Joe, David, et al.

I'n not a professional in these matters but my impression is that the
actual contrast in taxonomic assignments between proponents of these
alternative species concepts is miniscule with respect to the totality of
taxonomic categories recognized. The major disagreement seems one of
semantics: what level of the taxonomic tree is to be called the "species"?
The terminal contrast level or some level above that of terminal contrast
at which "gene pool isolation" seems to be well established. There seems
little real difference of judgment as to the actual SHAPE of the tree.
Examples such as that of the orioles seem to me minor differences of
interpretation or weighting of various subsets of the characteristic
contrasts between branches of the tree. Differences of opinion that one
might expect to find among proponents of a common theory of
classification, rather than evidence of contrasting "paradigms." The
ultimate goal of constructing a taxonomy to reflect as accurately as
possible the actual phylogenetic history of life seems common to both
sides of this fence. The assertion that reliance on "gross similarity"
judgments as an index of phylogenetic "distance" is fundamentally flawed
seems quite an exaggeration to me, as "gross similarity" judgements, i.e.,
"classical taxonomics," if done well, do more than count characters (ala
numerical taxonomy) but rather "intuitively" weigh the power of particular
types of phenotypic characters as predictors of the whole pattern of
variation, that is, they involve intuitive judgements of the "information"
each character entails. I seem to recall reading studies that show that
taxonomic judgments do not differ significantly among proponents of
competing theories.

Gene Hunn.

On Tue, 20 Jun 1995, Joe Morlan wrote:

> 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
>
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