American Society of Naturalists

Changde Cheng (成常德) and David Houle (Oct 2020)

Sexual dimorphism evolves more when sex-averaged trait values are selected than when dimorphism itself is selected

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Differences between male and female animals are a striking feature of many species, like the Flame Bowerbirds in the accompanying photo. Such sexual dimorphism can, for example, be explained if selection on males is dominated by females choosing to mate with males that have striking colors and behaviors, but selection on females favors individuals who avoid detection by predators. However, if you look closely, even traits with no obvious connection to mating often differ between the sexes. In a new paper in the American Naturalist, evolutionary biologists Changde Cheng and David Houle suggest that many of these differences between the sexes can be better explained by differences in the underlying inheritance of traits between the sexes, rather than by differences in natural selection. Using previously gathered data on the fruit fly, Cheng and Houle show that some traits may actually diverge faster when selection acts the same on both sexes than when selection favors differences between the sexes. Sexual dimorphism may often be better explained by differences in the way genes act in each sex, rather than by selection for the sexes to be different.

文章摘要

雌性和雄性为什么不一样？一种解释是不同的性别受不同的选择。比如说照片里的烈焰亭鸟，雄性的色彩为求偶而鲜艳夺目，雌性的色彩为生存而相对低调。但是进一步看，大多数性状在雄性和雌性间都会有差异，而这些差异不一定都是源自于性别间的选择差异。在最新一期的美国博物学家杂志上，演化生物学家成常德和David Houle在论文提出：遗传基础的差异比选择的差异能更好的解释广泛存在的性别二态性。即使选择在雌性和雄性中的作用完全一致，遗传基础上的差异也可以导致两性间的性状差异。根据果蝇中的研究数据，成常德和David Houle表明，当选择在雌性和雄性中发挥同样作用时，某些性状上在两性之间的分化速度甚至可以比在两性受差异选择时还要快。该文提供了一个新的性别二态性起源的解释：性二态性可能源于基因在不同性别中的不同作用，而不是选择的差异作用。

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Abstract

Sexual dimorphism is often assumed to result from balancing the strength of antagonistic selection in favor of dimorphism against the degree of constraint imposed by the shared genome of the sexes, reflected in the B matrix of genetic inter-sexual covariances. To investigate the totality of forces shaping dimorphism, we reparameterized the Lande equation to predict changes in trait averages and trait differences between the sexes. As genetic constraints on the evolution of dimorphism in response to antagonistic selection become larger, dimorphism will tend to respond more rapidly to concordant selection (which favors the same direction of change in male and female traits) than to antagonistic selection. When we apply this theory to four empirical estimates of B in Drosophila melanogaster, the indirect responses of dimorphism to concordant selection are of comparable or larger magnitude than the direct responses of dimorphism to antagonistic selection in two suites of traits with typical levels of inter-sex correlation. Antagonistic selection is more important in two suites of traits where the inter-sex correlations are unusually low. This suggests that the evolution of sexual dimorphism may sometimes be dominated by concordant selection, rather than antagonistic selection.

直接选择和间接选择对性二态演化的影响

性二态的演化通常被认为是两性冲突导致的反向选择，和彼此共有的基因组产生的遗传约束相平衡的结果；而在两性间作用一致的同向选择的影响是间接的，从而是次要的，甚至是无关的。但是我们通过Lande方程发现，当对反向选择的遗传约束增强时，同向选择变得更重要。这一结果得到果蝇中的实验数据支持。在具有典型的两性间遗传相关水平的两组性状中，性二态的演化对同向选择的间接响应不小于对反向选择的直接响应。其余两组的性间遗传相关极低，反向选择影响更大。结论是：性二态的演化有时会由两性一致的同向选择而非冲突导致的反向选择主导。