Niche expansion leads to small-scale adaptive divergence along an elevation gradient in a medium-sized passerine bird

Abstract

Niche expansion can lead to adaptive differentiation and speciation, but there are few examples from contemporary niche expansions about how this process is initiated. We assess the consequences of a niche expansion by Mexican jays (Aphelocoma ultramarina) along an elevation gradient. We predicted that jays at high elevation would have straighter bills adapted to feeding on pine cones, whereas jays at low elevation would have hooked bills adapted to feeding on acorns. We measured morphological and genetic variation of 95 adult jays and found significant differences in hook length between elevations in accordance with predictions, a pattern corroborated by analysis at the regional scale. Genetic results from microsatellite and mtDNA variation support phenotypic differentiation in the presence of gene flow coupled with weak, but detectable genetic differentiation between high-and low-elevation populations. These results demonstrate that niche expansion can lead to adaptive divergence despite gene flow between parapatric populations along an elevation gradient, providing information on a key precursor to ecological speciation.

The geographic scale of diversification on islands: genetic and morphological divergence at a very small spatial scale in the Mascarene grey white-eye (Aves: Zosterops borbonicus)

“…In Z. borbonicus , the altitudinal increase of size-related traits like wing, tail and tarsus length, combined with an opposite trend for diet-related traits like beak dimensions, suggests the role of natural selection in driving morphological divergence along steep altitudinal gradients, as has been documented in other similar avian systems [39-41]. Rapid morphological evolution has been documented in other Zosterops species [42,43], yet adaptive changes in morphology can often evolve too fast for neutral genetic markers to show congruent patterns of differentiation [40].…”

Fine-scale genetic population structure of an understory rainforest bird in Costa Rica

“…In Ecuador, Andean valley populations of rufous-collared sparrow (Zonotrichia capensis) separated by only 25 km displayed significant population genetic structure, and considerably different breeding phenology (Moore et al 2005). In Mexico, McCormack and Smith (2008) found morphological and genetic divergences related to foraging behavior among Mexican jay (Aphelocoma ultramarina) populations at different elevations. In the endangered habitat specialist Florida scrub jay (A. coerulescens), habitat fragmentation (both natural and anthropogenic) has led to significant population structure at small geographic scales .…”

Diversification across an altitudinal gradient in the Tiny Greenbul (Phyllastrephus debilis) from the Eastern Arc Mountains of Africa

“…[15-17]]. Although several studies have provided evidence of the importance of natural selection for the formation of new species even when gene flow occurs [18], research that connects presumably adaptive variation in traits (plumage in our case) with assessments of the phylogenetic relationships among populations/taxa, extent of genetic variation and gene flow are relatively few [19].…”