On the predictability of phenotypic divergence in geographic isolation

Do related populations that are separated by barriers predictably evolve
differences from one another over time, or is such divergence
idiosyncratic and unpredictable? We test these alternatives by
investigating patterns of trait evolution for 54 sister pairs of Andean
forest birds that live in similar environments on either side of the arid
Marañón Gap, a strong dispersal barrier for humid montane species. We
measured divergence in both sexual (song and plumage) and ecological (beak
size and beak shape) traits. Sexual traits evolve in a clock-like fashion,
with trait divergence positively correlated with genetic distance (r = 0.6
to 0.7). In contrast, divergence in ecological traits is uncorrelated or
only loosely correlated with genetic distance (r = 0.0 to 0.3). Thus, for
geographically isolated Andean montane forest birds that live in similar
environments, divergence is predictable in sexual traits, but not for
ecological traits. This means that sexual trait divergence occurs
independently of adaptive ecological divergence within the mega-diverse
tropical Andean avifauna. Last, we show that variation in genetic
divergence across a biogeographic barrier is associated with traits that
are proxies for species’ opportunities for dispersal (low elevation limit
and elevational niche breadth), but not with traits that are proxies for
species’ dispersal abilities (hand-wing index and foraging strata).