>There was never a "pure" race, even prior to beekeeping.  There has always been gene transfer between the races.

No, the so-called races arose as a result of isolation and the lack of gene exchange with bees from other regions. For example, it is assumed that the northern European black bees became distinct from the mediterranean types due to glaciation and their isolation in glacial refugia. 

Similarly, the divergence between African and European types occurred due to the lack of gene transfer from one continent to another. It is possible that these bees would have eventually become separate species just as the Asian bees diverged so far from the European bees to form separate species. 

Whether this occurs because of selection, adaptation or genetic drift is moot; generally once lineages speciate, they become unable to interbreed, and they can't go back. The following excerpt outlines the general principals, but some of the information has been updated in the subsequent twenty years.


> According to Ruttner (1992), 25 subspecies of A. mellifera can be recognized based on morphometric measurements, biogeography, and behavior. The high number of subspecies may result from a number of mechanisms that isolated populations and permitted the accumulation of genetic differences. In Europe such isolation probably accompanied refugia formation during Pleistocene glaciation. 

> The extent of genetic variability in group III may be the result of a great number of individuals in an ice age refugium or the existence of multiple small refugia where populations accumulated genetic changes during this time and then later coalesced into what is now considered A. m. mellifera. Subsequently, these honey bees colonized northern Europe.

> Based on sequence divergence, the group containing the African subspecies (group I) seems to be as old as the one containing A. m. mellifera (group III). Similarly, the north and south African subspecies may have had a common ancestor around 0.31 Myr ago.

Arias, M. C., and W. S. Sheppard. "Molecular Phylogenetics of Honey Bee Subspecies  (Apis mellifera L.) Inferred from Mitochondrial DNA Sequence." Molecular Phylogenetics and Evolution 5.3 (1996): 557-566.

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