The theory of sexual selection is the most widely accepted theory explaining the evolution of mating systems and secondary sexual characters. Polygyny is the most common mating system in mammals, and there is a strong correlation between the degree of polygyny and the degree of sexual size dimorphism skewed towards males. Sexual selection theory posits that polygyny in mammals has evolved through direct, precopulatory, intrasexual selection in males, and that sexual size dimorphism is a result of male competition for mates. New results that are being obtained with the use of molecular techniques and with comparative phylogenetic methods do not appear to support predictions from this classical model in full. In this article, an expansion of the classical model is presented that combines the effects of at least four forms of selection: natural, precopulatory intrasexual, postcopulatory intrasexual, and intersexual selection. This mixed model consists of an initial phase in which natural selection operates on body size, followed by a second phase dominated by sexual selection and involving increases in sexual dimorphism and coercive behaviour of males towards females. Sexual harassment induces female aggregation, thus creating social potential for polygyny. Males compete for access to the groups of females, following two possible evolutionary scenarios, directional or equilibrium sexual selection, both producing similar behavioural polygyny, but with differences in the intensity of intra‐male precopulatory sexual selection. Predictions of the mixed model are as follows: 1) polygyny can exist without high variance in male reproductive success (a fundamental requirement in the classical model); 2) extra‐group fertilisation can be common; 3) sexual size dimorphism evolved prior to polygyny; 4) sexual coercion is widespread; and 5) females reduce levels of sexual coercion by joining groups.

中文翻译：

---

哺乳动物一夫多妻和性别大小二态性进化的混合模型

性选择理论是解释交配系统和第二性征进化的最广为接受的理论。一夫多妻是哺乳动物中最常见的交配系统，一夫多妻的程度与性别大小二态性向雄性倾斜的程度之间存在很强的相关性。性选择理论认为，哺乳动物的一夫多妻是通过雄性的直接、交配前、性内选择进化而来的，而性别大小二态性是雄性争夺配偶的结果。使用分子技术和比较系统发育方法获得的新结果似乎并不完全支持这个经典模型的预测。在本文中，介绍了经典模型的扩展，该模型结合了至少四种选择形式的效果：自然、交配前的性内性选择、交配后的性内性选择和性间选择。这种混合模型包括一个初始阶段，其中自然选择对体型起作用，然后是第二个阶段，主要是性选择，涉及男性对女性的性别二态性和胁迫行为的增加。性骚扰会导致女性聚集，从而创造一夫多妻制的社会潜力。雄性竞争进入雌性群体，遵循两种可能的进化场景，定向或平衡性选择，两者都产生类似的行为一夫多妻制，但在男性内部预交性选择的强度上存在差异。混合模型的预测如下：1）一夫多妻制可以存在，而男性生殖成功率没有很大差异（经典模型中的基本要求）；2）群外受精很常见；3）性别大小二态性先于一夫多妻而进化；4) 性强迫普遍存在；5) 女性通过加入团体来降低性胁迫的程度。