Melanic polymorphism in ladybirds maintained by sexual selection

Sexual selection and the evolution of preferential mating in ladybirds I. Selection for high and low lines of female preference

“…The "rare male effect" so very often observed in experiments in which females are offered a choice between two different male genotypes or phenotypes will almost inevitably occur when females have specific mating preferences: rare males are preferred by a relatively greater proportion of females than common males; the selection is negatively frequency-dependent, becoming weaker as the preferred males become more common. This "rare male effect" has been observed very commonly in Drosophila (Ehrman, 1967;1970;1972;Spiess, 1968; 1969; Spiess and Ehrman, 1978), in a parasitic wasp Mormoniella vitripennis (Grant, Snyder and Glessner, 1974), in the guppy (Farr, 1977) and in the Two-spot Ladybird (Muggleton, 1979;O'Donald and Muggleton, 1979; O'Donald and Weir, l982a).…”

Non-random mating in the two-spot ladybird (Adalia bipunctata): I. A reassessment of the evidence

“…Other factors may include differential predation between phenotypes; indeed, it has even been suggested that ladybirds form an assemblage of Müllerian mimics (Rothschild, 1961;Muggleton, 1978;Brakefield, 1985). But as O'Donald and Muggleton (1979) point out, these putative selective factors do not necessarily give rise to the balance of forces required to maintain the poiymorphism. On the other hand, they suggested that frequency-dependent non-random mating would be sufficient to maintain the stability of the polymorphism.…”

“…But the analyses performed by Lusis and Creed were considered unsatisfactory by Muggleton (1979). He analysed Lusis and Meissner's data as well as some of his own, finding evidence of a two-sided frequency-dependent mating advantage.…”

“…A general mating advantage to melanics would produce fixation of one of the melanic phenotypes, so some additional selective force would be required to maintain the polymorphism. But Muggleton's frequency-dependent mating advantage could be sufficient to maintain the polymorphism (O'Donald and Muggleton, 1979), for it is the rarer phenotype, and not just melanic individuals alone, which would gain a mating advantage. A two-sided frequency-dependence such as this is an inevitable outcome of the expression of preference for each phenotype: each gains the advantage when rare and loses it when common.…”

“…In Nevertheless, a polymorphism can still be maintained if the frequency-dependence is balanced by natural selection against the melanic phenotype, provided of course that the mating advantage is greater than the adverse selection at low frequencies (O'Donald and Muggleton, 1979).…”