck models of evolution showed different rates of divergence between these groups; surprisingly, differentiation was greater among non-divide pairs. We interpret this finding as a result of variable rates of population blending and fusion between partially diverged forms. Ancient pairs of populations that subsequently fused are now observed as a single form, whereas those that did not fuse are observable as pairs and included in our study. We propose that fusion of two populations is more likely to occur when they have similar migratory routes and little other phenotypic differentiation that would cause reproductive isolation. By contrast, pairs with migratory divides are more likely to remain reproductively isolated, even when differing little in other phenotypic traits. These findings suggest that migratory differences may be one among several isolating barriers that prevent divergent populations from fusing and thereby increase the likelihood that they will continue differentiating as distinct species.

Keywords: comparative analysis; evolutionary rate; migratory divide; phenotypic divergence; seasonal migration; speciation.

Figure 1.

Modelled relationship between overall phenotypic divergence and migration category (no divide, divide). Model averaging with linear mixed effects models were run with a random variable of genus nested within family to control for phylogenetic relationships. Significant predictors in the final model included migration category, time since divergence, and body mass. The effect of migration category is plotted with these additional variables held at their medians. (Online version in colour.)

Figure 2.

Relationship between phenotypic divergence and time since divergence. Results are shown for all traits combined (a) and each set of traits separately (b–d). Time since divergence was measured using genetic distance; phenotypic divergence was measured using average Hedges' g. Curves show results from OU (Ornstein–Uhlenbeck) models of evolutionary change, fitting separate models for taxa that do (dotted line and open points) and do not (black line and filled points) form migratory divides.