Hi all
In connection with recent discussions I would like to point out that there are several types of populations of honey bees as evidenced by a recent study in California which compared old (up to 100 years) specimens to modern ones. 

In this study you have an example of an isolated population in the Humboldt County area, which was predominantly A. m. m. (western European bee) for a very long time. Then you have the general domestic bee industry stock in the Sacramento Valley which is a mix of western and eastern (Italian & Carniolan) stock which has shifted its proportions over time. Finally, the southern California which is a mixture of the mentioned lines plus a predominance of Africanization. 

The southern Californian bees are the most genetically diverse, of course, being a wild interbreeding population. The domestic bees are fairly homogenous but as genetically diverse as their counterparts in Europe. The isolated population in Humboldt shows the incursion of domestic stock and probably the loss of feral reserves due to varroa mite. 

To put this into perspective:

Positive selection causes a beneficial allele to
sweep to high prevalence or fixation (100%
prevalence) rapidly within a population. When
a beneficial allele and surrounding variants on
the same haplotype reach high prevalence together,
it produces a population-wide reduction
in genetic diversity (sometimes referred to
as heterozygosity, polymorphism, or variability)

from: Vitti, Joseph J., Sharon R. Grossman, and Pardis C. Sabeti. "Detecting natural selection in genomic data." Annual review of genetics 47 (2013): 97-120.

What this means is that natural selection causes beneficial traits to prevail at the cost of diversity. Selective breeding also selects for traits that the breeder deems to be beneficial, again reducing diversity. Applying it to the paragraph above, we see that the feral Humboldt population was very homogenous at one time, probably as result of isolation. 

The domestic population has more diversity due to the admixture of different lineages. but is vulnerable to homogenization. That's why people like Sheppard and Cobey have proposed bringing in stock from Europe. Finally, the southern California population is a vast natural experiment, where the influx of African bees causes diversity to increase, but in the long run natural selection may homogenize the population to be essentially African, as in Mexico & South America.

Now the question becomes which is better: stock selected for beneficial qualities such as mite resistance; stock that is naturally adapted to a region, like Humboldt County; or a highly diversified wild population like the bees in SoCal. There is evidence that diversity per se is beneficial -- vs. homogeneous, or worse, inbred lines.

 On the other hand, a highly diverse population may not have characteristics that beekeepers want. Finally, while African bees have many desirable qualities such as diversity and vigor, their many faults make them a poor candidate as a source of genetic input, despite claims to the contrary, such as this statement:

Our analysis uncovers the potential genetic signatures that
beekeeping practices and the introduction of Varroa have had on the changes
experienced by bee populations over time and suggest that southern Californian
populations may harbor higher genetic variation that could be harnessed to improve
the health of A. mellifera and its important role in the agriculture of California.

from: Cridland, Julie M., et al. "Genome sequencing of museum specimens reveals rapid changes in the genetic composition of honey bees in California." Genome biology and evolution (2018).

PLB

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