As I understand it, the matter of fertility is complicated, which is one
reason for specifying a population. I believe that there are some genetic
conditions (immunological ?) which will make two people infertile even
though they may be fertile with other members of the race. In many plants
there are anti-selfing loci which prevent fertilization by the plants own
pollen and by other pollen that is too similar genetically.
As to genetics, I recall an example from Goldschmidt back in the '40s,
before there was any genetic testing except by crosses. Lymantria (gypsy
moth) has a circumpolar "species." Neighboring populations can breed in
the laboratory, though I do not know if they do naturally. But members of
the extremes have infertile hybrids with strange morphologies.
At least some of the argument over "good species" springs from
populations that have natural barriers preventing interbreeding, but can
be crossed in the laboratory to produce viable hybrids. Both lumpers and
splitters have the answer to considering them one or two species.
Obviously, only those who agree with me are right, or even have a claim
to be rational.
A large part of the argument over species is, IMO, "hermeneutical." If
all species come directly from the hand of God, then a "true species"
should not interbreed with another one. If species evolved, then there
should be a continuum from simple interbreeding populations, through
populations with timing, food, geographical or other isolation mechanism
that can interbreed in the laboratory, to those with total chromosomal
mismatch. The practical question is what we find, with opponents yelling,
"'Tain't so!"
Another part springs from approach. A natural history approach paints
with a broad brush, while genomics is very restricted at the present
time, for we don't have that many species sequenced. But each specialty
tends to hold that, assuming the availability of evidence, theirs is the
appropriate definition. But, if species are in flux, any approach has a
problem with the assumption of stasis.
Dave
On Thu, 01 Feb 2001 10:55:16 -0500 george murphy <gmurphy@raex.com>
writes:
> Keith B Miller wrote:
>
> > >Pray, what is a species?
> > >
> > >This is a very serious question.
> >
> > There are several different definitions. The standard biological
> species
> > definition is "a population of interbreeding individuals that is
> > reproductively isolated from other such populations under natural
> > conditions." This provides a good theoretical definition but is
> often
> > difficult to apply in real situation in the field and is obviously
> not
> > applicable to the fossil record.
> >
> > In practice what is commonly used is a morphological definition.
> In this
> > case, anatomical variation within a species is less than that
> between
> > species. Maintaining anatomical distinctions between coexisting
> > populations (or those in direct contact) would seem to require a
> degree of
> > genetic isolation sufficient to prevent the mixing of gene pools
> (ie they
> > are reproductively isolated). the existence of populations (or
> fossil
> > collections) with statistically distinct anatomies is thus
> considered a
> > basis for species designation.
> >
> > There are now also those who advocate using genetic divergence as
> a
> > criteria for recognizing species.
>
> Is there any evidence that there is a specific amount (even
> approximately) of genetic divergence which results in reproductively
> isolated
> populations? I.e., is it likely that there is a 1-1 correlation
> between
> criterion 3 and criterion 1?
>
> Shalom,
>
> George
>
>
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