“…IT has been suggested by a number of theoretical and experimental population geneticists (Karlin, 1968;Scudo and Karlin, 1969;O'Donald, 1978aO'Donald, , 1978b) that models of sexual selection, and of assortative mating, should be considered in which the mating preferences of one sex (usually the female) are not rigidly expressed, but are to a certain extent influenced by behavioural characteristics and availability of the preferred type. Specifically, one can suppose that a female possessing a preference for one type of male will initially mate only with that type; but if after a certain number of encounters the desired male partner has not been met, then the level of stimulation will have been raised to the point where she will effectively mate at random.…”