ABSTRACT The colonization of America by Drosophila subobscura is a recent, large-scale natural experiment in evolution with two replicates. The species was first detected in the population of Puerto Montt (Chile) in 1978 and subsequently in Port Townsend (state of Washington) in 1982. This double colonization was studied from the outset using different genetic markers. The analysis of lethal genes was one which provided more information. The study of lethal genes enabled the effect of the founder event to be quantified and, with the aim of comparing with Palearctic populations, the parameters defining the genetic structure of populations could be computed. It was also possible to estimate the initial number of colonizers. Furthermore, lethal genes demonstrated the great resemblance between the two colonizing events, and allowed us to conclude that they were strongly related. The origin of the colonization remains obscure, but two hypotheses can be derived from lethal gene data and other genetic evidence: an eastern or a western Mediterranean origin. Finally, some lethal genes associated with chromosomal inversions have persisted over time. An analysis was carried out to determine the adaptive value of the inversions with which these lethal genes are associated and to quantify the heterotic effect of the genotypes carrying these inversions.
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