Life history divergence in response to ecological selection: A meta-analysis in live-bearing fishes
Life history traits, such as maternal investment in offspring size and number, are predicted to evolve in response to age-specific mortality risk. Although empirical studies have tested these predictions by considering how maternal investment responds to selective agents, the relative magnitude and direction of these responses remains largely unexplored across the literature. We conducted a meta-analysis on maternal investment responses to selective agents across the well-studied family, Poeciliidae. We asked: 1) Do offspring size and offspring number respond to different selective agents as predicted by theory, and do putative selective agents differ in the magnitude of their effect? 2) What is the relationship between responses in offspring size and offspring number across selective agents? A literature search of maternal investment in Poeciliidae produced 168 studies for potential inclusion in our meta-analysis. We assessed the standardized mean differences (Hedge’s G) in maternal investment (offspring size and number) to selective agents. We predicted that: 1) mothers would increase offspring size in response to selective agents associated with high juvenile mortality (e.g. density, H2S concentration), and increase offspring number in response to selective agents associated with high adult mortality (e.g. predation); 2) size and number responses would trade-off across selective agents.
Our literature search returned 53 studies across 18 species that could be included in the meta-analyses. Females tended to increase offspring size in response to high food availability (mean effect size, lower 95 CI%, upper 95% CI: 0.570, 0.055, 1.088) and high density (0.600, -0.014, 1.228), and decrease offspring size in response to high predation (-0.875, -1.417, -0.281). Females tended to increase offspring number in response to high food availability (1.128, 0.304, 1.960) and high predation (1.236, 0.349, 2.131), and decrease offspring number in response to high H2S concentrations (-1.889, -3.227, -0.521). These results indicate that maternal investment responds to putative selective agents as predicted by theory in many, but not all, cases. Additionally, we found support for a general trade-off between offspring size and number responses (slope = -0.204, -0.340, -0.070), whose slope was also significantly different than -1. These findings suggest that increases to offspring number are not associated with decreases in offspring size of the same magnitude. Overall, our results provide among the first quantitative assessments of the general strengths of life history responses to potential selective agents.