We know that a lot of organisms, from humans to bacteria to birds to bees, have lots of sex. But what has mystified scientists for years is WHY. I mean, it's fun and all (unless you're a poor beetle girl stuck with this), but what purpose does it serve? On the face of it, in fact, sex seems to be pretty BAD for about half of the population: the women. For example, there's a lizard out there than can reproduce both asexually and sexually. When it reproduces asexually, it producing nothing but girls, all of whom can reproduce both asexually and sexually. Net win. But when it reproduces sexually, it will produce about 50% BOYS, which can only reproduce sexually, and if they aren't lucky and their courtship messages suck, they won't even do that. Why bother?! Why not just keep reproducing asexually and passing your genes directly on (no mixing!) to hundreds of thousands of lizards?
For many years scientists puzzlesd over this one. But recently, a series of three new hypotheses has been formed. Welcome to the good, the bad, and the ugly.
I'm sure by now most of us are aware of how sexual reproduction works. You are usually diploid, having two sets of chromosomes. But for reproduction, you form a section of cells that are haploid, each only having one set. When this set meets a set from your chosen partner, recombination takes place, and you end up with an offspring that has half your genes and half your partner's. Good enough. But why should we do this? Why not pass on ALL our genes to our offspring, and not have to deal with other members of the species? Screw them, I want MY genes passed on!
Right now, there are three possible hypotheses as to why sexual reproduction takes place.
This hypothesis is based on the idea that sexual being, rather than asexual ones, can adapt to bad changing conditions faster. This is because, when you happen to get a nice beneficial gene, by combining it with the genes of someone else, you have the potential to get a REALLY beneficial set (you also have the potential to get a really crappy set, but you win some, you lose some). Asexual organisms, on the other hand, don't have this opportunity, and the beneficial genes will get passed on linearly, without potential for grouping up with other beneficial genes, unless those arise by chance. The net result of this is that you might end up with a slight advantage over your asexual peers. It doesn't sound like much, but when everyone's starving, that small gene and the potential for it to become even better, makes a big difference.
Of course, where there are good mutation, there can also be bad mutations. But sex may help us with those as well. All critters are going to pick up mutations as they go along, but asexual ones may pick ones up that are slightly deleterious, and these might accumulate in the population. Overtime, you could even get replacement of all the normal genes with deleterious genes, leading to a pretty crappy organism. And a crappy organism is a dead one.
How would sexual reproduction help this problem? The hint is being diploid. When you have TWO sets of genes, and one copy is crap, you can just use the other copy. This means that the bad mutations building up in the asexual population will take a lot longer to show in the sexual population, as they get covered up by their healthy counterparts.
What are the ugly? The ugly are parasites, bugs, illnesses. It's possible that being sexual could help us a lot in keeping parasites at bay. After all, usually an asexual population (think bacteria) starts up, and there's a population boom while the food is hot. Unfortunately, all the asexual guys are exactly alike. This means that, if one parasites knows the right place to hit, all the asexual animals are all vulnerable in exactly the same way. So you might end up with a "boom and bust" pattern, where the bacteria grow and grow and grow, and then get knocked down. The few that have good mutations and are resistant then grow and grow and grow, and get knocked down again. Lather, rinse, repeat.
Sexual organisms, on the other hand, might do better against pests. By constantly mixing genes and shuffling them up, they create new combinations, and new variations. And if a parasite happens to get at an organism in one specific way...well, that's not going to work as well. The end result is that a sexual population will stay a lot more stable than one that is asexual, as more sexual organisms are likely to be immune at one time.
So which is it? The good, the bad, or the ugly? Sci sure doesn't know, but she thinks it might be a clever combination of all three. Sex is fun AND useful. Get out there and mix your genes!
Zimmer, C. (2009). On the Origin of Sexual Reproduction Science, 324 (5932), 1254-1256 DOI: 10.1126/science.324_1254