Institute of Evolutionary Biology and Environmental Studies, University of Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland.
Proc Biol Sci. 2011 Oct 7;278(1720):2996-3002. doi: 10.1098/rspb.2010.2763. Epub 2011 Feb 23.
Sex ratio theory proposes that the equal sex ratio typically observed in birds and mammals is the result of natural selection. However, in species with chromosomal sex determination, the same 1 : 1 sex ratio is expected under random Mendelian segregation. Here, we present an analysis of 14 years of sex ratio data for a population of song sparrows (Melospiza melodia) on Mandarte Island, at the nestling stage and at independence from parental care. We test for the presence of variance in sex ratio over and above the binomial variance expected under Mendelian segregation, and thereby quantify the potential for selection to shape sex ratio. Furthermore, if sex ratio variation is to be shaped by selection, we expect some of this extra-binomial variation to have a genetic basis. Despite ample statistical power, we find no evidence for the existence of either genetic or environmentally induced variation in sex ratio, in the nest or at independence. Instead, the sex ratio variation observed matches that expected under random Mendelian segregation. Using one of the best datasets of its kind, we conclude that female song sparrows do not, and perhaps cannot, adjust the sex of their offspring. We discuss the implications of this finding and make suggestions for future research.
性别比例理论认为，鸟类和哺乳动物中通常观察到的相等性别比例是自然选择的结果。然而，在具有染色体性别决定的物种中，在随机孟德尔分离下，预期会出现相同的 1:1 性别比例。在这里，我们对曼达特岛的麻雀（Melospiza melodia）种群在巢雏期和脱离亲代养育时的 14 年性别比例数据进行了分析。我们检验了性别比例是否存在超出孟德尔分离预期的二项式方差的变异，从而量化了选择塑造性别比例的潜力。此外，如果性别比例的变化是由选择塑造的，我们期望这种额外的二项式变异中的一部分具有遗传基础。尽管具有充分的统计能力，但我们在巢或独立时都没有发现性别比例存在遗传或环境诱导变异的证据。相反，观察到的性别比例变异与随机孟德尔分离下预期的变异相匹配。使用同类中最好的数据集之一，我们得出的结论是，雌性麻雀不会，也可能无法，调整其后代的性别。我们讨论了这一发现的意义，并为未来的研究提出了建议。
