![]() Because this interdigital webbing has evolved repeatedly in the group, it was long hypothesized that this derived trait was an adaptation to their arboreal habit. For example, many tropical plethodontid salamanders are arboreal, and as adults, have extensive webbing on their hands and feet. In some instances, the mechanisms responsible for the evolution of phenotypic traits commonly considered to be adaptive are more intricate than was previously believed. #216 permute 3 fullSuch structural mechanisms commonly interact with selection to shape the course of evolution, and while phenotypic traits are often portrayed as being the result of either selection or constraints, a full appreciation of the evolutionary process requires understanding the contributions of both components. Likewise, patterns of genetic covariance and underlying developmental pathways can alter both the direction and extent of morphological change, and influence the degree to which selection can operate. For example, functional and biomechanical requirements can restrict the evolutionary response to selection, particularly when competing functional demands on the same trait cannot be simultaneously optimized. While phenotypic variation is often assumed to be adaptive and molded by natural selection, several additional mechanisms play important roles in influencing and constraining morphological change. These examples, and many others, provide strong evidence of adaptation, and speak to the power of selection in directing morphological change. Additionally, common selective pressures found in distinct locations can generate similar patterns of morphological evolution among unrelated groups, resulting in evolutionary convergence or parallelism. For instance, divergent selection often generates discontinuities among populations and species, enhancing adaptive differences through time. How can we explain the extent of phenotypic diversity observed in nature? Since Darwin, a myriad of studies have proposed adaptive explanations for phenotypic variation, and have focused on the role of natural and sexual selection in shaping patterns of diversification. Our findings underscore the importance of examining morphological adaptations from multiple perspectives, and emphasize that both selective hypotheses and developmental processes must be considered for a more comprehensive understanding of phenotypic evolution. Thus, multiple developmental solutions exist to the same evolutionary challenge. ![]() However, developmental trajectories arriving at this common morphology are distinct with some species displaying developmental stasis (isometry), while others show an increase in foot webbing during growth. The common adult phenotype exhibited among species reveals that selection plays an important part in generating patterns of foot diversity in the group. The results presented here provide evidence of a complex history of phenotypic evolution in this clade. Finally, interspecific variation in adult foot morphology was significantly reduced as compared to variation among juveniles, indicating that ontogenetic convergence had resulted in a common adult foot morphology across species. Allometric parameters of foot growth were most similar to those of a tropical species previously shown to be adapted for climbing. Phylogenetic analyses revealed that multiple evolutionary transitions between isometry and allometry of foot webbing have occurred in this lineage. By contrast, three species exhibited significant allometry, with an increase in interdigital webbing during growth. Using geometric morphometrics and other approaches, we found that developmental patterns in five species displayed little morphological change during growth (isometry), where the extensive interdigital webbing in adults was best explained as the retention of the juvenile morphological state. We investigated ontogenetic trajectories of foot morphology in the eight species of European plethodontid cave salamander to test the hypothesis that adult foot morphology was adapted for climbing. For many traits, the relative importance of selective and developmental components remains understudied. Both natural and sexual selection play a large role in generating phenotypic adaptations, with biomechanical requirements and developmental mechanisms mediating patterns of phenotypic evolution. A major goal in evolutionary biology is to understand the evolution of phenotypic diversity. ![]()
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