Drosophila melanogaster, like humans, originated in Africa, and that's where they show the most genetic diversity -- just as humans do. The flies are thought to have appeared in Europe about 50,000 years ago, along with modern humans. On the way, both humans and flies squeezed through genetic bottlenecks that reduced the diversity in the population. Over generations, different fly strains evolved quite specialized niches -- such as a fondness for colonizing breweries, for example.
But the African fly genomes show that just in the past couple of decades, flies similar to those found in Europe or the U.S. have established populations back in Africa, often in new environments such as urban and industrial developments. For example, where modern breweries and bottled beer have displaced traditional African brewing, "Europeanized" brewery flies have followed.

Full Article
http://www.sciencedaily.com/releases/2012/10/121012102605.htm