Twins from an adaptive point of view

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The biological anthropologist Rebecca Sear looks at the evolution of human twinning in a post for This View of Life: “Solving the Evolutionary Puzzle of Twinning”.

She reviews the results of a recent paper modeling the fitness costs and benefits of multiple ovulation in a cycle, or polyovulation.

Twinning is a relatively rare event, varying from around 0.6 to 4% of all births, and this new paper suggests that its rarity is because it is an ‘accident’ arising from a strategy of polyovulation to counteract very high rates of foetal loss. Many questions still remain about this phenomenon: such as why foetal loss is so high, and why twinning rates vary around the world – because of differences in the likelihood of foetal loss, perhaps? But regardless of the explanation for the existence of twins, mythology, literature and the human experience is much richer because of this fascinating phenomenon."

Humans have a very high late of early pregnancy loss, which is itself an evolutionary problem. The idea of a bet-hedging strategy revolving around polyovulation is one possible explanation.

But there are a couple of things that this explanation is less good for. I’m less convinced than the authors of the study about bet-hedging for older mothers. The fact that twinning increases with maternal age seems more consistent with a progressive decline in the strength of selection against it, rather than an adaptation for twinning for older mothers.

More critically in my view is the fact that monozygotic twinning in humans is approximately the same frequency as dizygotic twinning in many populations, even though they originate from very different mechanisms. Polyovulation should have no effect on the rate of monozygotic twins, so why should they be around the same frequency?

Knowing more precise numbers about the rate of these in different populations and the genetic correlates of them will be helpful.