WHY SEXUAL REPRODUCTION ONLY HAPPENS IN PAIRS: New insights into unresolved problems in evolutionary biology

New research analyses why across the natural world sexual reproduction never involves mixing the genetic material of three individuals of a species. The study by Professor Motty Perry and colleagues, published in the December 2017 issue of the Economic Journal, explores how there can be an advantage to ''biparental'' sex over asexual reproduction without there being an even greater advantage to ''triparental'' sex.

The researchers begin by noting that the breadth and variety of methods by which different species reproduce through sex is incredible. Nonetheless, sexual reproduction displays a stunning regularity: each sexually produced offspring of any known species is produced from the genetic material of precisely two individuals – that is, sex is always biparental.

The obvious, but overlooked, question is: why? In particular, why are there no
triparental species in which an offspring is composed of the genetic material of three individuals?

Like Sherlock Holmes'' question about the dog that did not bark in the night, taking seriously the counterfactual question raised here has significant implications for our understanding of reality, in particular concerning the purpose of sex, one of the most important unresolved problems in evolutionary biology.

The question of the purpose of sex remains unresolved not because there are too few explanations; the difficulty is that there are too many. What is needed is some new means by which one or more of these theories can be rejected.

The new study proposes the following: a complete theory of sex must strike a delicate balance.

On the one hand – as is well known – it must explain why genetic mixing is sufficiently beneficial so that biparental sex overcomes the twofold cost of males it suffers because an equally-sized asexual population would grow twice as fast (as discussed by John Maynard Smith).

On the other hand – and this is the central point of the new study – genetic mixing must not be so beneficial that a further increase in fitness would be obtained from even more of it through triparental sex.

Two especially prominent explanations of the purpose of sex are the ''red queen hypothesis'' (Bill Hamilton, for example) and the ''mutational deterministic hypothesis'' (Alexey Kondrashov).

Both theories exploit the fact that sex generates genetic mixing, although they are in sharp disagreement about precisely why genetic mixing is advantageous. Broadly speaking, the red queen hypothesis asserts that genetic mixing reduces the impact of parasitic attack by increasing genotypic variability; while the mutational hypothesis asserts that it reduces the rate at which harmful mutations accumulate.

The new study finds that of these two theories, only the red queen hypothesis can confer an advantage to biparental sex over asexual reproduction without conferring an even greater advantage to triparental sex.

''Why Sex? And Why Only in Pairs?'' by Motty Perry, Philip J. Reny and Arthur J. Robson is published in the December 2017 issue of the Economic Journal. Motty Perry is at the University of Warwick. Philip J. Reny is at the University of Chicago. Arthur J. Robson is at Simon Fraser University.