Here Oldroyd, et al, talk about changes in populations brought about by African incursion:

Africanized bees are a consequence of asymmetric
introgression of African nuclear and mitochondrial DNA
into North and South American populations of honeybees. 
In some feral populations, European mitochondrial haplotypes have
been nearly completely replaced by mitotypes of African
origin. Nuclear genotypes are also dominated by African alleles 

[The genetic consequences of the anthropogenic movement of social bees]

Here, they discuss the merits of mixed populations and increased diversity (as opposed to heavy selection):

While introgression can be detrimental to the conservation
of honey bees within their native ranges, it is
actively sought after in regions without native A. mellifera
populations, such as North America. 
It has been well documented that genetic
diversity is important to the health of colonies, and beekeepers
seek novel genotypes resistant to pests. Admixture has
been shown to increase levels of genetic diversity in honey
bees (Harpur et al. 2012, 2013) and beekeepers have been
intentionally interbreeding subspecies of honey bee for at
least a century in North America. 

[Assessing patterns of admixture and ancestry in Canadian honey bees]

To me, this shows that traits that confer some clear survival benefit will accrue in populations as a whole. Management by beekeepers tends to broaden the gene base, creating benefits that depend on diversity, while swamping particular traits such as hygienic behavior.

PLB

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