Why Do We Have Sex (Apart from the Obvious Reason)?

Excerpted from New Scientist: The Origin of (almost) Everything, written by Graham Lawton and illustrated by Jennifer Daniel.

The birds and the bees and, of course, the fleas. Plants, fungi and amoebas too. It sometimes seems like sex is everywhere. But in biological terms, it is a minority pursuit. For the first 2 billion years of life on Earth, it didn’t exist. Even now, the organisms that dominate the planet – bacteria and archaea –don’t bother.

The origin of sex, then, is a bit of a mystery. And if its origins are hard to understand, its function is just as baffling.

At first sight that seems ludicrous. Surely sex has an obvious function: it generates variation, the raw material for evolution. The reshuffling and recombining of genetic information helps species adapt. It can also help spread beneficial genes throughout a population and eliminate harmful ones.

But there are big problems with this common sense argument. The first is that sex is grossly inefficient. It makes much more sense to clone yourself. Cloning produces many more offspring than sex, which means that asexual species should rapidly drive sexual ones to extinction by dint of producing far more offspring competing for the same resources.

What’s more, each clone has a combination of genes that has already been shown to be fit for purpose. Sex, by contrast, creates new, untested and possibly inferior combinations. In fact, sexual recombination disrupts favourable gene combinations more often than it generates them.

Sure, sex should be an advantage in the long term, over thousands and millions of years. Asexual species eventually accumulate mutations that they can’t get rid of, and which drive them to extinction. But evolution doesn’t work like that. It doesn’t plan ahead. All it cares about is the here and now.

And the trials and tribulations don’t end there. Sexual species have to find a mate, fight off rivals, and risk catching sexually transmitted diseases.

Finally, if sex is so beneficial, why is it that bacteria and archaea never evolved it, even though they do exchange bits of DNA from time to time? Conversely, if asexual reproduction is so great, why do almost all eukaryotes reproduce sexually at least some of the time? All this makes sex one of the biggest head- scratchers in biology.

For many years the best answer was the Red Queen hypothesis, a subtle variant on the ‘sex means variety’ explanation. This imagines an arms race between parasites and their hosts. The parasites’ generation time is so short that they can out-evolve their hosts. By throwing up new mixtures of genes with each generation, sex enables at least a few individuals to survive. It is named after the Red Queen because, like Alice in Through the Looking Glass, we have to run fast just to stay in the same place.

Unfortunately, it does not solve the problem. Parasites give sex a decisive advantage only when parasite transmission is very high and their effects are very serious. Under normal circumstances, clones still win.

In recent years a new explanation has started to take hold. This is based on the discovery that all eukaryotes are, or at least were, sexual (there are plenty of species that multiply by cloning, but they evolved celibacy only very recently). The logical conclusion is that sex evolved very early on in the eukaryote lineage, in a common ancestor of all living eukaryotes around 2 billion years ago.

Aside from sex, the other thing that unites all eukaryotes is the possession of mitochondria, the cell’s power supply. The new explanation claims that this is no coincidence: mitochondria made the evolution of sex inevitable. How so? The key point is that mitochondria have their own genomes. This is a remnant of the complete genome of the free-living bacterium that was engulfed at the dawn of eukaryote evolution. We know that as the two co-evolved, most of the genes were transferred to the host’s genome. The symbiont also bombarded the host with parasitic jumping genes.

Love conquers all

In other words, the acquisition of mitochondria unleashed a bout of turbulent genetic disruption. Under such high mutation pressure, the balance was tilted and sex became more advantageous than asexual reproduction. Any early eukaryote that evolved it would have outcompeted its asexual rivals, which were succumbing to unsurvivable levels of mutation.

Mitochondria also explain why sex remains advantageous today. The mitochondrial genome encodes vital genes, but cannot do anything on its own. It relies on the nuclear genome to make proteins and replicate its DNA, for example. Close cooperation between the cells’ two genomes is therefore vital to the functioning of the cell, especially in the crucial task of energy generation.

That cooperation is what sex ensures. Because the mitochondrial genome accumulates mutations at a higher rate than the nuclear genome – about 10 times faster in mammals – the accord between the two genomes gradually breaks down. We and our mitochondria are drifting apart, and though it is the mitochondria’s fault, we are the ones who suffer.

Sex resolves this disharmony by throwing out new combinations of nuclear genes that are more compatible with the mitochondria’s needs.

That is the why of sex. The how, however, remains very unclear. The simplest eukaryotes – amoebas – have sex by splitting their genome in half and then cleaving themselves in two, with half a genome in each portion; these half-amoebas then merge with others to create new individuals. That may be how the first sex was done.

In broad brush terms, it is still how it is done. Sex just means ripping a genome in half and uniting it with another half-genome from someone else to create a new whole genome. Humans and most other animals achieve that by having two sexes, one of which dumps their half-genomes into the other through the act of copulation.

Who said romance was dead?