Randy writes:
In nature, hybrids are generally more poorly adapted than either parental
type.  If this weren't the case, then the hybrids would quickly replace the
parent stock, and we would then call the hybrid a parent line.

* My ideas were based mainly upon work by Harpur and Zayed, which I quote extensively here:

Most domesticated species show evidence of a bottleneck following domestication. For example, crop plants such as maize, oats, rice and soybeans have lost 30– 90% of the genetic variation of their progenitors. Chickens, rabbits and silk moths have lost 50–60% of the genetic variation of their progenitors.

Reduced genetic diversity is a common feature of domesticated animal and plant populations that has been implicated as a cause of colony declines in honey bees. However, we observe an opposite pattern: managed populations have more genetic diversity when compared with their progenitors in E. and W. Europe. 

Why are high levels of genetic diversity in managed honey bee populations retained despite apparently strong pressures from management? The answer lies in beekeeping practices and honey bee mating biology. Queen bees mate with 15–20 unrelated drones by flying through congregation areas containing drones from hundreds of surrounding colonies. Regular stock importation combined with artificial increases in colony density and movement of colonies over large spatial scales could then increase and diversify the gene pool of managed honey bees, leading to admixed populations. 

Recent studies in Australia and the United States found no reductions in genetic diversity of honey bees participating in a 25- and 10-year closed-population breeding programme utilizing naturally mated queens. Beekeepers may be, intentionally or unintentionally, selecting hybrid colonies, which tend to have higher fitness at some colony-level traits.

Domestication is associated with a reduction in genetic diversity, but we have shown the opposite in honey bees. Higher genetic diversity in managed populations is probably caused by the honey bee’s promiscuous mating biology combined with human mediated dispersal. 

Harpur, B. A., Minaei, S., Kent, C. F., & Zayed, A. (2012). Management increases genetic diversity of honey bees via admixture. Molecular ecology, 21(18), 4414-4421.

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