Free Women After All: Sex, Evolution, and the End of Male Supremacy by Melvin Konner Page B

is Trichomonas vaginalis, known as “trich”—pronounced trick, appropriately from the human viewpoint—the protozoan that gives vaginas and penises the wrong kind of itch. Ironically, trich is asexual, except for those gene swaps; yet it may have had a sexual ancestor. The way we know this holds a clue to the origin of sex.
    Trich is capable of meiosis ( my-oh-sis ). This bane of high school biology comes down to something simple. Cell division, or mitosis ( my-toe-sis ), yields two identical daughter cells. It goes on in many parts of our bodies all the time. Meiosis differs in that it randomly allots half the genes or chromosomes of the dividing cell to each of the two daughters. This happens when we make eggs or sperm, so that when they join they can create something new. Gene swaps without meiosis involve a risk of mismatch between the parts of the genomes traded and their future genetic hosts. Meiosis elegantly divides each partner’s genome in half so that the halves can combine with their complements in the mate; this reduces the mistakes made when mates mix their genes. It’s the same principle as egg and sperm, except that the combining halves are not so different. But the advantages of meiosis may help explain why sex evolved and stuck in so many species—it’s a better way to mix and match genes and get the gold ring of adaptive variation.
    The sexual version of this unfolded two billion years ago amongsome one-celled organisms, and most plants and animals have stuck with it. But some, like the vaginal trich and the virginal whiptail, somehow became asexual again. It’s difficult to know why, but in one rather startling animal a reversal has been followed in real time, and it gives us clues to how sex evolved in the first place.
    The creature is the asexual New Zealand mud snail. At least it used to be asexual. In a wonderful case of evolutionary change seen in a human lifetime—there are a lot of these, despite what creationists claim—these snails re-evolved sex. They became infected with a worm, called Microphallus because it sterilizes the snails—male or female, sexual or asexual. This, needless to say, is bad for the guy or gal and the species.
    In the lab, when snails are deliberately infected with this worm, both snails and worms evolve faster—the Red Queen running in place again. But the real power of this research is that it shows that in nature, in New Zealand lakes, a small minority of snails that became sexual outevolved their far more common asexual counterparts. Microphallus quickly wiped out the most common asexual clones, and the more resistant ones were outbred by those snails that (re)invented males. So the Red Queen favored the sexual snails, crowding out the offspring of what we might call virgin queens.
    Nevertheless, as we’ve seen, some all-female species do persist in nature. In fact, as Olivia Judson and Benjamin Normark point out in “Ancient Asexual Scandals,” some of those may never have been sexual. Other species consist of hermaphrodites—individuals that are both male and female, making both eggs and sperm—and these can teach us a lot about our own battle of the sexes. Ordinary garden snails are an example. In a pinch, they can fertilize themselves (“selfing,” for short), but instead they mostly seek an equally versatile partner. Now it gets interesting. Each two-sex snail tries to inject sperm into the other. They mate for hours, so presumably it’s fun. But they are competing, even if both succeed.
    In one of the strangest mating rituals in nature, they shoot love darts—that’s the scientific term—a few millimeters long into each other. The dart looks like a spear point, and when snails mate, one or both will have a dart lodged in its body, like an arrow sticking out of an enemy. The dart usually does no great harm, but it does seem to hurt and can leave lasting damage. So why inject it? Because it’s coated with hormone-containing mucus that aids