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A significant observation to emerge from certain recent experiments in population genetics with Drosophila has been the dependency of Darwinian fitness on the frequency of genotypes or karyotypes. Experimental populations in which chromosomal arrangement or gene-frequency changes can be interpreted satisfactorily as the result of frequency-dependent adaptive values have been described in D. persimilis by Spiess (1957), in D. pseudoobscura by Pavlovsky and Dobzhansky (1966), in D. melanogaster by Kojima and Yarborough (1967), and in D. ananassae by Tobari (1964) and by Tobari and Kojima (1967).In accounting for such frequency dependency a most interesting phenomenon derived from mating behavior studies can be invoked as a possible mechanism: Petit (1951) first observed in multiple-choice matings between Bar and wild-type D. rnelanogaster that Bar males mated with relatively greater success when they were rare than when common. Later (1958) she described in more detail variation in a "coefficient of sexual selection" for Bar versus wild and for white eye versus wild. For both mutant-type males, the coefficient was a function of mutant frequency in mating and showed a "minority advantage" for both, plus a high frequency advantage for white eye. In 1964 I observed a similar minority advantage for certain karyotypes (Whitney vs. Klamath) of D. persimilis (Spiess, 1964) and in 1965 Ehrman completed an extensive series of mating-behavior experiments (1965, 1966) with karyotypes (Arrowhead, Chiricahua, Standard, and Tree Line) and mutant strains of D. pseudoobscura and two ethologically isolated races of D. paulistorum.Briefly, Ehrman's technique included direct observation of mating pairs in chambers where the frequency of karyotypes or genotypes was varied in both sexes simultaneously. Minority advantage occurred in chambers wherever D. pseudoobscura strains of different origins, different karyotypes, mutants versus wild type, or flies of the same genotype raised under different temperatures were used. There was no difference between the minority advantage of mutant-versus wild-type flies raised together in the same culture and flies raised separately, so that mating success was independent of the culture medium in which flies developed. With ethologically isolated strains of D. paulistorum, however, no significant effect of numerical frequency was observed in mating success. In attempting to account for the * .10 > p > .05.

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