A fundamental question in evolutionary biology asks whether organisms experiencing similar selective pressures will evolve similar solutions or whether historical contingencies dominate the evolutionary process and yield disparate evolutionary outcomes. It is perhaps most likely that both shared selective forces as well as unique histories play key roles in the course of evolution. Consequently, when multiple species face a common environmental gradient, their patterns of divergence might exhibit both shared and unique elements. Here we describe a general framework for investigating and evaluating the relative importance of these contrasting features of diversification. We examined morphological diversification in three species of livebearing fishes across a predation gradient. All species (Gambusia affinis from the United States of America, Brachyrhaphis rhabdophora from Costa Rica, and Poecilia reticulata from Trinidad) exhibited more elongate bodies, a larger caudal peduncle, and a relatively lower position of the eye in predator populations. This shared response suggests that common selective pressures generated parallel outcomes within three different species. However, each species also exhibited unique features of divergence, which might reflect phylogenetic tendencies, chance events, or localized environmental differences. In this system, we found that shared aspects of divergence were of larger magnitude than unique elements, suggesting common natural selective forces have played a greater role than unique histories in producing the observed patterns of morphological diversification. Assessing the nature and relative importance of shared and unique responses should aid in elucidating the relative generality or peculiarity in evolutionary divergence.
Potential constraints on the evolution of phenotypic plasticity were tested using data from a previous study on predator‐induced morphology and life history in the freshwater snail Physa heterostropha. The benefit of plasticity can be reduced if facultative development is associated with energetic costs, developmental instability, or an impaired developmental range. I examined plasticity in two traits for 29 families of P. heterostropha to see if it was associated with growth rate or fecundity, within‐family phenotypic variance, or the potential to produce extreme phenotypes. Support was found for only one of the potential constraints. There was a strong negative selection gradient for growth rate associated with plasticity in shell shape (β = −0.3, P < 0.0001). This result was attributed to a genetic correlation between morphological plasticity and an antipredator behavior that restricts feeding. Thus, reduced growth associated with morphological plasticity may have had unmeasured fitness benefits. The growth reduction, therefore, is equivocal as a cost of plasticity. Using different fitness components (e.g., survival, fecundity, growth) to seek constraints on plasticity will yield different results in selection gradient analyses. Procedural and conceptual issues related to tests for costs and limits of plasticity are discussed, such as whether constraints on plasticity will be evolutionarily ephemeral and difficult to detect in nature.
We examined intraspecific morphological diversification between river channel and lagoon habitats for two Neotropical fish (Bryconops caudomaculatus, Characidae; Biotodoma wavrini, Cichlidae). We hypothesized that differences between habitats (e.g. flow regime, foraging opportunities) might create selective pressures resulting in morphological divergence between conspecific populations. We collected fish from four channel-lagoon habitat pairs in the Río Cinaruco, Venezuela, and compared body morphology using geometric morphometrics. There were two aspects of divergence in both species: (1) placement of maximum body depth and (2) orientation of the mouth. For both species, maximum body depth was positioned more anteriorly (i.e. fusiform) in the river channel than in lagoons. Both species exhibited a relatively terminal mouth in lagoons compared to the channel. The mouth of B. caudomaculatus was relatively upturned, whereas the mouth of B. wavrini was relatively subterminal, in channel habitats. Observed morphological patterns are consistent with functional morphological principles suggesting adaptive divergence. We also show that spatial distance between habitats, presumably reflecting rates of population mixing, appears to have constrained diversification. For both species, morphological divergence increased with distance between habitats. Thus morphological divergence between channel and lagoon habitats apparently reflects a balance between diversification driven by natural selection, and homogenization driven by population mixing.
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