We test for direct and social selection on male body size using a 10-year dataset on black-throated blue warblers (Setophaga caerulescens), a territorial species for which body size is hypothesized to mediate competition for mates. We apply a new sexual network framework to quantify each male's social environment as the mean body size of his primary competitors. In this study, we test whether variation in the social environment affects selection on individual phenotype. However, accounting for differences in selective pressures, such as variation in the social environment, can advance our understanding of how selection shapes individual traits and subsequent fitness.
Our understanding of trait evolution is built upon studies that examine the correlation between traits and fitness, most of which implicitly assume all individuals experience similar selective environments. Our study suggests that transgenerational effects are absent in this species, because confidence intervals exclude all biologically relevant effect sizes. This study shows that evolution has led to a remarkable robustness of zebra finches against undernourishment. In contrast, we found no indirect effects of parental or grandparental condition (r = ‐0.017‐0.002 meta‐analytic summary of 138 effect sizes), and mixed evidence for small benefits of matching environments between parents and offspring, as the latter was not robust to confirmatory testing in independent data sets. We find that drastic differences in early growth conditions (nestling body mass 8 days after hatching varied 7‐fold between 1.7 and 12.4 gram) had only moderate direct effects on adult morphology (95%CI: r = 0.19‐0.27) and small direct effects on adult fitness traits (r = 0.02‐0.12). We combine clear one‐tailed hypotheses with steps of validation, meta‐analytic summary of mean effect sizes, and independent confirmatory testing. To assess the strength of such transgenerational effects we propose a strategy aimed at overcoming the problem of type I errors when testing multiple proxies of stress in multiple ancestors against multiple offspring performance traits, and we apply it to a large observational data set on captive zebra finches (Taeniopygia guttata ). However, many studies report negative effects of stress experienced by one generation on the performance of subsequent generations. Evolution should render individuals resistant to stress and particularly to stress experienced by ancestors.
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