Fairbairn Daphne J, Preziosi Richard F
Department of Biology, Concordia University, 1455 de Maisonneuve Boulevard, West, Montreal, Quebec, H3G 1M8, Canada.
Department of Biology, McGill University, 1205 Docteur Penfield, Montreal, Quebec, H3A 1B1, Canada.
Evolution. 1996 Aug;50(4):1549-1559. doi: 10.1111/j.1558-5646.1996.tb03927.x.
Sexual size dimorphism (SSD) is often attributed to sexual selection, particularly when males are the larger sex. However, sexual selection favoring large males is common even in taxa where females are the larger sex, and is therefore not a sufficient explanation of patterns of SSD. As part of a more extensive study of the evolution of SSD in water striders (Heteroptera, Gerridae), we examine patterns of sexual selection and SSD in 12 populations of Aquarius remigis. We calculate univariate and multivariate selection gradients from samples of mating and single males, for two sexually dimorphic traits (total length and profemoral width) and two sexually monomorphic traits (mesofemoral length and wing form). The multivariate analyses reveal strong selection favoring larger males, in spite of the female-biased SSD for this trait, and weaker selection favoring aptery and reduced mesofemoral length. Selection is weakest on the most dimorphic trait, profemoral width, and is stabilizing rather than directional. The pattern of sexual selection on morphological traits is therefore not concordant with the pattern of SSD. The univariate selection gradients reveal little net selection (direct + indirect) on any of the traits, and suggest that evolution away from the plesiomorphic pattern of SSD is constrained by antagonistic patterns of selection acting on this suite of positively correlated morphological traits. We hypothesize that SSD in A. remigis is not in equilibrium, a hypothesis that is consistent with both theoretical models of the evolution of SSD and our previous studies of allometry for SSD. A negative interpopulation correlation between the intensity of sexual selection and the operational sex ratio supports the hypothesis that, as in several other water strider species, sexual selection in A. remigis occurs through generalized female reluctance rather than active female choice. The implications of this for patterns of sexual selection are discussed.
性大小二态性（SSD）通常归因于性选择，尤其是当雄性为较大性别时。然而，即使在雌性为较大性别的类群中，有利于大型雄性的性选择也很常见，因此它并不能充分解释SSD的模式。作为对水黾（异翅目，黾蝽科）SSD进化更广泛研究的一部分，我们研究了12个细纹黾蝽种群的性选择模式和SSD。我们从交配样本和单个雄性样本中计算了单变量和多变量选择梯度，涉及两个两性异形性状（全长和股节前宽度）和两个两性同形性状（股节中长度和翅形）。多变量分析显示，尽管该性状存在雌性偏向的SSD，但仍有强烈的选择有利于较大的雄性，而有利于无翅和股节中长度减小的选择较弱。对最具二态性的性状股节前宽度的选择最弱，且是稳定选择而非定向选择。因此，形态性状的性选择模式与SSD模式不一致。单变量选择梯度显示，对任何性状的净选择（直接 + 间接）都很小，并表明偏离SSD的近祖模式的进化受到作用于这组正相关形态性状的拮抗选择模式的限制。我们假设细纹黾蝽的SSD并非处于平衡状态，这一假设与SSD进化的理论模型以及我们之前对SSD异速生长的研究均一致。性选择强度与操作性别比之间的种群间负相关支持了这样的假设，即与其他几种水黾物种一样，细纹黾蝽的性选择是通过雌性普遍的不情愿而非主动的雌性选择发生的。文中讨论了这对性选择模式的影响。
