JW: I don't understand why this [TF lineages die off/blink out at higher rates] is true. Please explain to someone like me who knows little about genetics.
Fitness traits are not binary, but represent proportionally and marginally better survival. For example, a constellation of traits might confer 3% better year-over-year survival -- all other factors being equal. That 3% would represent a nearly unprecedented quantum jump in typical evolutionary fitness.
We have been conditioned to think of all traits like GMO-injected Round-up ready corn -- a binary state, this is not true of whatever the mechanisms are for genetic resistance to parasite vectored disease.
Various traits have been proposed:
Hygenic removal of diseased larvae in both the Minnesota and Louisiana lineages.
Grooming behavior (may be a single loci) in AHB influenced genotypes
Leg-biting behavior observed in Indiana and Mexican bees.
Swarm rate / Supersedure at 3 months (maintains young healthy queens)
Russian-style strategic brood breaks when disease levels peak
Enhanced immune response to virus (multiple loci involved)
Reduced development days. (AHB trait)
Multiple queen maintenance (AHB trait)
Direct resistance to particular virus.
Change of pheromone odors reduce mite homing and mating
Life history stage changes in bees (increased nurse, reduced foraging).
The selected trait(s) confer the narrowest margins of improved survival in a system of natural selection. In an evolutionary time-scale (where hundreds of thousands of years blink by) these razor thin margins can establish new genotypes, where an accident of isolation permits a local population to "fix" the genotype without dilution.
In a treatment-free apiary, 30-90+% of the trialed hives die. This includes the vast majority of colonies with marginally "improved" genotypes, they are good, just not perfect, and the mites overwhelm them despite their marginally better fitness. These lineages are now dead, extinct, unable to contribute to the next generation. The lines that survive are accidents of fate.
In a directed selection model, the best examples are preserved, treated and promoted as the progenitors of the F2 generation. Their lineages are intact and able to contribute genes.