 The population geneticist W.J. Ewens has written of the Hardy-Weinberg principle, “it does not often happen that the most important theorem in any subject is the easiest and most readily derived theorem for that subject” (1969, p. 1). The main importance of the principle, as Ewens stresses, is not the gain in mathematical simplicity that it permits, which is simply a beneficial side effect, but rather what it teaches us about the preservation of genetic variation in a sexually reproducing population.