Eavesdrop morning coffee at any major centre of evolutionary theory today, and you will find ‘parasite’ to be one of the commonest words in the language. Parasites are touted as prime movers in the evolution of sex, promising the final solution to that problem of problems, the puzzle that led G. C. Williams to proclaim in 1975 ‘a kind of crisis’ at hand in evolutionary biology (Hamilton, 1980; Tooby, 1982; Seger & Hamilton, 1988). Parasites seem to offer a plausible justification for the otherwise futile effort females put into choosing among posturing males (Hamilton & Zuk, 1982; but see Read, 1990). Frequency-dependent selection exerted by parasites is, according to one admittedly minority view, largely responsible for the high levels of diversity found in gene pools (Clarke, 1979). One might even extrapolate to a time when the entire metazoan body could come to be seen as a gigantic adaptation against microscopic pathogens.
