August 2, 2009

Why sexual reproduction is so popular


There are organisms that reproduce sexually and there are organisms that reproduce asexually. The former outnumber the latter by many orders of magnitude. Hell, even plants have sex on a pretty regular basis–though it’s a sort of kinky, bee-assisted sex. The question is why? Asexual reproduction is far more efficient than its more popular cousin.

In New Zealand, scientists found an astonishing opportunity to test one hypothesis. Snails are known to reproduce both sexually and asexually. In the fresh waters of New Zealand, both types of snail live side by side. So they could watch, over time, the changes in populations.

Here’s the thing. Evolution is affected by pressures. Behaviors and adaptations result for many different reasons, but the need to survive appears to be the primary factor in most adaptations–there are exceptions to this, of course. And so, the fact that sexual reproduction, which is terribly inefficient, is more popular than asexual reproduction is something of a mystery. Aside from the fact that it’s more fun.

They hypothesis that these researchers in New Zealand were testing has to do with parasites. Parasites infect all organisms. Consider that a population of snails that reproduces asexually essentially just clones itself off ad infinitum. Every child is genetically identical to its parent. It inherits all of its parent’s strengths, and, more importantly for this discussion, all of its weaknesses.

The offspring of a species that reproduces sexually is genetically unique, as it’s a sort of random mixture of its parents’ genes. So what’s a parasite likely to have an easier time with? A species that’s always the same, with the same defenses and the same weaknesses to exploit? Or a species whose members are always going to be different, have different chemistries, have different defenses?

The study showed that the population of asexually producing snails showed a marked reduction over time as a result of infection from parasites. The sexually reproducing snails showed a far more stable population trend and far less susceptibility to infection by parasites. The evidence seems to show that the evolution of sex was influenced strongly by parasites.

The thing that I find interesting about these findings is what they mean when examined in light of other things that we know about sex evolution.

Consider: Sex evolved as a defense against parasites. Once it became established as the dominant reproductive activity, sex took on a life of its own.

Peacocks have these long, ornate tails that serve absolutely no survival purpose whatsoever. They are purely a result of sexual selection. They are used to attract a member of the opposite sex. In fact, survival-wise, peacocks must strike a delicate balance between ability to attract mates and ability to escape from predators. The tails are a hindrance in a survival situation, but they ensure that their genes will be passed on. And so there are two completely separate pressures affecting the size of peacock tail feathers. One pressure creates a trend toward a smaller tail and other toward a larger, more showy tail. A very interesting tension.

But then look at humans. We are interesting because, not only are we social creatures, but we are also sentient. The most intelligent species on this planet. Sexual selection in humans is a far, far, far, more complicated affair than even that of peacocks. Look at the bizarre and strange rituals that humans adopt. Look at the prejudices, the tragedies, the arguments, the problems, that all arise as a result of a defense mechanism against parasites.

Sex brings out the absolute worst in humankind. I suppose it wouldn’t be poetic if it weren’t also true that sex can bring out the absolute best in humans as well. Poetry and literature as we know it would probably not be possible if it weren’t for sex.

I just got married. I couldn’t be happier about it. I don’t necessarily believe that monogamy is the only way that humans can (or should) interact on a sexual level, but I have made a choice to join in that sort of relationship. The fact that we can choose what sort of relationship we want to be in is probably one of our biggest problems because anyone that wants to have a different sort of relationship than what is deemed normal is often ostracized or worse.

Perhaps the best literary example of what came about as a result of sex is the Trojan War. Imagine it. Helen of Troy. The “face that launched a thousand ships.” And also spawned one terrible Wolfgang Petersen movie. All of that, because of parasites.

Maybe the Transformers are better off without it.

Maybe not.

Liebe ist in Ihrem Herzen.

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