exactly what is a species?

[Robin Panza]

Unfortunately, there is no one accepted definition of a species, partly because
there is no definition that will cover nearly all (forget about covering *all*)
the kinds of organisms known.  Even trying to cover *most*, one must broaden
the definition to the point of being so vague as to be of little use.  Life is
messy and doesn't follow the rules.

Personally, I ignore asexual organisms and use a definition based on a
population genetics point of view.  A species is a group of organisms with a
common evolutionary fate, due to a high level of gene flow among the members
and a low level of gene flow beyond the membership.

In my view of sexually reproducing organisms, a hypothetical map of shared
alleles and loci would look rather like Sewell Wright's "adaptive topography".
There would be "lumps", or areas of relatively high density (many shared
genes).  These would be species.  There may be some gene flow (due to
hybridizaton, due to transfer of genes by viruses/bacteria/parasites, etc.)
between "lumps", which causes the lumps to have width, rather like bell curves.
As long as the rate of gene flow into and out of a lump is relatively low (and
no, I don't try to define "low" and "high") compared to the rate within the
lump, it will remain reasonably discrete from other lumps.  This is a species.
In other cases, the rate of gene flow between two lumps is high enough to have
significant effect on the evolutionary fates of the lumps, and these lumps
might be subspecies.  If gene flow is high, the lumps will not remain discrete
for long, and might best be thought of as polymorphisms, or an aborted attempt
at speciation.  Either way, they should not be considered species in my world
view.

As for ramifications for conservation biology, the problem as I see it is that
we are so typological.  People who are willing to preserve "species" are not
necessarily willing to save populations.  However, there are so many organisms
that are located in "leaky lumps" with non-zero gene flow that don't fit into
a typological world view.  We are not prepared to deal with this situation.
Ideally, we would preserve all individuals, as we don't know which ones carry
alleles that may be evolutionarily important.  Obviously, we can't.  No life
form can exist without harming other life forms.  Some sort of priority system
must be made, but refusing to help a population because there is some small
amount of gene flow is terribly naive.  We should develop a sliding scale,
where more effort is given to more discrete lumps.  Politicians, lawyers, and
John Q. Public are not going to be comfortable with that, and selling it will
be extremely hard.

I prefer to point out to such people that, while this may not be a distinct
species from that large population over there, this one undoubtedly has a
unique set of genes adapting it to local conditions.  We cannot simply let this
population go extinct and then import some from there--they may be ill-adapted
to fit into the local ecology.  Even if this population is "just" a geographic
variant, we will not be able to recreate this gene pool once it's gone.

Robin K Panza                   panzar at clpgh.org
Section of Birds, Carnegie Museum of Natural History