No abstract

Parasites impose fitness costs on their hosts. Biologists therefore tend to assume that natural selection favors infection-resistant hosts. Yet, when the immune response itself is costly, theory suggests selection may instead favor loss of resistance. Immune costs are rarely documented in nature, and there are few examples of adaptive loss of resistance. Here, we show that when marine threespine stickleback colonized freshwater lakes they gained resistance to the freshwater-associated tapeworm, Schistocephalus solidus. Extensive peritoneal fibrosis and inflammation contribute to suppression of cestode growth and viability, but also impose a substantial cost of reduced fecundity. Combining genetic mapping and population genomics, we find that the immune differences between tolerant and resistant populations arise from opposing selection in both populations acting, respectively, to reduce and increase resistance consistent with divergent optimization.One Sentence SummaryRecently-evolved freshwater populations of stickleback frequently evolve increased resistance to tapeworms, involving extensive fibrosis that suppresses parasite growth; because this fibrosis greatly reduces fish fecundity, in some freshwater populations selection has favored an infection-tolerant strategy with fibrosis suppression.

word count: 220 Total word count: 3862 Abstract 1Sex differences in selection arise for two possible reasons: 1) differences originating from 2 anisogamy -the Darwin-Bateman paradigm -and 2) competition-driven ecological character 3 displacement (ECD), agnostic of anisogamy. Despite mounting evidence of ECD and increasing 4 focus on the ecological causes and consequences of sexual dimorphism, progress in 5 understanding the evolution of ecological sex differences has likely been hindered because 6 ecological dimorphisms are not exclusive to ECD. I argue that embracing non-exclusivity of 7 causal models of sexual dimorphism itself provides insights into evolution of sex differences. 8This integrated view of the evolution of sexual dimorphism leads to four predictions for how 9 sex-specific selection and phenotypic divergence between the sexes change over the course of 10

Intraspecific variation can have important consequences for the structure and function of ecological communities, and serves to link community ecology to evolutionary processes. Differences between the sexes are an overwhelmingly common form of intraspecific variation, but its community-level consequences have never been experimentally investigated. Here, we manipulate the sex ratio of a sexually dimorphic predacious newt in aquatic mesocosms, then track their impact on prey communities. Female and male newts preferentially forage in the benthic and pelagic zones, respectively, causing corresponding reductions in prey abundances in those habitats. Sex ratio differences also explained a large proportion (33%) of differences in the composition of entire pond communities. Ultimately, we demonstrate the impact of known patterns of sexual dimorphism in a predator on its prey, uncovering overlooked links between evolutionary adaptation and the structure of contemporary communities. Given the extreme prevalence of sexual dimorphism, we argue that the independent evolution of the sexes will often have important consequences for ecological communities.