Data from: Extra-pair paternity and the variance in male fitness in song sparrows (Melospiza melodia)

The variance in fitness across population members can influence major evolutionary processes. In socially monogamous but genetically polygynandrous species, extra-pair paternity (EPP) is widely hypothesized to increase the variance in male fitness compared to that arising given the socially monogamous mating system. This hypothesis has not been definitively tested because comprehensive data describing males’ apparent (social) and realized (genetic) fitness have been lacking. We used 16 years of comprehensive social and genetic paternity data for an entire free-living song sparrow (Melospiza melodia) population to quantify and compare variances in male apparent and realized fitness, and to quantify the contribution of the variances in within-pair and extra-pair reproductive success and their covariance to the variance in realized fitness. Overall, EPP increased the variance in male fitness by only 0-27% across different fitness and variance measures. This relatively small effect reflected the presence of socially unpaired males with zero apparent and low realized fitness, small covariance between within- and extra-pair reproductive success and large variance in within-pair reproductive success that was relatively unaffected by EPP. Therefore, although EPP altered individual males’ contributions to future generations, its impact on population-level parameters such as the opportunity for selection and effective population size was limited.

种群个体间适合度的差异可影响核心进化进程。在社会单配制但遗传上属于多配混交的物种中，学界普遍提出假说：相较于仅遵循社会单配制交配体系的情况，婚外父权（extra-pair paternity, EPP）会增大雄性适合度的变异程度。
该假说尚未得到确定性验证，此前始终缺乏能够同时描述雄性表观（社会层面）与实际（遗传层面）适合度的全面数据。本研究针对整个野生歌带鹀（Melospiza melodia）种群，收集了长达16年的全面社会与遗传父权数据，借此量化并比较了雄性表观适合度与实际适合度的变异幅度，同时分析了配对内生殖成功、配对外生殖成功的变异及其协方差对实际适合度变异的贡献。
总体而言，在不同的适合度与变异度量标准下，EPP仅使雄性适合度的变异提升了0%至27%。这一相对微弱的效应源于多重因素：存在社会上无配偶的雄性个体，其表观适合度为0且实际适合度较低；配对内与配对外生殖成功之间的协方差较小；且配对内生殖成功的变异幅度较大，几乎不受EPP的影响。
因此，尽管EPP改变了单个雄性对未来世代的遗传贡献，但其对种群水平参数（如选择机会与有效种群大小）的影响较为有限。