Niche divergence builds the case for ecological speciation in skinks of the Plestiodon skiltonianus species complex

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.

Keywords: Adaptation; environmental niche models; niche overlap; phenotype–environment association; thermal adaptation.

Figure 1

The three lizard species forming the P. skiltonianus complex fall into two distinctive morphospecies a large‐bodied, unicolored form and a geographically widespread, small‐bodied form. (A) Occurrence points for members of the P. skiltonianus complex. (B) Young individual of the small‐bodied form, P. skiltonianus (photograph by Jackson Shedd). (C). Body size distributions for species of the P. skiltonianus complex. Size bins on the x‐axis reflect snout‐vent lengths (SVL) for 415 specimens of P. skiltonianus and 560 P. gilberti. Mean SVL ± SD (no. of specimens) is provided for males and females of both species.

Figure 2

(A) Thermal niche space of P. gilberti and P. skiltonianus based on mechanistic models that incorporate thermal constraints during the adult active period (March to June). These panels depict the intersection of the realized climate (C, solid dark gray dots), the potential niche (P, solid dark red or solid dark blue), the IUCN delimited realized niche (N, open red or open medium blue), and the specimen delimited realized niche (R, open circles pink or pale blue). (B) The spatial distribution of the potential niche for each species.

Figure 3

(A). Predicted species' distributions based on correlative ecological niche models (dark gray); the ranges have been bounded using equal sensitivity and specificity. Each panel depicts the predicted distribution under the present climate. The IUCN range for each species is also provided (light gray). (B) Species identity tests. Each graph depicts the distribution generated from 100 pseudoreplicated MaxEnt runs, which pool and then parse the presence points randomly. The arrow indicates the actual niche overlap score between pairwise niche comparisons among species. The niche overlap value is smaller than the null distribution in all pairwise comparisons, indicating that the niches have diverged.