Abstract

Measuring individual reproductive success in the wild is often achieved by counting the number of descendants produced by individuals. In seeking to understand how reproductive success can inform us about natural selection, however, we are faced with a conundrum. In terms of timing, what is the most relevant measure for examining selection? We might count the number of offspring born, surviving to the termination of parental care, surviving to adulthood, or only those surviving to themselves reproduce. Clearly, only the latter are passing on genes and traits to future generations, but this estimate may not always be available. So, are different estimates of fitness consistent? Do they provide us with similar inferences of selection on phenotypic traits? We examined these questions on a 29-year long-term study of individually monitored male and female Columbian ground squirrels (Urocitellus columbianus). We used the long-term data to calculate male and female fitness based on an annual measure of adult survival and the yearly production of offspring counted at the stages of birth, weaning, and yearling age. We then decomposed fitness into its constitutive elements including (1) adult survival to the next spring, and (2) the yearly production of offspring counted at the stages previously mentioned. We then compared fitness metrics to evaluate if they provided similar or contrasting information in the wild. Next, we used those fitness metrics to test for selection on the date of emergence from annual hibernation, a phenotypic trait previously shown to be highly variable, heritable, and associated with reproduction. Finally, we partitioned selection on emergence date into additive episodes of selection by looking at how selection changed from reproduction measured at birth, weaning, and when offspring reached yearling age. Overall, fitness metrics were well correlated, but correlations weakened the further offspring were counted from birth. We generally found directional selection for earlier emergence dates both in males and females. The strength of selection depended on which fitness metric was used. Most of the selection gradient on emergence date was explained by offspring born, and the selection differential was stronger in males than females. We evaluate how the choice of fitness metrics in life-history studies may nuance our inferences about natural selection.

Significance statement

This study explores how our inferences about natural selection acting on organismal traits vary depending on our choice of fitness metrics. Focusing on the timing of emergence from hibernation in Columbian ground squirrels, we show that directional selection for earlier emergence dates occurs, but the strength of selection depends on whether fitness is evaluated from offspring counted at birth, at weaning, or later in life. These results show that the choice of timing for fitness measurements may nuance inferences about natural selection in life-history studies.

中文翻译：

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从长期实地研究中测量适合度和推断自然选择：不同的测量方法得出细微的结论

摘要

衡量个体在野外的繁殖成功率通常是通过计算个体产生的后代数量来实现的。然而，在试图了解繁殖成功如何让我们了解自然选择时，我们面临着一个难题。在时间方面，检查选择最相关的措施是什么？我们可以计算出生的后代数量，存活到父母照顾终止，存活到成年，或者只有那些存活到自己繁殖的后代。显然，只有后者将基因和性状传递给后代，但这种估计可能并不总是可用。所以，不同的健康估计是否一致？它们是否为我们提供了关于表型性状选择的类似推论？我们在一项针对单独监测的雄性和雌性哥伦比亚地松鼠的 29 年长期研究中检查了这些问题（哥伦比亚尿虫）。我们使用长期数据来计算男性和女性的健康状况，这些数据基于成年存活的年度衡量标准以及在出生、断奶和一岁年龄阶段计算的后代的年产量。然后，我们将适应度分解为其构成要素，包括（1）成虫存活到下一个春天，以及（2）在前面提到的阶段计算的后代的年产量。然后，我们比较了健身指标，以评估它们是否在野外提供了相似或对比的信息。接下来，我们使用这些适应度指标来测试从每年冬眠中出现的日期的选择，这是一种以前被证明具有高度可变性、可遗传性并与繁殖相关的表型特征。最后，我们通过观察选择如何从出生、断奶和后代达到一岁时测量的繁殖变化，将出现日期的选择划分为选择的附加事件。总体而言，健康指标具有良好的相关性，但相关性减弱了从出生开始计算的更多后代。我们通常发现男性和女性早期出现日期的方向选择。选择的强度取决于使用的适应度指标。出现日期的选择梯度大部分是由出生的后代解释的，雄性的选择差异比雌性强。我们评估了生活史研究中健康指标的选择如何影响我们对自然选择的推论。健康指标具有良好的相关性，但相关性减弱了从出生开始计算的更多后代。我们通常发现男性和女性早期出现日期的方向选择。选择的强度取决于使用的适应度指标。出现日期的选择梯度大部分是由出生的后代解释的，雄性的选择差异比雌性强。我们评估了生活史研究中健康指标的选择如何影响我们对自然选择的推论。健康指标具有良好的相关性，但相关性减弱了从出生开始计算的更多后代。我们通常发现男性和女性早期出现日期的方向选择。选择的强度取决于使用的适应度指标。出现日期的选择梯度大部分是由出生的后代解释的，雄性的选择差异比雌性强。我们评估了生活史研究中健康指标的选择如何影响我们对自然选择的推论。出现日期的选择梯度大部分是由出生的后代解释的，雄性的选择差异比雌性强。我们评估了生活史研究中健康指标的选择如何影响我们对自然选择的推论。出现日期的选择梯度大部分是由出生的后代解释的，雄性的选择差异比雌性强。我们评估了生活史研究中健康指标的选择如何影响我们对自然选择的推论。

意义陈述

本研究探讨了我们对自然选择作用于生物特征的推论如何根据我们对适应度指标的选择而变化。着眼于哥伦比亚地松鼠从冬眠中出现的时间，我们表明会发生早期出现日期的定向选择，但选择的强度取决于是否从出生时、断奶时或生命后期计算的后代中评估适应性。这些结果表明，健康测量时间的选择可能会影响生活史研究中自然选择的推论。