Map of life expectancy at birth from Global Education Project.

Friday, May 30, 2008


I'll do drugs and rock and roll later. So, as it turns out, no doubt much to your surprise, the whole subject of sex is very interesting. I would say, after a bit of rooting around in the wilds of biology, that the origin of sex is actually as big a puzzle as the origin of life, if not an even greater one. Creationists, in fact, are all over it - in their view, sex could not possibly have evolved, so there you go.

Biologists no only wonder about where sex came from, they wonder almost as much about why it has stuck around. However, I must say that even though I'm not a specialist in the subject, I don't find that puzzling at all. Maybe having a bit of distance makes it seem clearer. Understanding why sexual reproduction is so strongly maintained, and is so nearly ubiquitous among the eukaryotes and particularly the metazoa, is actually very helpful in thinking about how it got started, so let me begin with the easier question.

It seems, at first, that sexual reproduction is a big disadvantage. First of all, you have to find a mate, which requires luck and effort. Second, your own genes get diluted, so if evolution is about gene selection, that would seem a big strike against sexual reproduction. You have to invest in making gametes which has some energy cost, and the process of fusing gametes to make a zygote can fail - it often does, in fact - reducing your rate of reproduction. Sexual selection pressure can drive evolution in odd directions, such as making big showy tails that attract predators and slow you down, or investing a lot in unproductive behaviors. (Tell me about it.) And the close physical contact required for mating creates one more way for pathogens to get around. So why bother with the whole thing?

Imagine two lineages of similar creatures, one reproducing sexually, the other reproducing asexually. Both have to live with pathogens and parasites, most of which are asexually reproducing prokaryotes that have very fast generational turnover. The asexually reproducing lineage has almost no genetic diversity. Occasionally a random mutation will arise, which will probably kill the organism, though once in a while a mutation will be tolerable or even favorable and the mutated lineage will persist. Nevertheless, a mutation in one lineage can never get combined with a mutation in another; 100% of the animals are reproductively isolated. They don't mate and they don't mix their genes. If a pathogen evolves that can evade their immune systems, they're wiped out, extinct, so long, sayonara, good bye.

The sexually reproducing species, however, is genetically diverse. Harmless or favorable mutations that arise over time will eventually spread, in all possible mixtures, through the progeny. Some that might even be harmful if expressed survive because they are autosomal recessive. When the new pathogen comes along, it is likely that some of the individuals in the population will just happen to be resistant to it. They and their offspring will survive and go on to rebuild the population.

It gets even better than that because all that mixing and matching of genes creates all sorts of combinations that might just happen to be better than any one mutation by itself. A mutation that is harmful in the asexual lineage might just turn out to be favorable if paired with some other mutation, but the opportunity will only come along if there is sexual reproduction. In a nutshell, the sexually reproducing species can evolve faster, and create much more innovation. Hence it is no surprise that all complex, multicellular organisms reproduce sexually. If they didn't, they never would have developed such complexity, and that's why asexually reproducing organisms are all microscopic, one-celled creatures. Some multicellular organisms, notably plants, can also reproduce asexually, which is handy for producing lots of new individuals fast and covering a hillside with daffodils or whatever; but also having sexual reproduction gives them all the advantages noted above as well.

Now, as for how the whole thing got started, that's a tough one. Lots of speculation actually centers around infections and parasitism -- two organisms getting their genes all mixed up together and then having to sort them out and mix them up again every generation. Maybe that Y chromosome is all that's left of a parasite.

Anyway, I'm not going there because I don't know where I tread. But I do know that evolution is inventive, but also conservative. It can only work with what it's got, which is why we don't have three arms even though it might be useful, or an eye in back to see what's gaining on us. So we're a social species, with very complex social structures and behaviors, and there just happen to be two different kinds of us, since we reproduce sexually. Now, evolution does drive a certain amount of sexual dimorphism simply because of the reproductive function. Men and women have different parts because they have to. The equation of investment in offspring also differs between men and women, and there's nursing as well. But it would be surprising if other kinds of sexual differences, in capacities or behavior, that one way or another enhanced the success of the species didn't also emerge.

It was fashionable back in the 1970s and '80s to believe that the differences in gender roles were all socially determined, and that human societies were possible in which the only differences between men and women were the direct consequences of gestating and suckling. Now I think it's becoming clear that much about gender roles is indeed socially determined and is highly mutable, but some things are not. There are differences between male and female brains and behavioral proclivities -- statistical averages, not absolutes -- and proclivities for different patterns of mating behavior, attachment to children, etc. For a while, it wasn't really possible to study these matters, and trying to sort out what is culture and what is wiring was not even permitted. But now people are working on it, and I do believe this will help us to know ourselves better.

From here, it gets to be pretty tricky. Sensitivities, and dudgeon, are high.

And don't get me wrong: It's important to study these issues in part because we might just like the answers after all:

"The so-called gender gap in math skills seems to be at least partially correlated to environmental factors," says economist and study researcher Paola Sapienza, PhD of Northwestern University's Kellogg School of Management. "The gap doesn't exist in countries in which men and women have access to similar resources and opportunities."

No comments: