Department of Biology, DePauw University, Greencastle, IN, USA.
School of Biological Sciences, The University of Western Australia, Crawley, Western Australia, Australia.
Nat Ecol Evol. 2020 Jul;4(7):987-992. doi: 10.1038/s41559-020-1173-y. Epub 2020 May 11.
Dizygotic twinning, the simultaneous birth of siblings when multiple ova are released, is an evolutionary paradox. Twin-bearing mothers often have elevated fitness, but despite twinning being heritable, twin births occur only at low frequencies in human populations. We resolve this paradox by showing that twinning and non-twinning are not competing strategies; instead, dizygotic twinning is the outcome of an adaptive conditional ovulatory strategy of switching from single to double ovulation with increasing age. This conditional strategy, when coupled with the well-known decline in fertility as women age, maximizes reproductive success and explains the increase and subsequent decrease in the twinning rate with maternal age that is observed across human populations. We show that the most successful ovulatory strategy would be to always double ovulate as an insurance against early fetal loss, but to never bear twins. This finding supports the hypothesis that twinning is a by-product of selection for double ovulation rather than selection for twinning.
双卵孪生，即当多个卵子同时释放时，会同时诞生多个兄弟姐妹，这是一个进化悖论。怀有双胞胎的母亲通常具有更高的适应度，但尽管双胞胎具有遗传性，但其在人类中的出生率却很低。我们通过表明，双胞胎分娩和非双胞胎分娩并不是相互竞争的策略，而是通过随着年龄的增长从单排卵到双排卵的适应性条件排卵策略来解决这个悖论。这种条件策略，加上众所周知的女性随着年龄增长生育能力下降，最大限度地提高了生殖成功，并解释了在整个人类群体中观察到的与母亲年龄相关的双胞胎出生率的增加和随后的下降。我们表明，最成功的排卵策略将是始终进行双排卵，以防止早期胎儿丢失，但永远不要生双胞胎。这一发现支持了这样一种假设，即双胞胎是选择双排卵的副产品，而不是选择双胞胎的结果。
