Abstract
BackgroundEcological factors often have a strong impact on spatiotemporal patterns of biodiversity. The integration of spatial ecology and phylogenetics allows for rigorous tests of whether speciation is associated with niche conservatism (constraints on ecological divergence) or niche divergence. We address this question in a genus of livebearing fishes for which the role of sexual selection in speciation has long been studied, but in which the potential role of ecological divergence during speciation has not been tested.ResultsBy combining reconstruction of ancestral climate tolerances and disparity indices, we show that the earliest evolutionary split in Xiphophorus was associated with significant divergence for temperature variables. Niche evolution and present day niches were most closely associated with each species’ geographic distribution relative to a biogeographic barrier, the Trans-Mexican Volcanic Belt. Tests for similarity of the environmental backgrounds of closely related species suggested that the relative importance of niche conservatism and divergence during speciation varied among the primary clades of Xiphophorus. Closely related species in the two swordtail clades exhibited higher levels of niche overlap than expected given environmental background similarity indicative of niche conservatism. In contrast, almost all species of platyfish had significantly divergent niches compared to environmental backgrounds, which is indicative of niche divergence.ConclusionThe results suggest that the relative importance of niche conservatism and divergence differed among the clades of Xiphophorus and that traits associated with niche evolution may be more evolutionarily labile in the platyfishes. Our results ultimately suggest that the taxonomic scale of tests for conservatism and divergence could greatly influence inferences of their relative importance in the speciation process.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0593-4) contains supplementary material, which is available to authorized users.
Highlights
Ecological factors often have a strong impact on spatiotemporal patterns of biodiversity
The relationship between niche evolution and speciation remains widely studied, because conservatism and divergence might promote the emergence of new species [51]
The degree of niche conservatism observed across the genus Xiphophorus varied markedly among clades
Summary
Ecological factors often have a strong impact on spatiotemporal patterns of biodiversity. Ecological factors play a critical role in evolutionary processes by providing sources of selection that drive microevolutionary change and by imposing constraints that limit organismal performance [1,2,3]. They can shape phenotypic evolution and functional diversification and set the distributional limits for populations and species [4,5,6]. Ecological speciation models suggest that divergent natural selection drives the emergence of reproductive isolation as a byproduct of adaptation [7, 8] Speciation in this context is accompanied by divergence in niche occupation along at least one axis of multidimensional niche space [9, 10]. Constraints imposed by ecological factors can play a role in speciation, if evolutionary lineages become geographically isolated during periods of environmental change but retain ancestral niche affinities
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