Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544
Department of Ecology and Evolutionary Biology, Princeton University, Princeton, NJ 08544.
Proc Natl Acad Sci U S A. 2019 Jun 18;116(25):12373-12382. doi: 10.1073/pnas.1903838116. Epub 2019 Jun 3.
The adult sex ratio (ASR) is an important property of populations. Comparative phylogenetic analyses have shown that unequal sex ratios are associated with the frequency of changing mates, extrapair mating (EPM), mating system and parental care, sex-specific survival, and population dynamics. Comparative demographic analyses are needed to validate the inferences, and to identify the causes and consequences of sex ratio inequalities in changing environments. We tested expected consequences of biased sex ratios in two species of Darwin's finches in the Galápagos, where annual variation in rainfall, food supply, and survival is pronounced. Environmental perturbations cause sex ratios to become strongly male-biased, and when this happens, females have increased opportunities to choose high-quality males. The choice of a mate is influenced by early experience of parental morphology (sexual imprinting), and since morphological traits are highly heritable, mate choice is expressed as a positive correlation between mates. The expected assortative mating was demonstrated when the population was strongly male-biased, and not present in the contemporary population with an equal sex ratio. Initial effects of parental imprinting were subsequently overridden by other factors when females changed mates, some repeatedly. Females of both species were more frequently polyandrous in male-biased populations, and fledged more offspring by changing mates. The ASR ratio indirectly affected the frequency of EPM (and hybridization), but this did not lead to social mate choice. The study provides a strong demonstration of how mating patterns change when environmental fluctuations lead to altered sex ratios through differential mortality.
成体性别比例（ASR）是种群的一个重要特征。比较系统发育分析表明，性别比例的不平等与配偶更换频率、婚外交配（EPM）、交配系统和亲代养育、性别特异性存活以及种群动态有关。需要进行比较人口统计学分析来验证这些推断，并确定在不断变化的环境中性别比例不平等的原因和后果。我们在加拉帕戈斯群岛的两种达尔文雀中测试了偏性性别比例的预期后果，那里的年降雨量、食物供应和存活率变化明显。环境干扰会导致性别比例严重偏向雄性，当这种情况发生时，雌性有更多的机会选择高质量的雄性。伴侣的选择受到早期父母形态的经验（性印记）的影响，由于形态特征具有高度的遗传性，因此伴侣选择表现为伴侣之间的正相关关系。当种群严重偏向雄性时，就证明了预期的交配选择，而在当代性别比例相等的种群中则不存在这种情况。当雌性更换伴侣时，最初的父母印记效应随后被其他因素所取代，有些雌性会多次更换伴侣。在偏向雄性的种群中，两种雀类的雌性更频繁地一妻多夫，通过更换伴侣而育出更多的后代。ASR 比率间接影响 EPM（和杂交）的频率，但这并没有导致社会伴侣选择。该研究有力地证明了，当环境波动通过差异死亡率导致性别比例发生变化时，交配模式是如何发生变化的。
