from an article by Randolph E. Schmid, Associated Press:

…Charles W. Whitfield, an assistant professor of entomology at the University of Illinois at Urbana-Champaign studied the genes of bees in Africa, Europe, Asia and the Americas, focusing on regions where several bee invasions have occurred, such as Brazil and South America.

Whitfield's team found that when invading bees were interbreeding with those already present, the combined genes were not just joined randomly.

"We asked the question: Is hybridization an essentially random process?" co-author Amro Zayed said in a statement.

When the African honey bees mated with the western European honey bees that had been in South America for centuries, one might expect that the hybrid offspring would randomly pick up both the functional and non-functional parts of the genome, he said.

"But actually what we found was there was a preference for picking up functional parts of the western European genome over the non-functional parts."

This combination seemed to give the newcomers an advantage over their predecessors, though the researchers were not able to determine exactly how the new bees benefited…

http://www.usatoday.com/tech/science/genetics/2008-02-25-killer-bees-genes_N.htm