Rudoy Andrey, Ribera Ignacio
Institute of Evolutionary Biology, CSIC-Universitat Pompeu Fabra , Barcelona , Spain.
PeerJ. 2017 Mar 7;5:e3060. doi: 10.7717/peerj.3060. eCollection 2017.
Sexual size dimorphism (SSD) is widespread among animals, with larger females usually attributed to an optimization of resources in reproduction and larger males to sexual selection. A general pattern in the evolution of SSD is Rensch's rule, which states that SSD increases with body size in species with larger males but decreases when females are larger. We studied the evolution of SSD in the genus (Coleoptera, Hydraenidae), measuring SSD and male genital size and complexity of ca. 80% of its 150 species and reconstructing its evolution in a molecular phylogeny with 71 species. We found strong support for a higher evolutionary lability of male body size, which had an overall positive allometry with respect to females and higher evolutionary rates measured over the individual branches of the phylogeny. Increases in SSD were associated to increases in body size, but there were some exceptions with an increase associated to changes in only one sex. Secondary sexual characters (SSC) in the external morphology of males appeared several times independently, generally on species that had already increased their size. There was an overall significant correlation between SSD, male body size and male genital size and complexity, although some lineages with complex genitalia had low SSD, and some small species with complex genitalia had no SSD. Our results suggest that the origin of the higher evolutionary variance of male body size may be due to lack of constraints rather than to sexual selection, that may start to act in species with already larger males due to random variation.
两性体型差异（SSD）在动物中广泛存在，通常认为雌性体型较大是为了优化繁殖资源，而雄性体型较大则归因于性选择。SSD进化的一个普遍模式是伦施法则，该法则指出，在雄性体型较大的物种中，SSD随体型增大而增加；而在雌性体型较大的物种中则相反（SSD随体型增大而减小）。我们研究了 属（鞘翅目，沼甲科）中SSD的进化情况，测量了其150个物种中约80%的SSD、雄性生殖器大小及复杂性，并在包含71个物种的分子系统发育树中重建了其进化过程。我们发现，有充分证据支持雄性体型具有更高进化变异性，相对于雌性而言雄性体型总体呈正向异速生长，并且在系统发育树的各个分支上测量到更高进化速率。SSD的增加与体型增大相关，但也有一些例外情况，即SSD的增加仅与某一性别的变化有关。雄性外部形态的第二性征（SSC）多次独立出现，通常出现在体型已经增大的物种上。SSD、雄性体型以及雄性生殖器大小与复杂性之间总体存在显著相关性，尽管一些生殖器复杂的谱系SSD较低，一些生殖器复杂的小物种没有SSD。我们的研究结果表明，雄性体型更高进化方差的起源可能是由于缺乏限制而非性选择，性选择可能由于随机变异而在雄性体型已经较大物种中开始起作用。
