Joe Morlan
94-04-29 10:%:00
SPECIES...

Unfortunately, the concept of what constitutes a "species" is
still a matter of considerable debate among systematists.  There
are three basic species concepts, 1) morphological, 2)
biological, and 3) phylogenetic.

The morphological species concept defines a species as an
individual which conforms to a described "type."   This concept
has been almost entirely abandoned except that it forms a basis
for the phylogenetic species concept.

The biological species concept, says that a species is "a
population of interbreeding individuals, which is reproductively
isolated from other such populations."  Note that it is the
population and not the individual which constitutes a biological
species, unlike the morphological species concept where the
individual type is the species.  Note that complete reproductive
isolation is not required, only some measurable degree of
reproductive incompatibility.  Occasional hybrids between
biological species do not change their status as species under
this concept.

The phylogenetic species concept says the a species is any
population which possesses a unique set of derived characters.
This is very similar to the morphological species concept, except
that it emphasizes that a species is a population and not an
individual.

Which concept one uses, depends to some extent on the type of
organism one is studying.  The biological species concept works
well with natural populations of sexually reproducing
vertebrates, but is not very good at diagnosing plants and is
quite meaningles when applied to asexually reproducing taxa.  For
those types the phylogenetic species concept can be quite useful.

If one were to apply the phylogenetic species concept (PSC) to
breeds of dogs, each breed would probably constitute a valid
phylogenetic species.  But most vertebrate zoologists follow the
biological species concept (BSC).  The domestic dog does not
exist in a native wild state anywhere in the world, so any
hybridization experiments must be based on the results of captive
breeding. Biological species cannot be diagnosed reliably on the
results of captive breeding experiments because the way animals
behave in captivity is usually drastically different from their
behavior under natural conditions.

In general dogs prefer to breed with other dogs, than with wolves
or coyotes.  Although there may be occasional hybrids in the wild
between wolves and coyotes, they are still separate biological
species (semispecies) because such hybrids are rare. If
hybridization were to occur frequently, then wolf and coyote
would soon merge into one huge intermediate population. Since that
hasn't happened and since wolves and coyotes have ranges which
broadly overlap each other, they are considered biological
species (as well as phylogenetic species).

Domestic varieties of dog are merely the consequence of selective
breeding and a wonderful example of how selection and isolation
can cause evolution and even speciation.  E.g. it may be
physically impossible for a chihuahua to mate with a Russian
Wolfhound.  If these were wild animals, they might be regarded as
"reproductively isolated" and thus biological species.  However,
in this case, there are many intermediate sizes which do
interbreed with each other, so these size extremes would still be
regarded as part of the same species.