Subject: Re: species concepts
Date: Jun 20 20:43:31 1995
From: Joe Morlan - jmorlan at slip.net


On Tue, 20 Jun 1995, David B. Wright wrote:

> jm> I think this is a "straw man." The BSC is not a cladistic or
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > phylogenetic technique. It is simply a way of making sense of the
> > reality of polytypic species and defining the limits of existing gene
> > pools.
>
> I think this is the crux of the biscuit. The BSC may not be intended
> (or suited) diagnose phylogenetic units (taxa), but as it places some
> populations in one species and other populations in other species, it is
> a de facto systematic/phylogenetic technique. And it is because the BSC
> diagnoses phylogenetically/genealogically incoherent species taxa that
> phylogenetically minded biologists are looking for replacements.

Evolution basically works at the level of the population. Selection
works on the individual members of the population. Evolution is a change
in allele frequency in a population over time. The BSC has the advantage
in that it correctly diagnoses those populations which constitute
independent gene pools and thus independent evolutionary units. No other
species concept does this. No other species concept diagnoses species in
terms of isolated gene pools.

This is an important and perhaps unappreciated point. If there is gene
flow across hybrid zones then the taxa at the ends of the hybrid zone are
not independent evolutionary units. Natural selection of individuals at
one end will affect allele frequency at the other. If there is
constriction of gene flow between populations (reproductive isolation),
then selection of individuals at one end does not affect the other.

Why should past evolutionary history be congruent with current
evolutionary constraints?

> > Thus trees constructed by connecting living taxa are not, and cannot be a
> > genealogy.
>
> "Those other categories," namely genera, families, etc., are indeed
> arbitrary, but the species itself is a category and thus arbitrarily
> delimited. One decides what criteria one wants to use to delimit
> species-taxa -- BSC, or ESC, or PSC for example -- and applies them. The
> criteria may indeed be internally consistent and capable of being applied
> objectively, but the fact remains that one has to choose a set of rules to
> draw lines between populations or sets of populations to call
> "species."

Yes, I suppose one can define species any way one wants. But consider the
alternative of ignoring reproductive isolation. Suppose, for the sake of
argument, that all "species" were capable of interbreeding with all
others. Free hybridization is everywhere. What then is the unit of
evolution in such a situation? What would happen to the world's
diversity?

Simple application of the principle of natural selection would predict
that diversity would decrease as unfit alleles would be selected out. In
the end we would see only ONE highly adapted species. Natural selection
is a powerful force against biological diversity.

What then accounts for the current biological diversity? It is, in my
view, not anything having to do with natural selection, but rather another
phenomenon entirely and that phenomenon is SPECIATION. Without speciation
there is no diversity.

But speciation is only special if it does something special. Other
species concepts ignore a fundamental evolutionary dynamic which pits the
inexorable forces of natural selection against the opposing forces of
speciation.

Thus the biological species is not arbitrary. It is a fundamental
natural unit. To throw the concept away makes a total mess of biology
and makes the evolution of diversity utterly incomprehensible.

[deletions]

> What I am advocating is dropping the misleading hybridization and
> potential interbreeding criteria from species-level taxon decisions, and
> instead basing these decisions on study of genealogic relationships among
> the populations in question. Doing that, even if one continues to
> recognize subspecies, is a pretty big shift from the classic BSC, and
> is hardly within its confines.

Ok. Then I'm afraid I'll stand by my position. Past genealogic
relationships do not necessarily predict the units of independent
evolutionary futures. The BSC is a concept for the here and now and for
the future. Hybridization does not mislead. It identifies the current
boundaries of the gene pools which are, in fact, the true measure of
biological diversity. What more can one expect from any species concept?

And let me try to be clear that diversity if very different from variation.
The former is intraspecific and the latter is intraspecific. Adherence
to the PSC confuses the two beyond repair.

> At least we agree that this is an exciting time for evolutionary
> biology...

Absolutely!

----------
Joe Morlan
Albany, CA
jmorlan at slip.net