Via Danny Yee, I see the Economist's take on a recent paper in Nature Genetics tracking the relative rates of geographic spread and temporal turn-over of human mitochondrial DNA and Y chromosomes. (I'd link to the paper itself, but the Nature Genetics website appears, astonishingly, to be completely non-functional at the moment. I'll try to post an update when it works again.)
Since everyone inherits their mitochondria from their mother, and men inherit their Y chromosomes from their fathers, these give us some indication of sexual differences in reproductive behavior. In what should not come as a surprise to anyone, the effective male population size is smaller than the female population size. Any given generation is descended from a fraction of the women alive in the previous generation, and in most species (including human beings) this distribution is somewhat skewed, so that less than half of the women have more than half of the children. What this paper shows is that the distribution of men's offspring is, averaging over several tens of thousands of years, even more highly skewed. Since every child has, necessarily, exactly one mother and one father, it follows that many women must've had children with men who had children with many other women. (Actually, a slight caution is in order here. Strictly speaking, this work only shows that a disproportionately small fraction of men have fathered the next generation's males. I suppose they could've been under-represented among the fathers of the next generation's females, but it's hard to see how.) Slightly more informative is the finding that Y chromosomes tend to diffuse more rapidly over geographic space than do mitochondrial DNA.
All very well, you say, but where's the bourgeois sexual morality? Well, here's how the The Economist article ends:
Although a moment's thought shows the old canard that males are actually, on average, more promiscuous than females cannot be true (since every reproductive act involves one of each) biologists have known for a long time that in most species males want to be more promiscuous than females. What holds them back is that females are choosy. And females also tend to be similar in their tastes, which means some males get chosen far more often than others, and therefore have more offspring. Females, by contrast, tend to have about the same number of offspring each.Now, the explanations which came immediately to my mind had nothing to do with female choice and the attractiveness of "tall, dark strangers from another place" (or even strangers from the same place), but were polygyny and conquest.
One result which did surprise the researchers was that men's genes tend to travel farther than women's. Some 70% of the world's modern cultures practice patrilocality --- in which a woman moves from her native village to her husband's village when she marries. Until now, it has widely been assumed that this practice would result in women's genes migrating farther afield than men's. So, not only are fewer men than women procreating, but they are travelling farther to sow their oats. Clearly, the tall, dark stranger from another place has been an attractive proposition to women for quite some time.
As to the first, it's a simple matter of historical fact that many (maybe most) settled societies have practiced some degree of polygyny, generally restricted to the upper classes. Whether this practice is recognized through formal marriages (e.g., the case of my great-grandfather), or semi-official mistresses and concubines (e.g., M. Mitterand) or completely unofficial (e.g., Mr. Jefferson; Mr. Thurmond), it reduces the effective male population size, by making upper-class men the fathers of a excessively large share of the next generation. (I assume that these men are at least marginally able to keep other men from having reproductive sex with these women.) Female choice doesn't enter into it. However, the social conditions necessary for such practices probably don't obtain in foraging bands, in which we've spent most of our evolutionary history, and indeed in which female choice was probably rather important (see Sarah Blaffer Hrdy, Mother Nature).
Similarly for conquest. There is a reason that people from the Orkneys have Y chromosomes which resemble those of people from Scandinavia, while their mitochondrial DNA is not particularly similar, and that reason is not a lot of smooth-talking Norse traveling salesmen. (I am sadly unable to find online the classic Far Side cartoon in which the Viking chieftain exhorts his men "Dammit, how many times do I have to tell you? First rape and pillage, then burn!") In fact, conquest has often been accompanied by polygyny on the part of the conquerors and their descendants, which is why an astonishing fraction of men from Eurasia are direct, male-line descendants of a Mongol who lived around a thousand years ago.
It probably means something that the writer at The Economist and I can look at the same results, and where I interpret them as telling a horrifying story of force, subjection and oppression of the most brutal kind, they see a Harlequin romance novel (or are they still "Mills & Boon" in the UK?) which deviates slightly from the publisher's guidelines at the end. But it's clearly not my place to say what this signifies.
Posted at September 27, 2004 15:38 | permanent link