5 years ago

Isolation-by-distance, not incipient ecological speciation, explains genetic differentiation in an Andean songbird (Aves: Furnariidae: Cranioleuca antisiensis, Line-cheeked Spinetail) despite near three-fold body size change across an environmental gradient

Robb T. Brumfield, Glenn F. Seeholzer
During the process of ecological speciation, reproductive isolation results from divergent natural selection and leads to a positive correlation between genetic divergence and adaptive phenotypic divergence, i.e. isolation-by-adaptation (IBA). In natural populations, phenotypic differentiation is often autocorrelated with geographic distance, making IBA difficult to distinguish from the neutral expectation of isolation-by-distance (IBD). We examined these two alternatives in a dramatic case of clinal phenotypic variation in an Andean songbird, the Line-cheeked Spinetail (Cranioleuca antisiensis). At its geographic extremes, this species shows a near three-fold difference in body mass (11.5 to 31.0 g) with marked plumage differences. We analyzed phenotypic, environmental, and genetic data (5,154 SNPs) from 172 individuals and 19 populations sampled along its linear distribution in the Andes. We found that body mass was tightly correlated with environmental temperature, consistent with local adaptation as per Bergmann's Rule. Using a PST-FST analysis, we found additional support for natural selection driving body mass differentiation, but these results could also be explained by environment-mediated phenotypic plasticity. When we assessed the relative support for patterns of IBA and IBD using variance partitioning, we found that IBD was the best explanation for genetic differentiation along the cline. Adaptive phenotypic or environmental divergence can reduce gene flow, a pattern interpreted as evidence of ecological speciation's role in diversification. Our results provide a counterexample to this interpretation. Despite conditions conducive to ecological speciation, our results suggest that dramatic size and environmental differentiation within C. antisiensis are not limiting gene flow. This article is protected by copyright. All rights reserved.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/mec.14429

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