Given the opportunity, honey bees can adapt rapidly in order to overcome disease, parasites, and other adversity.
Swarming is the act of bee colony reproduction. Weakened colonies do not swarm, therefore the colonies with healthy traits will reproduce.
Also, weak, sick colonies with weak traits do not produce drones (male bees). Healthy colonies that do produce drones contribute to the gene pool of all the colonies for miles around. You may object to this idea due to the fact that when a colony becomes queenless, the female worker bees may in fact start producing male drones on their own as a last ditch effort to pass on their genes before the colony dies. Although this is a weak colony that is reprducing, it may be weak for reasons not related to their genetics. It could be that the queen was killed, or she never had sufficient sperm due to lack of a sufficient number of drones in the area.
Bees are better than beekeepers at picking good genetics for reproduction. Natural selection is better than artificial selection. Bees select naturally for survivability while beekeepers select artificially for primarily narrow, financially beneficial traits.






