>
> >It was always my belief that there were stocks of EHB that had had a
> sufficiently long evolutionary exposure to tmites for a balanced
> host/parasite adaption to occur.


Dan, the bee stocks in the U.S. were established from stocks that
apparently had not been exposed to tracheal mite, and imports were cut off
around 1920.  So I question whether there were alleles that had been
naturally selected for Tmite resistance extant in our population when the
Tmite invaded.

A point to keep in mind is that genes involved in resistance to parasites
tend to be the most rapidly evolving genes, due to the Red Queen
Hypothesis, which describes the continual battle between parasites and
their hosts.  Parasites tend to evolve more quickly than do other changes
in the environment, hence the rapid evolution of resistance mechanisms.

But there is a large element of randomness (stochasticity) involved.  The
rapidity of natural evolution of a host against the pressure of a novel
parasited depends largely upon what potential "tools" a species already
contains in its genome (preadaptation).  And then there is a huge amount of
chance involved--whether such tools happen to get passed on to the next
generation, and whether they have a downside (such as, in humans, sickle
cell anemia vs resistance to malaria).

So whether resistance develops quickly or slowly depends upon a number of
factors, mainly:

   1. The degree of selective pressure (how badly the parasite is hurting
   the host).
   2. The existing toolbox of genes and alleles in the host's genome.
   3. The roulette-type chance of a beneficial random mutation occurring,
   and then actually getting passed to the next generation.

As Pete has pointed out previously, evolution does not have a direction or
goal.  It is the result of largely random trial and error.  The end result
is not necessarily the "best" or "most efficient" result, but rather one
that tended "to solve the problem" (to an adequate extent).

Returning to varroa resistance, the ancestral and most genetically diverse
honey bee populations (from Africa) happened to possess traits that
preadapted them for resistance to varroa.  And perhaps even more
importantly, humans stepped aside and allowed natural selection to take its
course.  The result is various bee races in Africa and the Americas (in the
same species as our domestic stocks) that are strongly resistant to varroa.

This fact alone is strong evidence that *Apis mellifera *has the clear
potential to develop resistance to varroa via natural selection.
-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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