Mating Probability, Body Size, and Inversion Polymorphism in a Colonizing Population of Drosophila buzzatii

Abstract

Drosophila buzzatii is a colonizing species mainly associated with the cactus genus Opuntia. Originally from South America, it has spread over the world within historical times following these cacti (Carson, 1965; Barker and Mulley, 1976; Fontdevila et al., 1981). It shows a moderately high inversion polymorphism in two of the four major autosomes encompassing about 15% of the total euchromatin (Carson and Wasserman, 1965; Fontdevila et al., 1981, 1982; Ruiz et al., 1984, 1986; Fontdevila, this volume). We are trying to answer a very simple question: how does natural selection work on this polymorphism in the wild? The answer, though, may be not so simple. Of course, indirect evidence from natural populations and experimental work carried out during the last forty years, shows that inversions are adaptive devices and that selection may be of considerable magnitude (Dobzhansky, 1970; Anderson et al., 1975; Spiess, 1977; Sperlich and Pfriem, 1986). Field studies of natural selection, on the other hand, are scarce and it is by no means clear which is the relative importance of the different selection components and which is the magnitude of the selection coefficients in nature. We think that inversions continue to be a very useful tool to learn about natural selection in this regard.

“What we need is more knowledge about the ways in which populations, in fact, meet evolutionary challenges: What intensities of natural selection can they put up with, how far and how fast can they modify their phenotype (including their habitats)? Colonizing species are ones which we know to have been confronted by a challenge — that of their new location; and we often know, even quite precisely, how long they have been facing it.”

C.H. Waddington, 1965