What Polecat Was The Progenitor Of The Ferret?
 
Trying to decide which polecat was the ancestor of the domesticated
ferret shouldn't be too hard; after all, there are only two possible
candidates: the steppe polecat, (Mustela eversmannii), and the European
polecat, (Mustela putorius).  Over the last few centuries, the ferret
has had it's own species name, was considered a descendent of the steppe
polecat, was defined as a descendent of the European polecat, and now it
is back to it's old species name (Mustela furo).  In the earliest natural
history books written centuries ago authors speculated on the origin of
the ferret, and in the latest genetic studies the same type of
speculation has taken place.
 
Humans have mucked up the history of ferret domestication for such a
long time that it is possible it will never be fully known or understood.
The problem is a long history of ferret-polecat introgression (a
two-dollar word for hybridization).  Ferreters have a millennia-long
history of breeding the ferret back to polecats to improve various
characteristics, most notably hunting instincts, strength and vigor, body
size, and coloration.  Ferrets have been hybridized with both polecats:
the steppe polecat (Mustela eversmannii) and the European polecat
(Mustela putorius).  The main problem with introgression is that it
confuses the genetic tests done to determine familial relationships,
making it hard to determine the progenitor (a two-dollar word for
ancestor).  Ferrets can breed with either of the two polecats and produce
healthy, reproductively viable offspring, and such breeding is still an
accepted practice throughout Europe.  While in Europe, I saw both
European polecat_x_ferret AND steppe polecat_x_ferret hybrids; they are
not that uncommon.  In any case, there are several good references that
detail the introgression of the ferret to both polecats, as well as those
that document European polecat_x_steppe polecat hybrids within the
transition zone between the two species.
 
Just for the sake of argument, suppose ferrets were domesticated from
the steppe polecat about 2300 to 2500 years ago, mostly in southeastern
Europe surrounding the northeast Mediterranean region (ancient Macedonia,
eastward towards present day Bulgaria, and northward towards present day
Hungary).  Suppose sometime later as the practice of keeping the
domesticated ferret moved westward, the ferret was hybridized with the
European polecat.  Now, not only are the genetics confused, but also the
karyotype, which would vary from 38 (steppe polecat) to 39 (hybrid) to
40 (European polecat), depending on the amount of introgression and
subsequent breeding practices.  After a couple of hybridizations and
breeding for tameness, the ferret's karyotype would tend to stabilize
with either 38 or 40 chromosomes (there is evidence of decreased
fecundity and increased birth defects with odd number chromosome counts,
so they tend to rapidly breed out).  After a few decades of introgression
with the European polecat, most ferrets would have the higher karyotype
count of 40, but they would still have genes and mitochondrial DNA from
both species.  Also, just to confuse the issue even more, both the steppe
and European polecats hybridize along the geographic regions where they
overlap, so the effects of introgression confuse genetic markers between
those species as well, blurring lines of distinction.  It is this avenue
of gene flow that is part of a body of evidence that encouraging some
zoologists to consider the two polecats are the same species, but that
is another issue for another time.  There is the additional problem that
both polecats diverged from their common ancestor comparatively recently
(recent speciation), so the changes found between the two species are
limited.  Finally, toss in a couple of thousand years of ferret breeders
periodically crossing the ferret back to both polecats, repeatedly
introducing wild polecat genes back into the domestic animal.  It is no
wonder that recent genetic studies have concluded that the progenitor of
the domesticated ferret is unresolved.
 
So, assuming the domestication of the ferret took place using both
polecats, what you should have is a domesticated animal with
characteristics of both species.  Does this sound familiar?  If you
embarked on a comparison of the early descriptions of the ferret, you
would note the repeated observation that it displays characteristics
of both polecat species.  The ferret has many skull and dental
characteristics of the steppe polecat, but karyotype characteristics
of the European polecat.  Sable ferrets look remarkably like European
polecats, while Siamese cinnamon (sandy) ferrets look remarkably like
steppe polecats.  The natural history of feral ferrets resembles that of
the steppe polecat, but many of the physiological responses are more like
the European polecat.  Contemporary ferrets have a body size intermediate
between the two polecats.  The list of comparative attributes is long and
it is hard to say which polecat comes out the winner; remember the
"unresolved" statement?
 
One problem is the karyotype of the ferret shows 40 chromosomes
(scientifically stated as 2n = 40), while the karyotype of the steppe
polecat is 38 (scientifically stated as 2n = 38).  The problem begs
the question: "Is it possible for a species to successfully breed with
a different species having a dissimilar karyotype?"  The answer is
obviously true because hybridization has been documented for a long
time, by many different researchers.  The main genetic difference between
the steppe and European polecat is not so much one of the genome (the
genetic information on the chromosomes), but of the karyotype (how the
chromosomes look in terms of size, form, and number).  In other words,
the two polecats have very similar genetic information, but one polecat
has 38 chromosomes and the other has 40.  The difference in karyotype
between the two species is attributed to a single Robertsonian
rearrangement.
 
For this limited discussion, there are two basic types of Robertsonian
rearrangements.  A "chromosome" is actually a pair of chromosomes, one
each from the mother and father that are connected together so they look
like plump worms forming an 'X' or a 'V' (which is why the scientific way
to count them is 2n = 40; it means 20 pairs totaling 40 chromosomes).
One type of Robertsonian rearrangement is when two chromosomes have their
fusion or connection point (the centromere) near the end of the
chromosome, and they are fused together to form a single chromosome with
a fusion point near the middle.  The other is where a single chromosome
with a fusion point near the middle is split to form two chromosomes with
fusion points near the end.  Since the steppe polecat is probably the
older of the two polecats (based on paleontological evidence), it is
likely the type of Robertsonian rearrangement was one where a single
chromosome was divided into two, changing the karyotype from 38
chromosomes to 40.
[Posted in FML issue 4730]