Insects show relatively little genital variation within species compared to extraordinary and often rapid diversification among species. It has been suggested that selection for reproductive isolation through differences in genital shape might explain this phenomenon. This hypothesis predicts that populations diverge faster in genital shape than in genital size. We tested this prediction in males from 10 dung beetle species with known phylogenetic relationships from the genus Onthophagus (Coleoptera: Scarabaeidae), including four species for which we were able to sample multiple populations. Specifically, we compared intra- and interspecific differentiation in shape and relative sizes of genitalia and calculated their respective evolutionary rates. We compared these rates to two similarly sized non-genital traits, the head and the fore-tibia. We found significant intraspecific variation in genital shape in all four species for which multiple populations were sampled, but for three of them we also identified significant relative size variation. We also found that genital shape evolved at higher rates than relative genital size. Genital shape evolved faster than head shape, but not fore-tibia shape. However, shapes of all measured structures evolved faster than their relative size. We discuss the functional constraints that may bias the developmental evolution of relative size and shape of genitalia and other morphological traits.

中文翻译：

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相对性状大小和形状的演变：来自be虫生殖器的见解。

与物种之间异常且通常迅速的多样化相比，昆虫在物种内部的生殖器变异相对较小。有人建议通过生殖器形状的差异选择生殖隔离可能解释了这种现象。该假设预测，生殖器形状的差异比生殖器大小的差异快。我们在10种粪甲虫物种的雄性中测试了这种预测，这些雄性具有来自嗜食性昆虫属的已知系统发育关系（鞘翅目：Scarabaeidae），包括我们能够对多个种群进行采样的四个物种。具体来说，我们比较了生殖器形状和相对大小的种内和种间差异，并计算了它们各自的进化率。我们将这些比率与两个类似大小的非生殖器特征（头部和前胫骨）进行了比较。我们在所有四个物种的多个种群中发现生殖器形状的种内显着变化，但对于其中三个，我们还发现了相对大小的显着变化。我们还发现生殖器形状的进化速度要高于相对生殖器的大小。生殖器形状的进化比头部形状快，但前胫骨形状则不快。但是，所有测量结构的形状演​​化都快于它们的相对大小。