Webster Michael S, Pruett-Jones Stephen, Westneat David F, Arnold Stevan J
Department of Ecology and Evolution, University of Chicago, 1101 E. 57th Street, Chicago, Illinois, 60637.
Center for Ecology, Evolution, and Behavior, T. H. Morgan School of Biological Sciences, 101 Morgan Building, University of Kentucky, Lexington, Kentucky, 40506-0225.
Evolution. 1995 Dec;49(6):1147-1157. doi: 10.1111/j.1558-5646.1995.tb04441.x.
Sexual selection can act through variation in the number of social mates obtained, variation in mate quality, or variation in success at obtaining extra-pair fertilizations. Because within-pair fertilizations (WPF) and extra-pair fertilizations (EPF) are alternate routes of reproduction, they are additive, rather than multiplicative, components of fitness. We present a method for partitioning total variance in reproductive success (a measure of the opportunity for selection) when fitness components are both additive and multiplicative and use it to partition the variance into components that correspond to each mechanism of sexual selection. Computer simulations show that extra-pair fertilizations can either increase or decrease total variance, depending on the covariance between within-pair and extra-pair success. Simulations also suggest that for socially monogamous species, extra-pair fertilizations have a greater effect than variation in mate quality or pairing status on the opportunity for selection. Application of our model to data gathered for a population of red-winged blackbirds (Agelaius phoeniceus) indicates that most of the variance in male reproductive success was attributable to within-pair sources of variance. Nevertheless, extra-pair copulations increased the opportunity for selection because males varied both in the proportion of their social young that they sired and in the number of extra-pair mates that they obtained. Furthermore, large and positive covariances existed between the number of extra-pair mates a male obtained and both social pairing success and within-pair paternity, indicating that, in this population, males preferred as social mates also were preferred as extra-pair mates.
性选择可以通过获得的社会配偶数量的变化、配偶质量的变化或获得婚外受精成功的变化来起作用。由于配偶内受精（WPF）和婚外受精（EPF）是繁殖的替代途径，它们是适合度的相加而非相乘的组成部分。当适合度组成部分既有相加性又有相乘性时，我们提出了一种划分繁殖成功总方差（选择机会的一种度量）的方法，并利用它将方差划分为与每种性选择机制相对应的组成部分。计算机模拟表明，婚外受精可以增加或减少总方差，这取决于配偶内和婚外成功之间的协方差。模拟还表明，对于社会上实行一夫一妻制的物种，婚外受精对选择机会的影响比配偶质量或配对状态的变化更大。将我们的模型应用于为红翅黑鹂（Agelaius phoeniceus）种群收集的数据表明，雄性繁殖成功的大部分方差可归因于配偶内的方差来源。然而，婚外交配增加了选择机会，因为雄性在其养育的社会后代比例和获得的婚外配偶数量上都存在差异。此外，雄性获得的婚外配偶数量与社会配对成功和配偶内父权之间存在大的正协方差，这表明，在这个种群中，作为社会配偶被偏好的雄性也是作为婚外配偶被偏好的。
