There is a third point to consider, which is the rate of dispersion of
a recessive trait into a large general population.  Rates of dispersion
have been worked out for a number of genetic traits, mostly by physical
anthropologists attempting to work out the ancestry of human species.
They are also used by geneticists working out the history of genetic
disease in both human and domesticated species.  The problem is the only
way for the hypothesis that cystinuria is widespread in ferrets, only
masked by diet, to be true is for the disease to have become entrenched
in ferrets between 50 and 150 years ago; that is assuming all the ferrets
remained breeders.  The reason is because the trait is recessive, so
there has to be a lot of it in the population for the disease to show up.
Imagine a gumball machine holding a thousand pink gumballs and two blue
ones.  If the gumballs are well mixed, what are the chances the two blue
gumballs will come out of the lot at the same time?  For cystinuria to be
widespread, a lot of carriers have to be breeding with a lot of carriers,
and they have to have been doing it for quite a long time.  That means
you will see a much higher rate of disease than we currently see.
 
I could write for hours and falsify this hypothesis even more, but I
only need a single falsification, and I have given at least two.  If the
disease were widespread, then it would be showing up in roughly the same
rates as dogs.  It is only showing up in a miniscule, statistically
insignificant population, and that is not what you see in a recessive
disease that is widespread within the population.  Everything we see
suggests that in ferrets cystinuria is a recent development in a small
section of the population.
 
Here's the thing about recessive traits, either autosomal or X-chromosome
linked.  Because the expression of those traits is mathematically
predictable, you can use math equations to extrapolate the minimum and
maximum numbers of carriers in a given population, based on expressed
characteristics.  For example, because we know albinism is an autosomal
recessive trait, we could expect that for every 100 albinos, there may be
as many as 200 hidden carriers of the albino gene, simply based on
Mendel's patterns of inheritance.  This is because < will not carry the
gene, will carry one gene, and < will be carry two and be an albino.  The
real numbers are not as simple to deduce because of a number of factors,
but even if only one ferret out of a hundred with the trait showed
symptoms of cystinuria, if it was widespread in the population, there
would be more sick ferrets than we are seeing.
 
Let me put it another way.  This entire argument rests on a single
question: can ferrets that have undiagnosed cystinuria consume a 30-35%
protein diet without forming stones?  This question HAS to be true if
cystinuria were widespread and never seen.  But we know that the
precipitation cascade in animals with cystinuria forms stones when the
conditions are right, regardless of the protein levels of the food.  If
the disease were widespread, SOME ferrets would get dehydrated and form
stones, SOME would have acidic urine and make them, SOME would eat too
much protein and make them, SOME would eat too much salt and make them,
and SOME would just make them for no real reason.  There might be dozens
of other factors that would result in the precipitation of cystine.
These are real, current factors, not hypothetical.  So, where are the
sick ferrets?
 
You can't have it both ways.  If cystinuria was widespread, then more
ferrets would show the disease.  If it is not widespread, well, that
explains why more ferrets don't show the disease.
 
Q: "But I did want to point out what seems like a hole in your logic --
if ferrets are indeed domesticated, then the selective pressures are
those of the conditions humans impose, and not those acting on wild
weasels.  You did say 'cause stones in a normal, average ferret', and
I am talking about selective pressure that would allow toleration of
non-meat feed."
 
I am so glad you picked up on this, because it is a great question and
incisive point, one greatly discussed in zooarchaeology.  The answer is,
ALL domesticated animals, even those under human selection, are still
under natural selection.  Humans select for color, nature selects for
resistance to disease.  You cannot remove an animal from natural
selection, not even humans.  We are in a closed system and natural
selection is one of the rules under which the system operates.
 
Aside from that, can you breed animals where the breeding process results
in changes to basic, "core" physiology?
[Posted in FML issue 4789]