A gene variation that dramatically decreases survival of embryos occurs at high frequency in many human populations – and it seems as if natural selection favored the persistence of this fertility-reducing trait.

A new study looked at thousands of preimplantation human embryos and their parents and found a genetic variation that conveys an increased risk of aneuploidy, that is, having the wrong number of chromosomes, which is the primary cause of pregnancy loss. The authors speculate that:

…the mitotic-error phenotype may be maintained by conferring both a deleterious effect on maternal fecundity and a possible beneficial effect of obscured paternity via a reduction in the probability of successful pregnancy per intercourse. This hypothesis is based on the fact that humans possess a suite of traits (such as concealed ovulation and constant receptivity) that obscure paternity and may have evolved to increase paternal investment in offspring. Such a scenario could result in balancing selection by rewarding evolutionary “free riders” who do not possess the risk allele—and thus do not suffer fecundity costs—but benefit from paternity confusion in the population as a whole.

This finding may help explain why nearly a third of natural human conceptions do not go to full term. It may be part of a weird evolutionary tradeoff in which reduced fecundity can be a feature, not a bug.

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Image: A lithograph in the 1835 medical text Geburtshülflicher Atlas in 48 Tafeln und erklärendem Texte, via the Wellcome Collection

Study: Common variants spanning PLK4 are associated with mitotic-origin aneuploidy in human embryos, Rajiv C. McCoy & others, Science, 10 April 2015