Q: "One of my friends said they heard you discuss the difference between
albinos and black-eyed whites at [Ferrets 2000].  They said you indicated
albinism was proof of domestication....How?"
 
Ah!  A person who understands the difference between a philosophical
question, why, and a mechanistic question, how.  Can you please explain
the difference to the more rabid creationists?
 
Albinism in ferrets is due to a recessive allele on a single chromosome
(actually, a pair of recessive alleles on a pair of matched chromosomes).
Because the allele is recessive, it requires the ferret to have two copies
for the trait to be expressed.  This is a truly recessive allele in
ferrets, which means without two copies, there can be no expression of the
trait in any form.  In other words, this trait does not blend, nor it does
it partially express; you either have two copies and are an albino, or you
have zero or a single copy and you are not=97there are no in-betweens.  In
non-domesticated species, albinism is extremely rare because the chances
of getting two copies of the allele is so remote.  In the wild, it is more
commonly seen in species where the breeding population is small, but even
then, it is quite rare.  Even in those instances where albinism occurs in
the wild, it never persists as a trait, being bred out of the species
relatively rapidly, as will be explained below.
 
In ferrets, albinism is persistent; that is, the allele has reached a
stable frequency of expression within the population and would not breed
out during random mating (assuming no change in selective pressures).  In
other words, if you randomly selected 100 females out of a randomly mixed
breeding pool of ferrets, and mated them to 100 male ferrets, also randomly
selected from a randomly mixed breeding pool, then the offspring would
express albinism at approximately the same frequency as in the parents,
as would succeeding generations.  Ferrets are not the only animal which
has persistent albinism; it also occurs in rabbits, mice, rats, chickens,
doves, and other human-selected (or domesticated) animals.  Thus,
persistent albinism is evidence of domestication, and is only found when
humans selective breed for the trait.  Indeed, it can only occur in a
state of domestication; a recessive trait, albinism cannot persist in wild
populations, especially those subjected to both predation and starvation
selective pressures (like what would be expected with such a predator as
the ferret).  Since historic references can document the condition in
ferrets for most of the last 1000 years, that alone is pretty good evidence
ferrets are domesticated (most reports from earlier ages discuss ferrets,
but do not describe their outward or physical appearance).  In other words,
long term, persistent albinism is absolute evidence that ferrets have been
bred, housed and cared for by humans, and that hybridization to wild
species has been minimal for at least since the first report of the trait.
Persistent albinism is simply not a trait of a non-domesticated (wild)
population.
 
Such a long span of time, coupled with ancient husbandry methods, has
"bred out" many of the problems associated with the inbreeding necessary
to establish albinism as a persistent trait (let no imperfect animal go
unculled; culling was thought to maintain the perfection God required in
the animal).  This includes the worst expressions of some of the
characteristic aspects of albinism (nothing major, just a simple skewing
of the bell curve towards the "we can live with it" side).  For centuries,
albino ferrets were preferred throughout Europe for rabbiting because the
white coats were thought easier to spot exiting burrows.  Poachers also
preferred albinos because they could be seen easier in the dark.  Many 300+
year old reports on ferrets considered only the albino coloration to be a
true ferret, calling the "polecat color" either semi-domesticated or "wild
but tamed".  It doesn't take a lot of generations, if carefully culled, for
the really bad side effects of inbreeding to be removed from a breeding
population.  Nor does it take much longer to skew the normal distribution
of albinistic characters towards the beneficial side; not actually making
albinism beneficial, but rather reducing the negative impact so it is
easier on the ferret.  So, even though albinism causes some problems in
ferrets, generally speaking, they can easily live with them.  The result
is an animal with what is essentially a birth defect, but who doesn't
exhibit=97at a population level=97the great numbers of really bad side
effects which would be expected.  The reason I use the term "birth defect"
is because the albino "gene" is essentially a mutated allele of a normal
gene.  Strictly speaking, there is no gene that codes for albinism.
Rather, there is a mutated allele which codes for pigmentation, but which
doesn't work, and when you have two of them and no way to make pigment,
then you are an albino.
 
This is a completely different thing than the partial depigmentation that
can be caused by human selection for tameness.  While on the surface,
"albinism" and "depigmentation" appear synonymous, they are not strictly
so.  An albino ferret cannot make pigments, but may have genetic code for
any number of coat colors which could be expressed in a heterozygous
offspring.  A depigmented ferret is just that, a ferret which can code for
pigment, but for some reason, pigments are not being made or deposited.  An
animal can become depigmented for metabolic reasons (seasonal white fur,
gray hair), or because of a disruption in the timing of distribution of
pigmented cells during early embryonic development (blazes, white faces
and bibs, white feet).  For example, fox, snowshoe hares and ermine are
seasonally depigmented, as are ferrets to a minor degree.  Polar bears have
evolved white fur, but they are not depigmented.  Those animals (almost
exclusively domesticated, but including humans) having blazes, stripes,
piebald coloration, or white foreheads and bibs are developmentally
depigmented.  Most depigmentation occurs because of a metabolic disruption
of normal pigmentation, and is an evolved trait which produces seasonal
coat coloration changes.  This is typically tied to photoperiod cycles and
is regulated by hormones.  But in domesticated animals, something else is
happening; depigmentation occurs because the cells which produce pigments
are prevented from reaching their final destinations, leaving specific
areas void of pigmented cells.  See the difference?  In albinos, pigmented
cells are normally distributed, but they lack the ability to produce
pigments.  In developmentally depigmented animals, pigments are produced
normally, but some areas lack pigmented cells.  In domesticated animals,
developmental depigmentation is a result of breeding for tameness.
 
There is a fairly well documented hypothesis called the "Star Gene" which
ties the behavioral changes caused by domestication to piebalding, neotony,
and a host of other morphological changes, mostly of a visual and auditory
nature (The Star Gene is named after the star of white commonly seen on the
foreheads of animals possessing the trait).  In an extremely short and
simple explanation, breeding for tameness (reducing fight/flight distance
to zero) results in changes in the timing of brain development, interfering
with the migration of pigmented cells during early embryonic development.
This results in human friendly animals, but it also results in stars,
blazes or light colored stripes on the head and forehead, white faces,
chest blazes, white feet or knees, generalized piebalding; that sort of
thing.  It can also result in some significant hearing and vision problems,
far beyond those caused by albinism.  Interestingly enough, a ferret with
a strong expression of the "Star Gene" resulting in deafness and visual
problems, may also be an albino.  Because you don't see the piebalding
clues, some people can erroneously attribute the bad side effects to
problems caused by albinism, when in reality it is due to piebalding
(Waardenburg's Syndrome is closely associated with piebalding, the outward
expression of the Star Gene).
 
Current domestication theory more-or-less accepts the Star Gene hypothesis
as valid because it is the simplest explanation for nearly identical
changes in different species.  For example, most domesticated animals
have members in the population which exhibit piebalding (brown/black
coloration), shortened faces with exaggerated foreheads (cranial neotony),
persistence of juvenile behaviors (behavioral neotony), and changes in the
reproductive timing cycles (increased breeding) (Note: piebalding is an
irregular brown/black and white coloration=97that stupid computer "cow" box
is piebald).  All these traits=97and many more=97can be traced either to
changes in brain development due to domestication, OR to changes in genes
closely linked to the Star Gene.  This is still considered an hypothesis
because the Star Gene has yet to be found and the mechanisms of the Star
Gene Effect are still mostly unknown.  It may be a single gene, or more
likely, a series of genes that are linked together in some way (either
because they are adjacent to each other on the chromosome or because they
are developmentally linked), so that changes in the keystone gene "the
Star Gene" results in changes to the linked genes.  Most of these changes
are thought to be disruptions in embryonic timing, but some are due to
environmental influences (metabolic and biochemical changes).
 
One of the tests of the Star Gene hypothesis is that the results of
breeding should be somewhat unpredictable.  Because the Star Gene changes
the developmental timing in the embryo, extremely slight changes due to
environmental influences can result in major differences in expression.
In other words, just a few minutes in timing one way or another can result
in the extension of a blaze into a stripe, or the difference between
diminished hearing and deafness.  This makes breeding for these traits
somewhat unpredictable; you may know you will get a blaze of some type,
but you cannot predict the degree of expression for trait because it is
so environmentally sensitive.  (Note: we will never know exactly HOW much
hearing loss is present in ferrets because we only test for the presence or
absence of hearing, not for the degree of diminished hearing.  Thus it is
probable hearing disorders are far more common than generally perceived).
 
In ferrets, pandas, white faces, bibs, stripes, white mitts, white knees,
and blazes are all thought to be expressions of the Star Gene, which is why
ferrets with those markings are so subject to visual and auditory problems
(and other problems common to Waardenburg's Syndrome).  Silvers and dark
eyed whites are not due to albinism, nor because of the expression of the
Star Gene, but rather because of manipulation of the various genes (and
their alleles) which code for color (the genes for color are thought to
exist in at least three different locations, on at least two different
chromosomes).  In other words, a dark eyed white ferret is white for the
same reasons polar bears are white; because their color genes code for it,
NOT because they are albino or have a strong expression of the Star Gene,
although both are possible.  The way you can discern the difference between
depigmentation due to breeding for tameness from naturally occurring
instances is by coexistent changes, including cranial differences and
behavioral neotony, which do not exist in wild populations.  Star Gene
mediated depigmentation is only present in domesticated animals because
it is a result of breeding for tameness.
 
So, since we know persistent albinism and depigmentation caused by the
expression of the Star Gene is only found in domesticated animals, and
since we also know albinism in ferrets is at least 1000 years old and that
Star Gene depigmentation is common, then we know the ferret has had a long
period of domestication, with breeding for tameness common and under human
control.  In other words, the fact that you can commonly see albino
ferrets, or ferrets with blazes, stripes, or panda markings, we know
"beyond a shadow of a doubt" that ferrets are quite domesticated.  We also
know the genetics of ferret coloration is quite complicated, with three
major elements to be complicating factors in predicting the results of
breeding (genes controlling normal pigmentation, the migration of pigmented
cells, and coat coloration).
 
Bob C
 
>From: "Bruce Williams, DVM"
>Subject: Re: Bob C: Albinism, Depigmentation, and Domestication
>
>Definitely my vote for the most literate post of 2001!
>
>Thanks, Bob.  I ALWAYS get something out of your posts.
>
>Bruce
[Posted in FML issue 3406]