Sex: why do we bother?

kissing. Fernando Meyer/Flickr
Extatosoma tiaratum AKA the Giant Prickly Stick Insect AKA Macleay’s Spectre.

As a society, we seem to be obsessed by sex. So wondering out loud why we have it is likely to invite a highly bemused response.

And yet sex remains one of the great, enduring mysteries of evolutionary biology. Why? Because the costs seem to outweigh the benefits, which means it ought to be eliminated by natural selection.

Having sex can have serious costs. These include the possibility of contracting a sexually transmitted disease, the risk of attack by a predator (though admittedly more of an issue for animals than humans), or simply the time and resources necessary to acquire a partner that might be better directed elsewhere.

But the real killer is that sexual reproduction, which invariably involves males, is vastly less efficient than asexual reproduction, in which males are redundant.

Asexually reproducing females produce offspring through a process known as parthenogenesis, in which unfertilised eggs develop as clones of their mother. Aphids are a familiar example, especially to keen gardeners, who may despair at the speed with which they can overwhelm their roses.

Asexual reproduction provides a huge numerical advantage over sexual reproduction because asexuals produce daughters only; whereas sexually reproducing females must produce sons and daughters.

This effect is so overwhelming that it is usually referred to as the cost of sex.

And yet, paradoxically, the vast majority of complex organisms reproduce sexually — only 0.1% of animal species are exclusively asexual.

The sex conundrum

The nineteenth century biologist August Weismann was among the first to recognise sex as a problem for evolutionary biologists. He thought it provided potentially beneficial genetic variation that ensured the survival of the species.

This theme was subsequently championed by biologists, including the revered geneticist and statistician Sir Ronald Fisher.

Although intuitively appealing, such explanations (sometimes referred to as the “Vicar of Bray” ideas, after the English folk song and comic opera) miss a small but important point: natural selection favours traits that benefit individuals, not species.

Contemporary explanations for sex tend to compare the benefits to a mother from producing sexual or asexual offspring, and these benefits fall into two camps.

The first proposes that deleterious mutations are likely to accumulate over generations in asexually reproducing individuals. Sex, through shuffling genetic combinations, purges these mutations.

The second proposes that sex produces genetically variable offspring, some of whom may thrive in an uncertain, and dramatically different, future environment.

Given the numerous explanations for sex, why is it still a mystery? The reason, quite simply, is that testing these ideas has proved challenging.

As noted in Nature last year, experimental analyses of the evolution of sex within populations are extremely rare.

What can be so difficult about comparing the fitness, or reproductive success, of females that reproduce sexually and asexually?

The problem is that sexually and asexually reproducing females typically differ in other ways. For example, females that reproduce asexually are often the hybrid offspring of individuals from different species or even genetically distinct populations.

So comparing asexual females with their sexual counterparts says as much about differences between hybrids and non-hybrids as between their different modes of reproduction.

Nevertheless, females of some species, especially insects, can reproduce either sexually or asexually. These species seem promising models for study, but unfortunately the mode of reproduction typically differs according to the season, the location or the condition of the female.

Aphids, for example, switch from asexual to sexual reproduction in late autumn. In other species, sex depends on the female’s condition, with mothers in poor physical condition, for example through starvation, typically reproducing sexually.

While these species allow comparisons within populations, we are still faced with the problem of teasing apart the role of sex from other influences.

For example, if females in good condition reproduce asexually and females in poor condition reproduce sexually, then any differences in offspring survival can be explained by both mode of conception and maternal condition.

Stick insects

Luckily, females of Macleay’s Spectre, an Australian stick insect found in Queensland rainforests, pet shops globally and online, can lay viable eggs with or without the benefit of a male.

This ability to produce sexual or asexual offspring allows females to avoid reproductive failure if a male doesn’t turn up. It also allows us to compare the viability of their sexual or asexual progeny, without the ambiguity of earlier studies.

Why do we need to know?

Is it important, beyond academic interest, that we solve this biological riddle? Quite possibly. It turns out that insect pests are over-represented by species capable of asexual reproduction.

Why this should be so is not clear, but this pattern has implications for the way we think about controlling these pests — especially as the mode of reproduction will influence the speed of evolution of resistant strains.

Understanding the costs and benefits of different modes of reproduction will be helpful, even for stick insects as some species are pests of native forests.

Of course, males will remain useful for our own species, unless females develop the capacity to reproduce asexually.

The chances of that are remote, but in time we should know why we bother with sex.

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