Why is there sex?

Sex is retained by the overwhelming majority of plants and animals. Around a tenth of one per cent of named animal species are reckoned exclusively asexual, and under one per cent of flowering plants substantially asexual. Purely asexual lineages exist and are usually young: virtually all of them sit at the tips of the tree of life rather than as deep branches.

That pattern suggests asexual lineages arise often and go extinct quickly, which is itself evidence about what sex is for.

Recombination breaks up combinations of genes. That lets a beneficial mutation spread without carrying its neighbours along, and lets a harmful one be removed without discarding the good genes beside it.

In an asexual lineage, mutations accumulate in the genome they arose in and cannot be separated from it. H. J. Muller saw what follows, in 1964. In a finite population, drift can remove the least-damaged class of individuals, and a sexual population can rebuild that class by recombining what is left while an asexual one has to wait for the right back mutation. The asexual line’s fitness decays, and the process is called Muller’s ratchet.

In 2011 a study took populations of a nematode that can both self-fertilise and outcross, and exposed them to a bacterial pathogen that was itself allowed to evolve against them.

Outcrossing rose from about a fifth to over seventy per cent in the first eight generations whether the pathogen was coevolving or fixed. Only the populations facing the coevolving one held it there. Against a fixed pathogen the rate slid back to the level of populations facing nothing at all.

Obligately self-fertilising populations exposed to the coevolving parasite were extinct within twenty generations, and none died out under the fixed pathogen or under no pathogen at all. That is the Red Queen hypothesis, which Jaenike, Hamilton and Lloyd put forward around 1980, tested rather than argued, in a laboratory, with an outcome.

Experiments have tested both families. A coevolving bacterial pathogen kept nematode populations outcrossing. Populations of the same worm put under a raised mutation rate also favoured outcrossing over selfing, and the more a population outcrossed, the better it held its fitness.

Both effects are real in the laboratory. Whether either is large enough in a natural population to pay a twofold cost is the open part: Otto’s reading, in a 2009 review, is that there is likely to be some truth in each of the explanations she surveys, depending on the organism, and that too little is known from natural populations to settle it.

Many organisms reproduce sexually only sometimes: aphids and water fleas turn to sex when their condition is poor, and a large fraction of plants can spread by runners or set asexual seed while still flowering.

In Otto’s models a modifier that raises sex to a low level spreads even against a twofold cost, and one that raises it to a high level does not, because the cost climbs with the amount of sex while the benefit does not. That makes the interesting question not why sex exists but why obligate sex exists in animals like us.