Phenotypic differentiation in a heterogeneous environment: morphological and life-history responses to ecological gradients in a livebearing fish

Predicting how environmental variation drives phenotypic diversification is one of the main aims of evolutionary ecology. Yet, we still only have a limited understanding of how it drives diversity, especially when multiple factors interact. To address this issue, the superfetating livebearing fish Phalloptychus januarius (Poeciliidae) was repeatedly sampled (over a 2-year period) in four coastal lagoons in Brazil to investigate the relative contribution of different environmental factors on phenotypic patterns. We further compare our results to those reported for another poeciliid (Poecilia vivipara), which inhabits some of the same lagoons but has a drastically different reproductive strategy (no superfetation). We used a model-averaging approach to estimate the relative importance of differences in predation pressure, salinity, dissolved oxygen, pH, temperature and food availability in driving variation in body shape, life histories, and life-history proxies. We found consistent population differences in several traits, but also seasonal variation within each lagoon. Specifically, predation, oxygen availability and pH affected several different traits and played important roles in driving phenotypic differences between and amongst populations. Moreover, our study reveals differential responses in phenotypic traits to the same environmental gradients between P. januarius (this study) and P. vivipara (previous studies), and we suggest that future work should further inves- tigate differential phenotypic responses to single versus multiple concomitant selective forces, and how this affects different species.