Abstract
Natural selection often results in profound differences in body shape among populations from divergent selective environments. Predation is a well-studied driver of divergence, with predators having a strong effect on the evolution of prey body shape, especially for traits related to escape behavior. Comparative studies, both at the population level and between species, show that the presence or absence of predators can alter prey morphology. Although this pattern is well documented in various species or population pairs, few studies have tested for similar patterns of body shape evolution at multiple stages of divergence within a taxonomic group. Here, we examine morphological divergence associated with predation environment in the livebearing fish genus Brachyrhaphis. We compare differences in body shape between populations of B. rhabdophora from different predation environments to differences in body shape between B. roseni and B. terrabensis (sister species) from predator and predator free habitats, respectively. We found that in each lineage, shape differed between predation environments, consistent with the hypothesis that locomotor function is optimized for either steady swimming (predator free) or escape behavior (predator). Although differences in body shape were greatest between B. roseni and B. terrabensis, we found that much of the total morphological diversification between these species had already been achieved within B. rhabdophora (29% in females and 47% in males). Interestingly, at both levels of divergence we found that early in ontogenetic development, females differed in shape between predation environments; however, as females matured, their body shapes converged on a similar phenotype, likely due to the constraints of pregnancy. Finally, we found that body shape varies with body size in a similar way, regardless of predation environment, in each lineage. Our findings are important because they provide evidence that the same source of selection can drive similar phenotypic divergence independently at multiple divergence levels.
Highlights
Numerous studies have documented adaptation to divergent natural selection regimes [1,2,3,4,5,6,7,8]
Effects of Predation Environment on Body Shape Consistent with the predictions in our first hypothesis, we found that body shape differed between predation environments both within Brachyrhaphis rhabdophora and between B. roseni and B. terrabensis
The phenotypic change vector analysis (PCVA) revealed that the magnitude of shape variation was greater in the B. roseni/B. terrabensis species group (MD = 0.0348; P = 0.001); the trajectories differed in orientation (h = 80.14u; P = 0.001)
Summary
Numerous studies have documented adaptation to divergent natural selection regimes [1,2,3,4,5,6,7,8]. Few studies have looked at the evolution of adaptive strategies across a speciation continuum (i.e., both within and between species) with the intent of determining how much diversification takes place across different stages of speciation [9,10,11] The paucity of such studies may be due to the difficulty of identifying systems where divergent selection regimes have driven or are driving divergence at multiple taxonomic levels. These studies are valuable to our understanding of evolutionary diversification, and can help explain how predictable phenotypic divergence is when populations or species are subject to similar selective environments. Comparative studies of taxa from different ‘predation environments,’ both between populations within species and between species pairs, show a strong link between the presence of predators and overall prey morphology [13,20,31,32,33,34,35,36]
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