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Multiple paternity (MP) increases offspring's genetic variability, which could be linked to invasive species' evolvability in novel distribution ranges. Shifts in MP can be adaptive, with greater MP in harsher/colder environments or towards the end of the reproductive season, but climate could also affect MP indirectly via its effect on reproductive life histories. We tested these hypotheses by genotyping N = 2,903 offspring from N = 306 broods of two closely related livebearing fishes, Gambusia holbrooki and Gambusia affinis. We sampled pregnant females across latitudinal gradients in their invasive ranges in Europe and China, and found more sires per brood and a greater reproductive skew towards northern sampling sites. Moreover, examining monthly sampling from two G. affinis populations, we found MP rates to vary across the reproductive season in a northern Chinese, but not in a southern Chinese population. While our results confirm an increase of MP in harsher/more unpredictable environments, path analysis indicated that, in both cases, the effects of climate are likely to be indirect, mediated by altered life histories. In both species, which rank amongst the 100 most invasive species worldwide, higher MP at the northern edge of their distribution probably increases their invasive potential and favours range expansions, especially in light of the predicted temperature increases due to global climate changes.

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 investigate differential phenotypic responses to single versus multiple concomitant selective forces, and how this affects different species.