Data from: Does variation in the intensity and duration of predation drive evolutionary changes in senescence?

1. The evolutionary theory of senescence predicts that increased rates of extrinsic mortality select for faster declines in fertility and survival with age. One predicted mechanism is that increased mortality favors alleles that enhance fitness early in life at the expense of survival or reproduction later in life (antagonistic pleiotropy). 2. We tested these predictions in natural populations of Daphnia ambigua from lakes that vary in the severity and duration of fish predation. Daphnia are found in lakes with: (1) anadromous alewife (Alosa pseudoharengus) that migrate between marine and freshwater, (2) permanent landlocked alewife, and (3) no alewife. Daphnia are rare year-round in ‘landlocked lakes’ and are seasonally eliminated from the water column in ‘anadromous lakes’ due to the very strong predatory impact of anadromous alewife on populations of zooplankton. Previous work has also shown that intense seasonal bouts of predation by anadromous alewife has selected for increased allocation towards growth and reproduction in Daphnia found in lakes with anadromous alewife. Such variation in the risk of mortality and the expression of life history traits early in life provides the raw materials to test the evolutionary theory of ageing. 3. We reared replicate populations of Daphnia from all three lake types and quantified lifetime rates of offspring production and intrinsic survival. We found no differences in age-related declines in fertility or survival. Daphnia from anadromous lakes produced significantly more offspring throughout their lifetime despite no differences in lifespan or in the number of reproductive bouts compared with Daphnia from lakes with landlocked and no alewife. 4. The lack of divergence in ageing contradicts the prediction that elevated mortality rates drive evolutionary shifts in ageing. We reconcile these results by considering the predictions of theoretical frameworks that incorporate feedbacks associated with increased mortality such as density- and condition-dependent processes. Our results, which are better explained by a consideration of these processes, thus call for a greater consideration of models that more explicitly consider the ecology of focal organisms.

1. 衰老进化理论预测，外源性死亡率（extrinsic mortality）的升高会驱动自然选择，使得种群随年龄增长的生育力与存活率衰退速率加快。其中一种被提出的作用机制为：死亡率升高会青睐那些在生命早期提升个体适合度，却以生命后期的存活或繁殖为代价的等位基因，即拮抗多效性（antagonistic pleiotropy）。
2. 我们在鱼类捕食压力严重程度与持续时长存在差异的湖泊的自然种群中，检验了上述预测，研究对象为模糊水蚤（Daphnia ambigua）。该水蚤的栖息湖泊可分为三类：(1) 存在溯河洄游西鲱（anadromous alewife，Alosa pseudoharengus）的湖泊——该物种可在海洋与淡水环境间洄游；(2) 存在永久陆封西鲱的湖泊；(3) 无西鲱的湖泊。在‘陆封湖泊’中，模糊水蚤全年种群密度均处于较低水平；而在‘溯河洄游湖泊’中，由于溯河洄游西鲱对浮游动物（zooplankton）种群具有极强的捕食作用，水蚤种群会在每年的特定季节从水体中被完全清除。既往研究已证实，溯河洄游西鲱带来的季节性高强度捕食压力，会选择出在生命早期更多分配资源用于生长与繁殖的模糊水蚤类群。这类死亡率风险与早期生活史特征表达的差异，为检验衰老进化理论提供了充足的实验材料。
3. 我们对来自上述三类湖泊的模糊水蚤建立了重复种群并进行饲养，定量测定了其终生子代生产速率与内在存活率。结果显示，两类水蚤的生育力与存活率的年龄相关性衰退并无显著差异。不过，与来自陆封西鲱湖泊和无西鲱湖泊的水蚤相比，溯河洄游湖泊中的水蚤在整个生命周期内的子代总产量显著更高，但其寿命与繁殖周期次数并无明显区别。
4. 衰老相关性状未出现分化这一结果，与‘死亡率升高会推动衰老的进化改变’的预测相悖。我们通过纳入与死亡率升高相关的反馈过程（如密度依赖与条件依赖过程）的理论框架，对该结果进行了整合阐释。我们的研究结果更符合这类纳入生态过程的模型的预测，因此呼吁学界更多关注那些更明确纳入焦点生物生态学特征的进化模型。