Life-history traits are directly linked to fitness, and therefore, can be highly adaptive. Livebearers have been used as models for understanding the evolution of life histories due to their wide diversity in these traits. Several different selective pressures, including population density, predation, and resource levels, can shape life-history traits. However, these selective pressures are usually considered independently in livebearers and we lack a clear understanding of how they interact in shaping life-history evolution. Furthermore, selective pressures such as interspecific competition are rarely considered as drivers of life-history evolution in poeciliids. Here we test the simultaneous effects of several potential selective pressures on life-history traits in the livebearing fish Poeciliopsis prolifica. We employ a multi-model inference approach. We focus on four known agents of selection: resource availability, stream velocity, population density, and interspecific competition, and their effect on four life-history traits: reproductive allocation, superfetation, number of embryos, and individual embryo size. We found that models with population density and interspecific competition alone were strongly supported in our data and, hence, indicated that these two factors are the most important selective agents for most life-history traits, except for embryo size. When population density and interspecific competition increase there is an increase in each of the three life-history traits (reproductive allocation, superfetation, and number of embryos). For individual embryo size, we found that all single-agent models were equivalent and it was unclear which selective agent best explained variation. We also found that models that included population density and interspecific competition as direct effects were better supported than those that included them as indirect effects through their influence on resource availability. Our study underscores the importance of interspecific competitive interactions on shaping life-history traits and suggests that these interactions should be considered in future life-history studies.

中文翻译：

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是什么驱动了活生鱼 Poeciliopsis profilifica 的生活史变化？多个推定选择代理的评估

生活史特征与健康直接相关，因此具有高度适应性。由于这些特征的广泛多样性，活体动物已被用作了解生活史演变的模型。几种不同的选择压力，包括人口密度、捕食和资源水平，可以塑造生活史特征。然而，这些选择压力通常在生活承载者中被独立考虑，我们对它们如何相互作用在塑造生活史进化方面缺乏清晰的了解。此外，诸如种间竞争之类的选择压力很少被认为是 poeciliids 生活史进化的驱动因素。在这里，我们测试了几种潜在选择压力对活鱼 Poeciliopsis prolifica 生活史特征的同时影响。我们采用多模型推理方法。我们关注四种已知的选择因素：资源可用性、流速、种群密度和种间竞争，以及它们对四种生活史特征的影响：生殖分配、超胎、胚胎数量和个体胚胎大小。我们发现，我们的数据强烈支持仅具有种群密度和种间竞争的模型，因此，这两个因素是除胚胎大小外的大多数生活史特征的最重要选择因子。当种群密度和种间竞争增加时，三个生活史特征（生殖分配、超胎和胚胎数量）中的每一个都会增加。对于个体胚胎大小，我们发现所有单代理模型都是等效的，目前还不清楚哪种选择代理最能解释变异。我们还发现，通过对资源可用性的影响，将人口密度和种间竞争作为直接影响的模型比将它们作为间接影响的模型得到更好的支持。我们的研究强调了种间竞争相互作用对塑造生活史特征的重要性，并建议在未来的生活史研究中应考虑这些相互作用。