Abstract
BackgroundA major goal in evolutionary biology is to understand the evolution of phenotypic diversity. Both natural and sexual selection play a large role in generating phenotypic adaptations, with biomechanical requirements and developmental mechanisms mediating patterns of phenotypic evolution. For many traits, the relative importance of selective and developmental components remains understudied.ResultsWe 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. 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. By contrast, three species exhibited significant allometry, with an increase in interdigital webbing during growth. Phylogenetic analyses revealed that multiple evolutionary transitions between isometry and allometry of foot webbing have occurred in this lineage. Allometric parameters of foot growth were most similar to those of a tropical species previously shown to be adapted for climbing. 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.ConclusionsThe results presented here provide evidence of a complex history of phenotypic evolution in this clade. The common adult phenotype exhibited among species reveals that selection plays an important part in generating patterns of foot diversity in the group. 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. Thus, multiple developmental solutions exist to the same evolutionary challenge. 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.
Highlights
A major goal in evolutionary biology is to understand the evolution of phenotypic diversity
Foot morphology was characterized using a variety of measures (Figure 1B), including sinuosity, and foot shape derived from a set of nine landmarks and geometric morphometric methods [39,40,41]
An analysis of covariance (ANCOVA) revealed that developmental trajectories differed among species (Table A1A), with most species displaying no change in foot webbing through ontogeny (Table 2)
Summary
A major goal in evolutionary biology is to understand the evolution of phenotypic diversity. Patterns of genetic covariance [18,19,20] and underlying developmental pathways [21,22] can alter both the direction and extent of morphological change, and influence the degree to which selection can operate Such 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 [23]. A single webbed species (the cave-dwelling Chiropterotriton magnipes) had a growth trajectory where foot area increased relative to body size in a manner consistent with adaptation This example demonstrates the importance of considering both selective and structural mechanisms when assessing patterns of morphological change
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