Abstract
Adaptation to different thermal environments has the potential to cause evolutionary changes that are sufficient to drive ecological speciation. Here, we examine whether climate-based niche divergence in lizards of the Plestiodon skiltonianus species complex is consistent with the outcomes of such a process. Previous work on this group shows that a mechanical sexual barrier has evolved between species that differ mainly in body size and that the barrier may be a by-product of selection for increased body size in lineages that have invaded xeric environments; however, baseline information on niche divergence among members of the group is lacking. We quantified the climatic niche using mechanistic physiological and correlative niche models and then estimated niche differences among species using ordination techniques and tests of niche overlap and equivalency. Our results show that the thermal niches of size-divergent, reproductively isolated morphospecies are significantly differentiated and that precipitation may have been as important as temperature in causing increased shifts in body size in xeric habitats. While these findings alone do not demonstrate thermal adaptation or identify the cause of speciation, their integration with earlier genetic and behavioral studies provides a useful test of phenotype-environment associations that further support the case for ecological speciation in these lizards.
Highlights
Ecological speciation occurs when natural selection causes divergence in traits that influence reproductive compatibility, eventually leading to a cessation of gene flow between lineages that have adapted to different environments (Dobzhansky 1937; Mayr 1942; Schluter 2000)
Ecology and Evolution published by John Wiley & Sons Ltd
We focused on comparisons between the two morphospecies rather than among lineages within the species complex because (1) size is the fundamental trait responsible for the mechanical reproductive barrier and (2) it is necessary to rule out niche conservatism between the two main morphs in order to set precedence for thermal adaptation as a promoter of ecological speciation
Summary
Ecological speciation occurs when natural selection causes divergence in traits that influence reproductive compatibility, eventually leading to a cessation of gene flow between lineages that have adapted to different environments (Dobzhansky 1937; Mayr 1942; Schluter 2000). Spatially explicit environmental data have been increasingly used to test for ecology’s role in speciation, with comparisons of climate-based environmental niche models between sibling species representing the key analytical framework (Graham et al 2004; Kozak et al 2008; Warren et al 2008; McCormack et al 2010) This approach represents one of several potential steps in demonstrating a role for thermal adaptation in the speciation process, given that an organism’s optimal performance is expected to match the climate regime it experiences most often (Angilletta 2009; Keller and Seehausen 2012).
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