On Tue, 20 Jun 1995, Joe Morlan wrote:
> On Mon, 19 Jun 1995, David B. Wright wrote:
>dw> Joe, you are still trying to defend the BSC by attacking the=20
> > PSC, and we are not going to get anywhere arguing about the=20
> > PSC. The BSC'=92s weaknesses are independent of the PSC or any=20
> > other species concept. =20
>=20
jm> Sorry, but it seemed to me that you were trying to defend the PSC
> (phylogenetic species concept) by attacking the BSC (biological species
> concept). Fair is fair.=20

Well, actually I was just trying to point out that because the BSC is=20
flawed we need to consider alternatives. An extreme version of the PSC=20
has never been to my taste. I did answer your criticisms of the PSC in=20
cases for the sake of argument. But arguing about the PSC is tiresome. =20
Let's focus on the BSC's failings.

>[dw oriole example snipped]
=20
jm> The oriole case has been resolved by splitting the Baltimore and=20
> Bullock's Orioles into separate species. This change will become=20
> official in the next AOU Check-List supplement. The problem was not with=
=20
> the BSC but with the interpretation of incomplete data on hybridization. =
=20

And was this "re-evaluation" by any chance prompted by Freeman's finding?=
=20

>dw > This flaw and=20
> > the previous one exist in the BSC simply because its architects=20
> > were not very concerned with genealogy. Your claim that the=20
> > BSC does no=92t make phylogenetic statements is simply wrong -- it=20
> > is precisely because the BSC makes *misleading* phylogenetic=20
> > statements (e.g., relationship of Bullock'=92s, Altamira and=20
> > Baltimore orioles in above example) that many systematists are=20
> > looking for alternatives. =20
=20
jm> I think this is a "straw man." The BSC is not a cladistic or=20
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> phylogenetic technique. It is simply a way of making sense of the=20
> reality of polytypic species and defining the limits of existing gene=20
> pools. =20

I think this is the crux of the biscuit. The BSC may not be intended=20
(or suited) diagnose phylogenetic units (taxa), but as it places some=20
populations in one species and other populations in other species, it is=20
a de facto systematic/phylogenetic technique. And it is because the BSC=
=20
diagnoses phylogenetically/genealogically incoherent species taxa that=20
phylogenetically minded biologists are looking for replacements. =20

> The level of relationships can be refined by resorting to higher=20
> and lower categories to construct a hierarchical tree, but those other=20
> categories are admittedly arbitrary and do not require or depend on the=
=20
> BSC for their existence. Furthermore, any such classification deals only=
=20
> with the tips of the branches and not with the known common ancestors. =
=20
> Thus trees constructed by connecting living taxa are not, and cannot be a=
=20
> genealogy.

"Those other categories," namely genera, families, etc., are indeed=20
arbitrary, but the species itself is a category and thus arbitrarily=20
delimited. One decides what criteria one wants to use to delimit=20
species-taxa -- BSC, or ESC, or PSC for example -- and applies them. The=
=20
criteria may indeed be internally consistent and capable of being applied=
=20
objectively, but the fact remains that one has to choose a set of rules to=
=20
draw lines between populations or sets of populations to call=20
"species." And it is important not to confuse *categories* with *taxa*,=20
i.e., clades. Clades -- the branches on the tree of life -- exist in natur=
e=20
independent of our ability to recognize them. Calling a particular=20
branch a genus or family (or species or subspecies) is a matter of=20
assignment, i.e., it is arbitrary. It is true that species can at least=20
be defined more *objectively* than the other categories, but it still on=20
the basis of criteria we choose (and one could argue that the PSC's=20
simple definition of a species as the smallest diagnosably distinct=20
population [the tips of the branches] is the most objective of them all...)

The trees in question are not genealogies in the sense of having every=20
ancesor and descendant identified, no. They are genealogic in the sense=20
that Darwin used the word, i.e., organisms are related by propinquity=20
of descent: common ancestry.

>dw > Moving away from the classic BSC does no=92t mean we have to=20
> > ignore gene flow among populations, etc., and rely on a starck=20
> > version of the PSC. A reasonable approach is to actually make=20
> > phylogenetic trees that relate populations in question to one=20
> > another and use these trees as a context for evaluating=20
> > significance of gene flow among populations, etc. You could probably=
=20
> > even do this and hang onto subspecies, which is what I mean by=20
> > "revamping the BSC." =20
>=20
jm> "Revamping" is much too strong a word. What you are advocating is bein=
g=20
> done already within the BSC as the recent splits voted by the AOU=20
> demonstrate. My advocacy of the continuance of the BSC and opposition to=
=20
> the PSC is not an advocacy to avoid or deny the results of new research. =
I=20
> embrace all the new techniques which are clarifying the phylogeny of the=
=20
> world's avifauna. This is an exciting time.

What I am advocating is dropping the misleading hybridization and=20
potential interbreeding criteria from species-level taxon decisions, and=20
instead basing these decisions on study of genealogic relationships among=
=20
the populations in question. Doing that, even if one continues to=20
recognize subspecies, is a pretty big shift from the classic BSC, and=20
is hardly within its confines.
=20
At least we agree that this is an exciting time for evolutionary=20
biology...

David Wright
dwright at u.washington.edu