Sanghvi Krish, Henshaw Jonathan M, Kacelnik Alex, Janicke Tim, Sepil Irem
Department of Biology, University of Oxford, Oxford, United Kingdom.
Institute of Biology, University of Freiburg, Freiburg, Germany.
Evolution. 2025 Oct 6;79(9):1937-1953. doi: 10.1093/evolut/qpaf127.
The Bateman gradient is a fundamental metric of sexual selection, often interpreted as the fitness advantage individuals gain by increasing their number of mates. However, it is recognized that any traits influencing both mating and reproductive success can confound the gradient, misrepresenting the strength of precopulatory sexual selection. Yet, the magnitude of this misrepresentation across different biological systems (e.g., differing in anisogamy or strength of sperm competition), which covariates are most problematic, or how confounded relationships can be diagnosed to better interpret the Bateman gradient, remains largely unexplored. To address these gaps, we simulate 9 plausible biological scenarios where the effect of male mating success on reproductive success is confounded. We find that covariances between male mating success and female fecundity or egg allocation confound male Bateman gradients more strongly than covariances between male mating success and ejaculate traits. These differences in the impact of male-female vs. male-male covariances are especially pronounced in systems with high levels of anisogamy and no sperm competition. We provide guidelines for empiricists to visually identify such covariances by recording mating order, and suggest that researchers explicitly state causal assumptions when interpreting Bateman gradients. Additionally, when the covariate is a confounder, not a mediator, we demonstrate that partial Bateman gradients allow better interpretation of the strength of precopulatory sexual selection. These insights into the mechanisms driving variation in the Bateman gradient allow us to clarify its meaning, identify scenarios where its interpretation might be problematic, and offer practical solutions to address this.
贝特曼梯度是性选择的一个基本指标，通常被解释为个体通过增加配偶数量而获得的适应性优势。然而，人们认识到，任何影响交配和繁殖成功的性状都可能混淆该梯度，从而错误地呈现交配前性选择的强度。然而，这种错误呈现在不同生物系统（例如，在配子大小异型或精子竞争强度方面存在差异）中的程度、哪些协变量最成问题，或者如何诊断混淆关系以更好地解释贝特曼梯度，在很大程度上仍未得到探索。为了填补这些空白，我们模拟了9种合理的生物学情景，其中雄性交配成功对繁殖成功的影响被混淆了。我们发现，雄性交配成功与雌性繁殖力或卵子分配之间的协方差比雄性交配成功与射精性状之间的协方差更强烈地混淆了雄性贝特曼梯度。在配子大小异型程度高且不存在精子竞争的系统中，雌雄协方差与雄雄协方差影响的这些差异尤为明显。我们为实证研究人员提供了通过记录交配顺序直观识别此类协方差的指导方针，并建议研究人员在解释贝特曼梯度时明确说明因果假设。此外，当协变量是一个混杂因素而非中介变量时，我们证明部分贝特曼梯度能够更好地解释交配前性选择的强度。这些对驱动贝特曼梯度变化机制的见解使我们能够阐明其含义，识别其解释可能存在问题的情景，并提供实际解决方案来解决这一问题。
