Hi all
I have noticed that people that raise bees for sale tend to have a very high opinion of their own bees, and make very optimistic claims about the potential for honey bee improvement. On the other hand, the people that buy them tend to have far less enthusiasm for these claims. Norman Carreck is a little more realistic when he writes:

> Bee breeding is of course a continuous process, and one should not underestimate the practical difficulties of maintaining a desired strain of bee. Leslie Bailey of Rothamsted pointed out some years ago (Bailey, 1999) that much of the honey bee's inherent resistance to disease stems from its genetic diversity, so that breeding for resistance to one pest or disease may increase susceptibility to others.

About survivor stock he writes:

> 150 honey bee colonies ... were left untreated, and monitored for the next six years. As expected, there were heavy losses initially, and few swarms, but by 2005 (Fries et al., 2006) there were 13 colonies remaining.

> Did the bees become more resistant, or did the mites become less virulent, or a combination of both? This is difficult to determine, but is of vital importance, when one considers that the life cycle of a varroa mite is only a few weeks, whilst in contrast, that of the honey bee is several years, so one can make much more rapid progress in breeding less virulent mites than in breeding more tolerant bees.

> The conclusion was that two mechanisms were involved: firstly, the Bond colonies produced less brood than control colonies, and secondly they had a lower proportion of the mites in the sealed brood compared to control colonies.

So this points to a second drawback of breeding for one trait: you may wind up with colonies that survive but don't do much else. 

Brother Adam, on the other hand, was constantly aiming for a hybrid bee which would be produced by combining disparate lines. Again, such an approach is a challenge due to the "the practical difficulties of maintaining a desired strain of bee." 

> In their search for the "best bee" Polish beekeepers readily use imported material, seeing it as a method for quickly increasing honey yield of their bees. The bee, which has already been considered in Poland [Troszkiewicz 1992] and has recently gained increasingly more popularity, is the Buckfast. 

> The present authors analysed Buckfast bees in an area where the nectar flow culminated in the first half of the season, in a stationary apiary, i.e. in standard conditions for an average Polish apiary. It is in such untoward conditions for the Buckfast that they exhibited higher efficiency than the indigenous European Black bees.

Here I would point out that a bee that is profitable is generally a better one to have than one that survives but doesn't produce much for the owner. Unless you are keeping them as pets.

sources: 

Carreck, N. L. (2011). Breeding honey bees for varroa tolerance. Carreck, NL (Ed.), Varroa-still a problem in the 21st century

Olszewski, K., Borsuk, G., Paleolog, J., & Strachecka, A. (2012). Evaluation of economic traits in Buckfast bees in comparison with the hybrids of European Black bees and Caucasian bees. 

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