Genetic history of a colonizing population: D rosophila buzzatii ( D iptera: D rosophilidae) in A ustralia

D rosophila buzzatii   P atterson & W heeler, a cactophilic species that feeds and breeds in the rotting tissues of various O puntia cactus species, was inadvertently introduced to A ustralia from A rgentina sometime during the period 1931–1936. After a bottleneck at introduction, its spread through the cactus distribution was probably very rapid as a result of natural dispersal from the site of introduction and from three other foci colonized from the introduction site by human intervention. By 1940, the O puntia distribution and consequently that of D . buzzatii was reduced to spatially isolated populations, with probable further bottlenecking of at least some of the D . buzzatii populations. Allozyme data (primarily six polymorphic loci) from flies collected during A pril 1972 to F ebruary 1996 at 67 localities were used to examine current population differentiation and relationships, as well as to infer aspects of their demographic history. Although there is significant isolation‐by‐distance, genetic relationships among the populations are not simply related to geographical distance, implying that genetic drift has contributed to population differentiation. However, the biotic and, to an extent, the physical environment are not the same in A ustralia as in A rgentina. Consequently, exposure to novel environments has led to local adaptation and further population differentiation. Genetic variation and the structure of Australian populations apparently are determined by founder effects (drift) at the level of individual breeding sites (cactus rots), by diversifying selection among rots within a locality, as well as by drift and geographically varying selection among localities. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society , 2013, 109 , 682–698.