Experimental evidence for the evolution of the mammalian baculum by sexual selection

Male genitalia exhibit a taxonomically widespread pattern of rapid and divergent evolution. Sexual selection is generally believed to be responsible for these patterns of evolutionary divergence, although empirical support for the sexual selection hypothesis comes mainly from studies of insects. Here we show that sexual selection is responsible for an evolutionary divergence in baculum morphology among populations of house mice Mus domesticus. We sourced mice from three isolated populations known to be subject to differing strengths of postcopulatory sexual selection and bred them under common-garden conditions. Mice from populations with strong postcopulatory sexual selection had bacula that were relatively thicker compared with mice from populations with weak selection. We used experimental evolution to determine whether these patterns of divergence could be ascribed to postcopulatory sexual selection. After 27 generations of experimental evolution, populations of mice subjected to postcopulatory sexual selection evolved bacula that were relatively thicker than populations subjected to enforced monogamy. Our data thereby provide evidence that postcopulatory sexual selection underlies an evolutionary divergence in the mammalian baculum and supports the hypothesis that sexual selection plays a general role in the evolution of male genital morphology across evolutionary diverse taxonomic groups.

雄性生殖器在分类学上呈现出广泛存在的快速演化与趋异演化模式。尽管关于性选择假说的实证支持主要来自昆虫类群的研究，但学界普遍认为性选择是造成这类演化趋异的原因。本研究证实，家鼠（Mus domesticus）不同种群间的阴茎骨（baculum）形态演化趋异正是由性选择导致的。我们从三个已知受到不同强度交配后性选择作用的隔离种群中采集家鼠，并在同质园饲养条件下开展繁育。相较于来自弱交配后性选择种群的家鼠，来自强交配后性选择种群的家鼠其阴茎骨相对更粗壮。我们通过实验演化探究这类形态趋异模式是否可归因于交配后性选择。经过27代实验演化后，经历交配后性选择的家鼠种群其阴茎骨相对粗壮程度显著高于实施强制单配制的对照组种群。综上，本研究数据证实交配后性选择是哺乳类阴茎骨形态演化趋异的关键成因，并支持性选择在不同演化分类类群的雄性生殖器形态演化中发挥普遍作用这一假说。