Subject: Re: species concepts
Date: Jun 20 10:30:28 1995
From: Joe Morlan - jmorlan at slip.net


On Mon, 19 Jun 1995, David B. Wright wrote:

> 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

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

> Whether we settle on something that looks like Cracraft'=92s=20
> version of the PSC or something that is more explicitly=20
> phylogenetic remains to be seen. =20

I've read Cracraft's reorganization of the classification of the birds of
the world dealing with higher categories (Auk 98:681-714). I also recall
S. Olson wrote a scathing review of Cracraft which made me wonder how it
ever got published in the "Auk" in the first place. I sure seemed like
Cracraft didn't follow his own professed methodology.=20

=20
> Consider the following real-world example involving orioles:
>=20
> ---------------Bullock'=92s *
> |
> |
> ---------------Streak-backed
> -----|
> | ---------Altamira
> | |
> ------|
> |
> ---------Baltimore *
>=20
> The diagram above is based on Freeman'=92s 1991 (U Washington PhD=20
> diss.) analysis of mtDNA restriction sites. Baltimore and=20
> Bullock'=92s orioles hybridize with one another in the Great=20
> Plains and were lumped together as a single species on that=20
> basis -- the Northern Oriole. But Baltimore is genealogically=20
> closer to Altamira, which is regarded as a distinct species,=20
> that it is to Bullock'=92s. In other words, one BSC subspecies=20
> (Baltimore) is more closely related to a different species=20
> (Altamira) than it is to a conspecific subspecies (Bullock'=92s). =20
> This simply makes no sense. Similar examples of the=20
> genealogic flaws of the BSC'=92s hybridization criterion exist in=20
> chickadees (Gill et al. 1993) and Towhees (cited by Zink in a=20
> talk here; ref?). =20

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

As I said in my previous post, it is not unexpected that distinct species
may hybridize occasionally for a variety of reasons. Sometimes isolating
mechanisms in long separated taxa may not have time to develop under
conditions of recent secondary contact. This is what I think happened in
the case of the orioles.=20

I agree that hybridization is not the best way to measure genetic=20
distance. However it is a good measure of the boundaries of gene pools. =
=20
The PSC has no mechanism to measure gene pool boundaries and doesn't care=
=20
about them.

> You still have no=92t told us why we should accept the BSC's
> lumping of phenotypically similar allopatric populations on=20
> the basis of "potential interbreeding," when we know that=20
> gross phenotypic similarity is a notoriously unreliable=20
> indicator of genealogic relationships. =20

The specific status of morphologically similar "semispecies" is a problem=
=20
for the BSC because there is no possible test of sympatry. However=20
behavioral and biochemical data can lead to an informed opinion using=20
BSC criteria. In the past, it was considered good taxonomy to lump=20
morphologically similar allopatric populations in order to use the=20
subspecies concept to convey biological information. Identifying these=20
taxa as subspecies has the advantage of indicating, by their names, that=20
the taxa are a) allopatric and b) more closely related to each other than=
=20
to other members of their genus. =20

You are correct, that this practice went overboard, and some of the=20
excesses are now being corrected without resorting to abandoning the=20
entire concept of polytypic biological species. =20

> 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

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. 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.

I feel that your attack on the BSC as failing to provide a "true=20
genealogy" is misplaced at best.

> 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

"Revamping" is much too strong a word. What you are advocating is being=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.

> The Zink paper (Evolution, 1994) cited by Dennis in an earlier=20
> message in this discussion shows than Zink has retreated from=20
> the "one barbule" position of McKittrick & Zink 1988. =20
> Basically he is looking for agreement in disparate sets of=20
> characters (for example, plumage and mtDNA) in diagnosing=20
> different populations as species). This "retreat" simply puts=20
> Zink more in line with Donoghue=92s version of the PSC. =20

I'm sorry to see that Zink has backed off of his earlier position. That=20
position had the advantage of being completely consistent. If I=20
understand you, the new version will require judgment as to what level of=
=20
"agreement" will diagnose species. This will lead to eventual=20
disagreement among informed researchers and we will be back to the same=20
problem posed by the BSC which the PSC was supposed to solve. =20

This would be the worst of both worlds. We would have all the problems of
the BSC and the PSC without the befefits of either.=20

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