Population genetic analyses of the past two decades confirmed an earlier hypothesis by L Tsacas and D Lachaise that the cosmopolitan species Drosophila melanogaster has an Afrotropical origin, and that it colonized the rest of the world only very recently. Maximum likelihood analyses based on multilocus data suggest that the putative ancestral African population expanded its size about 60,000 years ago (ya). These demographic changes were accompanied by the fixation of numerous beneficial mutations, as revealed by signatures of positive directional selection in the genome (selective sweeps). The estimated rate of adaptive substitution on the X chromosome is in the order of 10(-11) per nucleotide site per generation. Comparable (but not significantly higher) substitution rates are found in derived populations that colonized new habitats outside Africa, such as in a European population that branched off from the African lineage about 16,000 ya.