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.

中文翻译：

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年龄依赖性排卵策略可解释人体内双卵双生的进化。

同卵双生，即释放多个卵时同胞的同时出生，是一种进化悖论。双胞胎母亲通常有较高的身体素质，但尽管双胞胎是可遗传的，但双胞胎的产生仅在人口中很少发生。通过证明孪生和非孪生不是竞争策略，我们解决了这个矛盾。相反，同卵双生是适应性排卵策略的结果，该策略随着年龄的增长从单排卵转换为双排卵。这种有条件的策略，加上众所周知的随着年龄增长而生育力下降的方法，可使生殖成功最大化，并解释了在全人类中观察到的与产妇年龄相关的孪生率的上升和下降。我们证明，最成功的排卵策略是始终排卵以防止早产，但绝不生育双胞胎。这一发现支持以下假设：双胎是双排卵选择的副产品，而不是双胎选择的副产品。