Drosophila melanogaster originated in tropical Africa but has achieved a 
cosmopolitan distribution in association with human habitation. Cosmopolitan 
populations of D. melanogaster are known to have reduced genetic variation, 
particularly on the X chromosome. However, the relative importance of population 
bottlenecks and selective sweeps in explaining this reduction is uncertain. We 
surveyed variation at 31 microsatellites across a 330-kb section of the X 
chromosome located between the white and kirre genes. Two linked clusters of 
loci were observed with reduced variation and a skew toward rare alleles in both 
an Ecuador and a Zimbabwe population sample. Examining Zimbabwe DNA sequence 
polymorphism within one of these regions allowed us to localize a selective 
sweep to a 361-bp window within the 5' regulatory region of the roughest gene, 
with one nucleotide substitution representing the best candidate for the target 
of selection. Estimates of sweep age suggested that this fixation event occurred 
prior to the expansion of D. melanogaster from sub-Saharan Africa. For both 
putative sweep regions in our data set, cosmopolitan populations showed wider 
footprints of selection compared to those in Zimbabwe. This pattern appears 
consistent with the demographic amplification of preexisting sweep signals due 
to one or more population bottlenecks.