The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [41]
There were more males in lakes. About twelve per cent of snails in the average lake are male, compared to two per cent in the average stream. So the Vicar of Bray is ruled out. But there are also more parasites in lakes, so the Red Queen is not ruled out. Indeed, the closer he looked the more promising the Red Queen seemed to be. There were no highly sexual populations without parasites.59
But Lively could not dismiss the tangled bank. So he returned to New Zealand and repeated his survey this time intent on finding out whether the snails and their parasites were genetically adapted to each other. He took parasites from one lake and tried to infect snails from another lake the other side of the Southern Alps. In every case, the parasites were better at infecting snails from their own lake. At first this sounds like bad news for the Red Queen, but Lively realized it was not. It is a very host-centred view to expect greater resistance in the home lake. The parasite is constantly trying to outwit the snail’s defences so it is likely to be only one molecular step behind the snail in changing its keys to suit the snail’s locks. Snails from another lake have altogether different locks. But since the parasite in question, a little creature called Microphallus, actually castrates the snail, it grants enormous relative success to the snails with new locks. Lively is now doing the crucial experiment in the laboratory, to see whether the presence of parasites actually prevents an asexual snail from displacing a sexual one.60
The case of the New Zealand snails has done much to satisfy critics of the Red Queen, but they have been even more impressed by another of Lively’s studies, of a little fish in Mexico called the topminnow. The topminnow sometimes hybridizes with another similar fish to produce a triploid hybrid (i.e., a fish that stores its genes in triplicate, like a bureaucrat). The hybrid fish are incapable of sexual reproduction, but each female will, as a virgin, produce virgin clones of herself so long as she receives sperm from a normal fish. Lively and Robert Vrijenhoek of Rutgers University in New Jersey caught topminnows in each of three different pools and counted the number of cysts caused by black-spot disease, a form of worm infection. The bigger the fish, the more black spots. But in the first pool, Log pool, the hybrids had far more spots than the sexual topminnows especially when large. In the second pool, Sandal pool, where two different asexual clones coexisted, those from the commoner clone were the more parasitized; the rarer clones and the sexual topminnows were largely immune. This was what Lively had predicted, for he reasoned that the worms would adjust their keys to the commonest locks in the pond, which would be those of the commonest clone. Why? Because a worm would always have a greater chance of encountering the commonest lock than of encountering any other lock. The rare clone would be safe, as would the sexual topminnows, each of which had a different lock.
But even more intriguing was the third pool, Heart pool. This pool had dried up in a drought in 1976 and had been recolonized two years later by just a few topminnows. So by 1983 all the topminnows there were highly inbred and the sexual ones were more susceptible to black spots than the clones in the same pool. Soon more than ninety-five per cent of the topminnows in Heart pool were asexual clones. This, too, fits the Red Queen theory, for sex is no good if there is no genetic variety: it’s no good changing the locks if there is only one type of lock available. Lively and Vrijenhoek introduced some more sexual female