Evidence for Local Specialization in a Widespread Lizard

Figure 1 . (A) Widespread species may be comprised of populations (dashed lines) exhibiting traits generalized across all habitats or (B) capable of specializing to unique habitats throughout their range.

Widespread species are expected to be successful in natural environments because of their ability to generalize across a variety of habitats. Throughout their range, widespread species may experience a variety of habitat types and may subsequently exhibit similar patterns of morphology and performance capabilities. In this sense, widespread species could encounter a “jack-of-all trades but master of none” trade-off in that a population may not be optimally adapted to a certain environment (Figure 1A). By contrast, we hypothesized in a recent paper published in Evolution that local specialization could be driving the broad-scale success of a widespread species. By adapting to a specific habitat, natural selection could produce unique fitness surfaces and phenotypic variation between populations (Figure 1B).

In this study, my collaborators and I conducted this study on four distinct populations of Urosaurus ornatus, a widespread lizard found throughout the American southwest (Figure 2), to determine whether success is a result of ecological generalism or local specialization. Urosaurus ornatus is a small, polymorphic lizard that primarily occupies desert habitats. While the common name is the ornate tree lizard, this species can naturally be found on a wide variety of substrates, including tree limbs, tree trunks, boulders, shrubs, snags, canyon walls, and the ground. We focused on populations found in one of two microhabitat types, tree-dominated or boulder-dominated, to assess habitat-specific differences in natural selection.

Figure 2. Male (left) and female (right) Urosaurus ornatus on a natural perch.

Morphological characters and performance capacity are ideal traits for this experiment due to their sensitivity to ecological and environmental characteristics. Thus, our results show striking differences in selection on these traits by sex, supporting the notion of divergent ecological pressures within a shared environment. This, coupled with the heterogeneity in selection between habitat types, leads us to believe that local adaptation is driving the success of this widespread species. In the past, evidence for generalism at the species level has masked the underlying affects of the environment and local adaptation. Here we are able to tease apart some of these traits and determine how selection varies at the population level in order to extrapolate to the species level.

So what do tree lizards have to do with anoles? In short, the similarities between Urosaurus and Anolis are plentiful. While there may be significant differences in habitat, both genera contain species that are wonderful models for a plethora of different ecological, evolutionary, and genomic questions. The wide breadth of anole literature has influenced our findings in this study and contributed significantly to its impact and viability. For that, we thank the many anole researchers from around the world!