An introduction to sex


‘Nothing in life is more important, more interesting – or more troublesome.’ So says Dr Tatiana, sex expert to all creation, and here she explains why it’s something most of us can’t live without.

If not for sex, much of what is flamboyant and beautiful in nature would not exist. Plants would not bloom. Birds would not sing. Deer would not sprout antlers. Hearts would not beat so fast. But ask an assortment of creatures, what is sex? and they will give you different answers. Humans and many other species will say copulation. Frogs and most fish will say the squirting of eggs and sperm in joint shudders of spawning. Scorpions, millipedes, and salamanders will tell you that sex is packets of sperm deposited on the ground for the female to sit on so they’ll explode into her reproductive tract. A sea urchin will say sex is releasing eggs and sperm into the sea in the hope that they will, somehow, find each other in the waves. For flowering plants, sex is trusting the wind or an insect to carry pollen to a receptive female flower.

The means to an end
To succeed, each of these methods requires a suite of different features. A male flower who wishes to be a Lothario and have his pollen strewn to as many mates as possible must seduce not female flowers but bees. Other creatures must wear gaudy costumes, be they fancy feathers or frivolous fins; they must sing and dance for hours and hours; they must perform prodigious feats, building and rebuilding nests and bowers. In short, they must expend enormous energy shouting, ‘Choose me, choose me.’ And all for – what?

In truth, these various practices are just the means to an end. The ultimate sex act – the act that all these antics have evolved to accomplish – is the mixing of genes, the creation of an individual with a new genetic make-up. To a miserable organism sitting alone in a singles bar, genetic mixing might not seem worth the bother. Yet it is fundamental to the grand scheme of things. To see why, let’s take a step back and think about how evolution works.

Survival of the fittest
For most of us, caught up in the hurly-burly of our daily struggles, the purpose of life may seem elusive. But from an evolutionary point of view, the purposes of life are clear: survival and reproduction. If you fail at either, your genes go with you to the grave. If you succeed at both, you pass your genes on to your children. Inevitably – such is life – some organisms do better than others at surviving and reproducing. If everyone had identical genes, then differences in survival and reproduction would be due to luck, not genes. But usually individuals have different genes. And insofar as a particular gene confers an advantage in terms of survival or reproduction, that gene will spread.

This simple process, discovered by Charles Darwin and Alfred Russell Wallace in the nineteenth century, is the principal mechanism of evolution. It is known as natural selection. Sometimes the process is fast and easy to see. Suppose a poison – an antibiotic, say, or an insecticide – appears in the environment. And suppose that surviving the poison depends on having a particular gene. Those who do not have the gene will die, their genes ‘deselected’ from the population. In the extreme case, no one has the resistance gene, everyone dies, and the population becomes extinct. More often than not, however, some individuals are fortunate and have a gene to resist the poison. Since these individuals are the only ones to survive and reproduce, the genetic makeup of the population will shift to one where everybody is resistant.

Mutation and sex
Thus, genetic variation is crucial: no genetic variation, no evolution. But where does genetic variation come from? There are two main sources: mutation and sex. Mutation, or random changes to information contained in genes, is the more primitive of the two. Mutations arise from errors made by the cell’s genetic copying machinery. Since no scribe is perfect, some errors are inevitable – which is just as well. Whereas sex produces new combinations of genes that already exist, mutation creates altogether new genes – and thus generates the raw material of evolution. Without mutation, evolution would grind to a halt.

Yet mutation by itself is not enough. From time to time organisms evolve to give up sex, reproducing asexually instead. When this happens, any genetic differences between a parent and child are, by definition, due to mutation only. At first, asexual organisms often flourish. But their glory is fleeting. For reasons that remain mysterious, the loss of sex is almost always followed by swift extinction. Apparently, without sex you are doomed.

Seduction and survival
Which is not to say that sex makes life easy. No matter how good your survival skills are – you can be the champion at evading predators, or have the best nose for finding food, or be immune to every disease – it will all be for naught if you cannot find, impress and seduce a mate. Worse, success at seduction is often at odds with survival. If you’re a bird, flaunting an enormous tail may make you quite the cock among hens – but it may also make you lunch for a cat. Worse still, the competition for mates is often exceedingly stiff.

The upshot of all this is that the need to find and seduce a mate is among the most powerful forces in evolution. Perhaps nothing in life generates a more ecstatic diversity of tactics and stratagems, a more surprising array of forms and behaviours. In comparison, tricks to avoid predators seem predictable and limited. They typically include one or more of the following features: going about in groups, moving fast, blending in with your surroundings, looking scary, sporting a shell or sharp bits, or tasting revolting. But tricks to seduce a partner – ah, here the variety is endless.

How did sex begin?
Alas, we may never know for sure. After all, some sort of genetic exchange probably started soon after life appeared about four billion years ago – and looking that far back in time is an uncertain business at best. But there are lots of wacky ideas. Let’s take a quick glance at some of them.

Microbes not that different from modern bacteria seem to have evolved shortly after the origin of life, so it’s tempting to imagine that these primordial beasties started swapping strings of DNA to replace or repair genes that had broken. A second, more exotic idea is that sex was simply infectious. In other words, it arose because a segment of DNA promoted gene exchange in order to spread itself through the population. To use an analogy, it’s as though the common cold caused humans to be promiscuous – an effect that would clearly enhance its transmission.

However speculative the origins of bacterial sex, though, the ideas look like a mighty edifice compared with how little we know about the origins of the sort of sex that humans, birds, bees, fleas, green algae, and other eukaryotes (those requiring that each parent donate one complete set of his or her genes) conduct. Probably it evolved only once. But exactly how or why is a deep mystery. Some say it arose as a result of cannibalism – one cell eats another and then collects its DNA. Others plump for DNA repair. And still others argue that this sort of sex, too, originated as a disease, with infectious genetic elements promoting their own spread.