Abstract
BackgroundDisentangling the selective factors shaping adaptive trait variation is an important but challenging task. Many studies—especially in Drosophila—have documented trait variation along latitudinal or altitudinal clines, but frequently lack resolution about specific environmental gradients that could be causal selective agents, and often do not investigate covariation between traits simultaneously. Here we examined variation in multiple macroecological factors across geographic space and their associations with variation in three physiological traits (desiccation resistance, UV resistance, and pigmentation) at both population and species scales, to address the role of abiotic environment in shaping trait variation.ResultsUsing environmental data from collection locations of three North American Drosophila species—D. americana americana, D. americana texana and D. novamexicana—we identified two primary axes of macroecological variation; these differentiated species habitats and were strongly loaded for precipitation and moisture variables. In nine focal populations (three per species) assayed for each trait, we detected significant species-level variation for both desiccation resistance and pigmentation, but not for UV resistance. Species-level trait variation was consistent with differential natural selection imposed by variation in habitat water availability, although patterns of variation differed between desiccation resistance and pigmentation, and we found little evidence for pleiotropy between traits.ConclusionsOur multi-faceted approach enabled us to identify potential agents of natural selection and examine how they might influence the evolution of multiple traits at different evolutionary scales. Our findings highlight that environmental factors influence functional trait variation in ways that can be complex, and point to the importance of studies that examine these relationships at both population- and species-levels.
Highlights
Disentangling the selective factors shaping adaptive trait variation is an important but challenging task
Species differed along major axes of environmental variation Our final dataset consisted of 149 latitude and longitude collection records across the United States (10 D. novamexicana, 71 D. a. americana, and 68 D. a. texana) for Desiccation Resistance Pigmentation
For PC1, D. novamexicana focal populations differed from both D. a. americana (P = 0.033) and D. a. texana (P = 0.003), the two americana subspecies populations did not differ (P = 0.070); for PC2, D. novamexicana differed from D. a. americana (P = 0.045), but the other two contrasts (D.nov – D. tex: P = 0.093; D.am – D.tex: P = 0.840) were not significant
Summary
Disentangling the selective factors shaping adaptive trait variation is an important but challenging task. Latitudinal or altitudinal clines have received particular attention in numerous systems, including Drosophila, Arabidopsis thaliana, and humans, where analyses indicate strong trait-environment associations for several physiological and other variants [1, 17, 18]. Even among these well-characterized examples, the underlying cause of clinal variation is still not always clear, when trait variation is surveyed across generalized geographic space as opposed to specific environmental gradients. Using environmental data from GIS-based databases, these approaches quantify the direction and magnitude of bioclimatic variation across species ranges Investigating how these macroecological factors co-vary with trait
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