Cullen Thomas M, Fraser Danielle, Rybczynski Natalia, Schröder-Adams Claudia
Department of Earth Sciences, Carleton University, Ottawa, Ontario, K1S 5B6, Canada.
Evolution. 2014 May;68(5):1469-84. doi: 10.1111/evo.12360. Epub 2014 Feb 18.
Sexual selection is one of the earliest areas of interest in evolutionary biology. And yet, the evolutionary history of sexually dimorphic traits remains poorly characterized for most vertebrate lineages. Here, we report on evidence for the early evolution of dimorphism within a model mammal group, the pinnipeds. Pinnipeds show a range of sexual dimorphism and mating systems that span the extremes of modern mammals, from monomorphic taxa with isolated and dispersed mating to extreme size dimorphism with highly ordered polygynous harem systems. In addition, the degree of dimorphism in pinnipeds is closely tied to mating system, with strongly dimorphic taxa always exhibiting a polygynous system, and more monomorphic taxa possessing weakly polygynous systems. We perform a comparative morphological description, and provide evidence of extreme sexual dimorphism (similar to sea lions), in the Miocene-aged basal pinniped taxon Enaliarctos emlongi. Using a geometric morphometric approach and combining both modern and fossil taxa we show a close correlation between mating system and sex-related cranial dimorphism, and also reconstruct the ancestral mating system of extant pinnipeds as highly polygynous. The results suggest that sexual dimorphism and extreme polygyny in pinnipeds arose by 27 Ma, in association with changing climatic conditions.
性选择是进化生物学中最早受到关注的领域之一。然而，对于大多数脊椎动物谱系而言，两性异形性状的进化历史仍知之甚少。在此，我们报告了关于一种典型哺乳动物类群——鳍足类动物中两性异形早期进化的证据。鳍足类动物展现出一系列的两性异形和交配系统，涵盖了现代哺乳动物的各种极端情况，从具有孤立和分散交配行为的单态分类群到具有高度有序多配偶制后宫系统的极端体型差异。此外，鳍足类动物的两性异形程度与交配系统紧密相关，两性差异强烈的分类群总是呈现多配偶制系统，而单态性更强的分类群则拥有较弱的多配偶制系统。我们进行了比较形态学描述，并提供了中新世时期基部鳍足类分类群恩氏海熊兽（Enaliarctos emlongi）存在极端两性异形（类似于海狮）的证据。通过几何形态测量方法，并结合现代和化石分类群，我们展示了交配系统与性别相关的颅骨异形之间的紧密关联，同时将现存鳍足类动物的祖先交配系统重建为高度多配偶制。结果表明，鳍足类动物的两性异形和极端多配偶制在2700万年前就已出现，这与气候条件的变化有关。
