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The other good thing about sex

DAVID P. BARASH, an evolutionary biologist, is professor of psychology at the University of Washington.

IN SAMUEL BECKETT’S “Waiting for Godot,” two tramps -- Vladimir and Estragon -- wait to see if Godot will arrive. Today, in evolution’s worldwide theater of the worrisome and real, we’re all waiting to see if the bird flu virus will get around to attacking us big time. Godot never showed up; H5N1 just might.

On the other hand, if the dreaded bird flu pandemic doesn’t appear, it may be due to luck, or the quarantine and slaughter of infected animals, or other timely and effective public health measures (of which admittedly there have been precious few thus far), or -- oddly enough -- sex.

There appears to be a curious connection between sex and disease, one that evolutionary biologists have only recently come to appreciate, and that cuts against the grain of conventional wisdom -- which equates sex with sexually transmitted diseases. Thus, biologists have long scratched their collective heads about sex, starting with this conundrum: Sex isn’t necessary.

Lots of living things reproduce by parthenogenesis (the development of unfertilized eggs) or simply by sending out shoots or buds. Not only does asexual reproduction avoid the many direct hassles of sex -- the need to find a suitable mate, the time and effort of courtship, the risk of being injured or infected during the act -- it also gets around a huge genetic drawback: Genes within a sexually reproducing creature enjoy only a 50% chance that they will be transmitted to any given offspring, whereas asexual reproduction guarantees that each gene has a 100% certainty of being projected into the future. And projecting genes into the future is what evolution is all about.

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This 50% cost imposed by sexual reproduction had long troubled evolutionary scientists. Until, that is, British biologist William D. Hamilton came up with the idea that sexual reproduction might be a tactic in an evolutionary arms race between hosts and their diseases.

First, let’s face the disconcerting fact that there are many more of them (pathogens and parasites) than us (free-living organisms). After all, every multicellular critter is home to thousands, often millions, of internal free-loaders. Considering just one group of worms, invertebrate biologist Ralph Buchsbaum has suggested that “if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable.”

Nematodes and all the rest seek to live at our expense; we, in turn, seek to thwart them. If we, the unwitting and unwilling hosts, stay genetically the same from one generation to the next, then we are sitting ducks, easy targets for “them.”

Enter sex. By mixing and matching our genes, sexually reproducing creatures bob and weave, creating new genetic combinations with every bout of reproduction, confounding -- or at least challenging -- our pathogens and parasites by creating moving targets instead of sitting ducks. Because of their generally short life spans, pathogens can evolve rapidly compared with ourselves; via the diversity-creating mechanism of sex, we level -- somewhat -- the evolutionary playing field. At least some bacteria, worms and viruses are unable to draw a bead on our descendants.

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Maybe -- unlike Godot -- a bird flu plague will arrive after all. While we wait to find out, at least we have something to do to amuse ourselves. And if we succeed in dodging the bullet, we might want to offer thanks -- not only to veterinarians, virologists, public health workers and the gods of our choice -- but also to sex.


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