A virus is a small collection of genes

What is the purpose of sexual reproduction ? Adel Fattah, Developmental Biologist The Naked Scientists

SEX: A short word. Often used. Often used to sell products in fact. Yet it is one of our base instincts, one of our prime motivations in life. I can't remember how many times I chose a seat on the train because a good-looking girl sat nearby, or bought a magazine because of the attractive woman on the cover. So why do we find the opposite sex so attractive? I mean, the act of sex itself is a pretty bizarre thing to do, all that jigging around and what about the mess? I mean we all look pretty hilarious whilst we do it (and the only reason they don't look daft in films is because they are not really doing it). If we didn't enjoy it so much would we really bother?

Would you go through all that if it weren't for the orgasm at the end? After all, there is a lot more to it than just the act of sex itself. There is the whole elaborate (and expensive) courtship display beforehand: the "asking her out", the "first date", a bit of food, a bit of wine, one thing leads to another... (Not on the first date, of course). Then there's the "I'd like to do this again sometime" and the 'should I call today, or wait a few days? I don't want to seem too keen'. Countered by "I'm washing my hair tonight". And so back to square one, until you get past the 'hair wash barrier'. Now you're in with a chance!

So basically we are all driven with a deep desire to mate and we will go through just about anything to achieve it (girls, the best time to ask a guy for that expensive treat is just beforehand). I remember all the embarrassing episodes that happened when I was a teenager. Many left me scarred for life, but none stopped my adolescent craving to 'go all the way'.

Propagation of Species

So a lot of questions, Adel, but what point are you trying to make? Well, I'm trying to make the point that sex is meant to be all about propagation of species or more specifically, it is all about propagation of our genetic material. But is the theory that we are simply vessels for the propagation of genetic material a little simplistic? As I alluded to above, there is a lot more to finding a mate than simply sex. If passing on genetic material is what it is all about why don't we just clone ourselves? No mess, no fuss, just pure transmission of your own genetic material! Many primitive animals do it every day. Some do it several times a day!

Which begs the question is there an advantage to sex? The whole point of sex is to mix the genes in the gene pool allowing the transmission of 50% your own and 50% your partner's genes to your young. The point of mixing the genes is to allow for variation in the gene pool. As genes are mixed new combinations arise some are useful to the survival of a species, others are detrimental. Those that carry detrimental genes are disadvantaged at mating and are thus less likely to pass on those detrimental genes. In contrast, those that possess genes, which confer an advantage, are more likely to survive predators and beat their competitors in the race to find a mate, and their genes are more likely to be passed on. This is the basis of Darwin's theory of evolution and the mechanism by which it happens is termed Natural Selection.

Monogamy versus Polygamy

So if the whole point of sex is to mix up the gene pool why not have multiple partners rather than a monogamous relationship? Why are humans (generally) monogamous, are we the only creatures to spend the majority of our lives with one partner? Is that what puts us apart from other animals? Not so. Take love birds for example, they remain in lifelong monogamous pairs, but they seem the exception rather than the rule. In contrast, many animals belong to the 'sowing wild oats' school of thought, for example, the male chimpanzee often invites females to mate by typically spreading his legs to reveal a bright red, erect penis that stands out against the black scrotum. Not recommended behaviour down the pub on a Friday. Similarly, some human males are of the 'kebab theory of women': a great idea after ten pints down the pub, but you wouldn't want to wake up next to one every day.

More seriously, creatures such as fish and especially sea urchins release their eggs and sperm into the sea and hope that some of each meet up and fertilise. Not strictly polygamy, granted. But if humans mated with numerous partners their genes would be spread further and as such, is monogamy not a disadvantage? The answer here may be quality not quantity. Monogamy in humans may have evolved because we need to nurse our young for many years before they become independent. A stable family life is important in order to make our offspring high achievers and thus attractive to other high achievers. In other words, by improving the quality of young we increase the chances of propagating our own genes successfully.

What's Love got to do with it?

OK. I've established we need sex and that for humans monogamy is preferred. So where does love fit into all this? Pah! I hear you say. "Love? That's not very scientific, is it? This essay is about the science behind sex, not the spiritual and mysterious nature of love.

"Aha!" I reply. But has love evolved to maintain monogamy? Those possessed by love will do many (often strange) things and unless you experience it yourself, one cannot explain how it can be an incredibly strong motivator. Arguably stronger than lust, and that's saying something! Many people have married 'the One', citing that they just knew it in their heart 'that was it'. Love is strong enough to keep couples together through thick and thin. 'All you need is love'. I wonder if there is some physiological basis to love? Perhaps it could be a balance of hormones that subtly affect our mental perceptions of our chosen partner versus other acquaintances. A little like the release of endorphins during pleasurable activity. But I suspect it is a higher cerebral function modified by social conditioning and cultural values. I'm no neuroscientist so I won't explore avenues I know little about.

What I want to argue is that the whole point of falling in love is a complex form of mate selection. A bit like the female peacock (peahen) that picks the peacock with the biggest and best-kept tail. As I hinted at before, it is women that are the choosy ones; the men are less so (driven by a deep desire to distribute their genes far and wide). So men need to attract women, and women need to choose a good husband and father. It is not something taken lightly and so love has evolved as a mechanism to secure a good mate and keep them together, at least until the kids have flown the nest.

The Evolutionary Arms Race

But why should we evolve? What's the point? Is there some spiritual force behind it all pushing species to evolve to perfection? Possibly, but it is not for the scientific method to explain matters in that way, that is for the philosophers to argue. One interesting theory raised in the book 'The Red Queen' is that sex has evolved as a protection against parasites. Daft, eh? Well, no. Let's break it down a little. Sex evolved to allow genetic variation. Genetic variation allows evolution to select the most hardy. Why do we need to be the most hardy. The answer is to beat infections, especially 'genetic infections': the viruses.

A virus is a small collection of genes that hijacks cells to take over their machinery to reproduce. Cells evolved mechanisms to keep viruses out, the viruses struck back by evolving ways around these defences. Cells retaliate by creating new and better defences. So the arms race continues. Such models have been run through computers comparing sexual versus asexual species and how they cope with a parasite invader. In all cases, asexual populations were wiped out within a few generations whilst sexual populations survived.

Of course there are other theories, such as sex was useful millions of years ago to ensure survival and was something we've been stuck with since, a sort of evolutionary left over. But the parasite theory is one that has a very convincing basis.