Abstract
BackgroundMany cave-dwelling animal species display similar morphologies (troglomorphism) that have evolved convergent within and among lineages under the similar selective pressures imposed by cave habitats. Here we study such ecomorphological evolution in cave-dwelling Sclerobuninae harvestmen (Opiliones) from the western United States, providing general insights into morphological homoplasy, rates of morphological change, and the temporal context of cave evolution.Methodology/Principal FindingsWe gathered DNA sequence data from three independent gene regions, and combined these data with Bayesian hypothesis testing, morphometrics analysis, study of penis morphology, and relaxed molecular clock analyses. Using multivariate morphometric analysis, we find that phylogenetically unrelated taxa have convergently evolved troglomorphism; alternative phylogenetic hypotheses involving less morphological convergence are not supported by Bayesian hypothesis testing. In one instance, this morphology is found in specimens from a high-elevation stony debris habitat, suggesting that troglomorphism can evolve in non-cave habitats. We discovered a strong positive relationship between troglomorphy index and relative divergence time, making it possible to predict taxon age from morphology. Most of our time estimates for the origin of highly-troglomorphic cave forms predate the Pleistocene.Conclusions/SignificanceWhile several regions in the eastern and central United States are well-known hotspots for cave evolution, few modern phylogenetic studies have addressed the evolution of cave-obligate species in the western United States. Our integrative studies reveal the recurrent evolution of troglomorphism in a perhaps unexpected geographic region, at surprisingly deep time depths, and in sometimes surprising habitats. Because some newly discovered troglomorphic populations represent undescribed species, our findings stress the need for further biological exploration, integrative systematic research, and conservation efforts in western US cave habitats.
Highlights
Cave habitats have long interested evolutionary biologists and ecologists – such habitats combine geographic isolation, promoting speciation and endemicity, with selective similarities, promoting convergence in life history and morphological form [1]
An example is the study of ecomorphological evolution, where ecomorphs are defined as ecological forms characterized by suites of phenotypic traits that have some conspicuous correlation with ecological attributes [7]
Taxon Sampling Most of the specimens used in this study were collected in recent fieldwork conducted by the authors - when collecting a series of individuals, some were preserved in 80% EtOH for morphological analysis, whereas specimens destined for molecular analysis were preserved in 100% EtOH
Summary
Cave habitats have long interested evolutionary biologists and ecologists – such habitats combine geographic isolation, promoting speciation and endemicity, with selective similarities, promoting convergence in life history and morphological form [1]. This combination of divergence and selective constraints leads to evolutionary trends that are predictable and often replicated independently within and among animal lineages, making cave organisms excellent candidates for comparative evolutionary analyses [2,3,4,5,6]. We study such ecomorphological evolution in cave-dwelling Sclerobuninae harvestmen (Opiliones) from the western United States, providing general insights into morphological homoplasy, rates of morphological change, and the temporal context of cave evolution
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