"Is hybridization an essentially random process?" Zayed said. 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 nonfunctional
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 nonfunctional parts."

In repeated invasions of a new territory, the honey bee, Apis
mellifera, can benefit from the genetic endowment of those bees that
arrived in earlier territorial expansions. It appeared that the
Africanized bees that kept some of the functional western European
genes were gaining an advantage, Whitfield said. "Those African bees
are doing better because there were western European honey bees there
for them to mix with"

http://www.news.uiuc.edu/news/08/0225honeybees.html

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