How Males Shape Up

Understanding how environmental forces, genetic variation, and developmental process combine to mold adaptations remains a core challenge in evolutionary biology. Our research is answering this challenge. More specifically we focused on how these three drivers of adaptation have shaped the evolution of body morphology in poeciliid fish. The body morphology of poeciliid fish has been shown to be under strong ecological selection and sexual selection. In the poecilid fish Poecilia reticulata, body morphology can evolve rapidly, over a span of four to five years. These attributes makes it an ideal system to study the processes leading to adaptive evolution. Here we studied this adaptive evolution in poeciliid fish at two scales: among populations of a single species and within populations of a single species. At the broadest scale, we investigated local adaptation among populations of a single poeciliid species, Heterandria formosa. We quantified patterns in morphological variation among populations and tested for associations between this variation and ecological data, which are derived from long term population censuses. Results from this study illustrate the complicated construction of multivariate phenotypic variation and suggest that different agents of selection have acted on different components of body morphology. These patterns in inter-population phenotypic variation can be evidence of local adaptation; however, they can also be reflective of patterns in phenotypic plasticity induced by environmental or maternal effects. The role of maternal effects are especially relevant in H. formosa as females are live bearing and provide nutrients to developing embryos via a placenta. We used a common garden experiment and a large factorial breeding experiment to explicitly test for genetically based differences among populations in their responses to environmental variation (norms of reaction). This laboratory work allowed a definitive diagnosis of which features actually represented local adaptations among populations of Heterandria formosa. Results showed that male body morphology has a significant genetic component and signs of population specific response to both the environment during post-parturition development and in response to the maternal environment during embyronic development. The narrowest scope of our work focuses on the evolution of body morphology within an experimental population of the Trinidadian guppy. Interestingly, the strength and direction of selection on phenotypic variation is not the same among all individuals within a population. In particular, genetic correlations between the sexes can produce intralocus sexual conflicts (ISC) when selection favors different trait values in the genders. This form of sexual conflict can inhibit the evolution of males and females so that neither sex can reach its optimal trait value. Theory suggests that ISC will have a minimal effect in populations off there adaptive peak such as those in a novel or variable environment. However, ISC is likely to have an inhibitory effect in populations in a stable environment near their adaptive, thereby limiting adaptive evolution. Here we used an experimental population of Trinidadian guppies in a novel environment to unravel the emergence of sexual conflict as populations adapt to novel environments. Guppies from a high predation environment were translocated to a drainage with minimal predation. The phenotypic evolution of the population was tracked monthly. This work incorporates mark-recapture methods, complete pedigree reconstruction using high throughput sequencing, and geometric morphometric shape analysis. Results indicate that there are fitness differences among individuals and different optimal values in males and females but the role of sexual conflict within this population is currently negligible. To conclude, our work investigated the environmental and genetic factors influencing the evolution of body morphology in poeciliid fish. We demonstrate the complicated nature of selection with different selective agents acting on different aspects of body morphology. We found that populations of fish have unique environmental maternal effects which may ultimately be adaptive. We also conclude that while there are optimal trait value differences in males and female guppies but being off the adaptive peak limits the influence of ISC on morphological evolution.