Borgstede Matthias
University of Bamberg Bamberg Germany.
Ecol Evol. 2021 Aug 17;11(18):12676-12685. doi: 10.1002/ece3.8012. eCollection 2021 Sep.
The Trivers-Willard hypothesis (TWH) states that parents in good condition tend to bias their offspring sex ratio toward the sex with a higher variation in reproductive value, whereas parents in bad condition favor the opposite sex. Although the TWH has been generalized to predict various Trivers-Willard effects (TWE) depending on the life cycle of a species, existing work does not sufficiently acknowledge that sex-specific reproductive values depend on the relative abundances of males and females in the population. If parents adjust their offspring sex ratio according to the TWE, offspring reproductive values will also change. This should affect the long-term evolutionary dynamics and might lead to considerable deviations from the original predictions. In this paper, I model the full evolutionary dynamics of the TWE, using a published two-sex integral projection model for the Columbian ground squirrel (). Offspring sex ratio is treated as a nonparametric continuous function of maternal condition. Evolutionary change is treated as the successive invasion of mutant strategies. The simulation is performed with varying starting conditions until an evolutionarily stable strategy (ESS) is reached. The results show that the magnitude of the evolving TWE can be far greater than previously predicted. Furthermore, evolutionary dynamics show considerable nonlinearities before settling at an ESS. The nonlinear effects depend on the starting conditions and indicate that evolutionary change is fastest when starting at an extremely biased sex ratio and that evolutionary change is weaker for parents of high condition. The results show neither a tendency to maximize average population fitness nor to minimize the deviation between offspring sex ratio and offspring reproductive value ratio. The study highlights the importance of dynamic feedback in models of natural selection and provides a new methodological framework for analyzing the evolution of continuous strategies in structured populations.
特里弗斯-威拉德假说（TWH）指出，状况良好的父母往往会使后代的性别比例偏向于生殖价值变化较大的性别，而状况不佳的父母则偏爱相反的性别。尽管TWH已被推广用于预测取决于物种生命周期的各种特里弗斯-威拉德效应（TWE），但现有研究并未充分认识到特定性别的生殖价值取决于种群中雄性和雌性的相对丰度。如果父母根据TWE调整后代的性别比例，后代的生殖价值也会发生变化。这应该会影响长期的进化动态，并可能导致与原始预测产生相当大的偏差。在本文中，我使用已发表的哥伦比亚地松鼠两性积分投影模型，对TWE的完整进化动态进行建模。后代性别比例被视为母体状况的非参数连续函数。进化变化被视为突变策略的连续入侵。在不同的初始条件下进行模拟，直到达到进化稳定策略（ESS）。结果表明，不断演变的TWE的幅度可能远大于先前的预测。此外，进化动态在达到ESS之前表现出相当大的非线性。非线性效应取决于初始条件，表明从极端偏向的性别比例开始时进化变化最快，而对于状况良好的父母来说进化变化较弱。结果既没有显示出使平均种群适应性最大化的趋势，也没有显示出使后代性别比例与后代生殖价值比例之间的偏差最小化的趋势。这项研究突出了动态反馈在自然选择模型中的重要性，并为分析结构化种群中连续策略的进化提供了一个新的方法框架。
