Webster Michael S
Section of Neurobiology and Behavior, Seeley G. Mudd Hall, Cornell University, Ithaca, NY, 14853, USA.
Evolution. 1992 Dec;46(6):1621-1641. doi: 10.1111/j.1558-5646.1992.tb01158.x.
Although sexual selection is widely accepted as a primary functional cause of sexual size dimorphism in birds and mammals, results from some comparative studies have cast doubt on this conclusion. Chief among these contradictory results is the widespread association between body size and size dimorphism-large species tend to be more dimorphic than small species. This correlation is not directly predicted by the normal sexual selection scenario, and many hypotheses have been advanced to explain it. This paper reviews these hypotheses and evaluates them using data for the New World blackbirds (Icterinae). In this avian subfamily, (1) body size correlates with the intensity of sexual selection (as measured by mean harem size), and (2) size does not correlate with dimorphism if the effects of mating system are removed. Similar results are obtained when controlling for the confounding influence of phylogeny. Further, body size and mating system are associated with nesting dispersion. These results strongly argue that sexual dimorphism is a product of sexual selection in this subfamily, and suggest that either: (1) large body size itself, or the ecology of large species, promotes the development of coloniality and a polygynous mating system; or (2) polygyny and/or coloniality lead to the evolution of large size in both males and females. None of the other hypotheses examined predict an association between size and mating system, and all predict that size will correlate with dimorphism after the effects of mating system are removed. Thus, none of the other hypotheses seem applicable in this case. These results are compared to those obtained for other avian and mammalian taxa. Difficulties of analysis present in previous studies are discussed. I argue that it is inappropriate to assume that associations between a trait and body size or phylogeny are evidence of nonadaptive evolutionary "constraints."
尽管性选择被广泛认为是鸟类和哺乳动物两性体型差异的主要功能性原因，但一些比较研究的结果对此结论提出了质疑。这些相互矛盾的结果中最主要的是体型与体型差异之间的广泛关联——大型物种往往比小型物种的两性差异更大。这种相关性并非正常性选择情形所直接预测的，因此人们提出了许多假说对此进行解释。本文回顾了这些假说，并利用新大陆黑鹂（拟黄鹂亚科）的数据对其进行评估。在这个鸟类亚科中，（1）体型与性选择强度相关（以平均妻妾群大小衡量），并且（2）如果去除交配系统的影响，体型与两性差异不相关。在控制系统发育的混杂影响时也得到了类似结果。此外，体型和交配系统与筑巢分布有关。这些结果有力地表明，两性差异是该亚科中性选择的产物，并表明要么：（1）大体型本身，或者大型物种的生态环境，促进了集群性和一夫多妻制交配系统的发展；要么（2）一夫多妻制和/或集群性导致了雄性和雌性体型的进化。所检验的其他假说均未预测到体型与交配系统之间的关联，并且都预测在去除交配系统的影响后体型将与两性差异相关。因此，其他假说在此情况下似乎都不适用。本文将这些结果与其他鸟类和哺乳动物类群的结果进行了比较。讨论了先前研究中存在的分析困难。我认为，假设一个性状与体型或系统发育之间的关联是非适应性进化“限制”的证据是不合适的。
