This type of genetic change is not that uncommon; it has been estimated
that some type of Robertsonian rearrangement occurs in 1 out of 900 human
births.  Because of the risks of birth defects and a general lack of
reproductive success in individuals having an odd number of chromosomes,
within a few generations the karyotypes tend to stabilize to the even
numbers; in this case either 38 or 40.  Because of this, Robertsonian
rearrangements are viewed as a major factor in rapid species change.  It
also confuses biologists, making hybrid zones seem narrower than they
really are.  There can be a tremendous amount of introgression between
two species having similar genetics but different karyotypes, but it is
hard to see.  A female jill might have 2 kits with 40 chromosomes, 3
with 38 chromosomes, and 3 with 39 chromosomes.  Because the 3 kits
with 39 chromosomes tend to have lower survivorships, when you sample
the population, you mostly see polecats having 38 or 40 chromosomes, so
hybridization appears to be less common than in reality.
 
What this means in terms of domestication is that hybrid ferrets would
also have karyotypes of 38, 39, or 40 chromosomes, but the ones with the
odd number would tend to die out, either because of birth defects or a
lack of reproductive virility.  Descendents of these hybrids would
therefore tend to have either 38 or 40 chromosomes.  Continue to breed
those descendants to European polecats, and in a surprisingly short
number of generations, most of the descendents would have 40 chromosomes
even though they have genes from both polecats.  Reverse it by breeding
to steppe polecats, and the karyotype would tend to stabilize after a
number of generations at 38 chromosomes.  This is why it is a mistake to
try to use the karyotype of the domesticated ferret to determine the
ancestral progenitor.  All you are looking at is the last few hundred
years of breeding practices, not necessarily true ancestral origins.
 
The actual genetic structure of the two species is quite similar.
Geneticists can find genetic differences between the two polecats, but
the real question isn't one of differences.  Rather, it is one of
similarities.  For example, a lot is made of the differences in culture
between European Americans and African Americans, and a number of real
differences exist.  The question is, are those differences significant?
If you are a separatist, you say they are, but that is an arbitrary
distinction, not a scientific one.  A better way of looking at the
distinction is to measure the degree of variation with European American
culture, that within African American culture, and that between European
American and African American cultures.  When you do it that way, you
discover the differences between cultures are less than those within
cultures.  In other words, there are more differences within the European
American culture than there are between European American and African
American cultures.  So, now how different are they?  That is the problem
when defining a species.  When you measure the variation within European
polecats and steppe polecats, then measure the difference between the
polecats, you discover there is more variation within the groups than
between the groups.  In other words, they are far more alike than they
are different.
 
These findings have been confirmed in several genetic studies, suggesting
the two species of polecats are genetically quite similar.  Paleontology
also confirms the close relationship of the polecats, suggesting the
separation between species is less than 500,000 years.  The willingness
and commonness of hybridization also suggests a very close relationship.
The more the relationship between the polecats is studied, the more the
distinction becomes blurred.  If the distinctions between the polecats
are blurred, how easy can it be to determine which polecat is the
ancestor of the ferret?
 
This is why the authors of recent genetic studies clearly stated they
could not determine the progenitor of the ferret.  The authors of the
genetic studies went a step further than just suggesting the ancestor
of the ferret was unknown.  The suggested reason suggested for the
difficulty of progenitor determination was introgression; that is,
hybridization.  In other words, the reason given for the difficulty of
finding the ancestor of the domesticated ferret was that it had been
bred back to the two polecats so frequently that it made the distinction
too hard to resolve (at least with the research methodology used).
 
If genetic studies cannot help us to determine the ancestor of the ferret
(at least at this time), what other lines of evidence can be used to
determine the progenitor?  Historic records can be of some value.  If
you look at the oldest possible references of the ferret, those from
Aristotle, Aesop, and Aristophanes, they are all Greek.  All three
authors use the term "Iktis" (or Ictis), which has been translated as
polecat, weasel, marten, house-weasel, and ferret, depending on the
translator.  No one is really sure of what animal Aristotle is talking
about; a polecat, marten, mongoose, or ferret--it is hard to tell,
although linguistic evidence suggests it is probably either a polecat or
ferret.  You can argue this because both Aesop and Aristophanes use the
same word in their writings and it is unlikely that the word would have
shifted meaning in such a short period of time.  The translation in
Aesop's writing is confusing because of the tendency of medieval period
writers to change the animals mentioned in the fables to those of a
biblical nature, so in many modern translations you see "asp" or "fox"
in places where iktis was originally penned.  In the most modern
translations of Aesop from the Greek, iktis is translated as
"house-weasel" or "house-marten."  Of the three, Aristophanes is the most
meaningful because of the nature of the writings.  Aristophanes was a
popular playwright and political satirist.  Aristophanes used iktis in at
least 4 surviving plays to describe an animal that was associated with
people and their homes, an animal that was known for it's odor, and who
would steal food.  The animal was common enough for people to be able to
recognize the satirical reference, so the implication is the animal was
common, recognizable, and kept in homes.  Later references to the word
iktis invariably refer to the polecat or ferret, so linguistic continuity
suggests the word is referring to either the ferret or the polecat.
 
The classic objection to using the early Greek writers to support an
early ferret domestication is that in later writings iktis is used to
name the polecat.  On the surface, this would be a valid objection; early
writers used iktis for polecat, later writers used iktis for polecat, how
can it be used for ferret?  However, the objection ignores the time it
takes for new words to be invented to replace the existing names in the
everyday vernacular.  For example, the names of both domesticated and
wild forms of mice, rats, chinchillas, hamsters, gerbils, skunks, mink,
rabbits, and fox are exactly the same, even though the domesticated forms
are clearly domesticated, and some have been for centuries.  Also, some
languages conserve the basic form of the word, using prefixes or suffixes
to modify it for greater understanding.  For example, in Hungarian the
word "gvriny" means polecat, "vadaszgvriny" means domesticated ferret,
"feketelabz gvriny" means black-footed ferret, "molnargvriny" means
steppe polecat, and "tigrisgvriny" means vormela (AKA: the tiger or
striped polecat).  Gvriny has been conserved and is interchangeable with
ferret or polecat, essentially meaning both.  It is the prefix that
defines the way the word is to be interpreted.  One of the reasons you
know the ferret has had a long domestication history is because it has
a different name than the polecat.  To the ancient Greeks, ferret
domestication would have been comparatively recent; it would not be
surprising to find they used the same word for both polecat and ferret,
allowing the context of the message to define the exact meaning of the
word.
[Posted in FML issue 4730]