Subject: Re: species concepts (was VCR Fox Sparrows)
Date: Jun 19 12:05:56 1995
From: Joe Morlan - jmorlan at slip.net


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

> There is no need for "subspecies" with the PSC. Well-
> diagnosed subspecies under the BSC would simply be species
> under the PSC. Poorly diagnosed subspecies under the BSC
> would not be recognized under the PSC.

What is "poorly diagnosed?" Consider the Northern Flicker which has
very pronounced subspecies and some not-so-pronounced? How many species
of Northern Flicker would be recognized? How many of Downy Woodpecker?
How many of Horned Lark or Song Sparrow?

I think there is a significant advantage in the current hierarchy which
distinguishes levels of geographic variation within a larger polytypic
biological species. The PSC makes no distinction between the Salt Marsh
Song Sparrow and the Heerman's Song Sparrow vs. Swamp Sparrow. They all
just become equally full species despite the close affinity of the first
two vs the last.

All it takes is a well defined population with an extra barbule on a
single feather to diagnose a species according to Zink. As long as a
unique suite of derived characters can be found, you have a species.
That's my understanding of Zink's position. Am I wrong?

> I doubt that even the most fervid PSCer would split humans
> into separate species, especially on the basis of distance (as opposed
> to cladistic) data.

Why not? The PSC diagnoses species based entirely on the existence of a
unique set of derived characters. Certainly the mDNA is such a character.
It is measurable, derived and unique to the population. Additional
characters also associate with African Human, Japanese Human and
Caucasian Human.

> But since you regard partitioning polytypic
> species into subspecies as crucial to a good species concept, Joe,
> perhaps you will show how the BSC subspecies criteria are applied to these
> human populations.

The biological species concept does not partition all geographic
variation into named subspecies. It recognizes that certain types of
intraspecific variation such as mosaic variation or clinal variation are
not amenable to named subspecies. The PSC however, insists that any
trivial set of derived locally distributed characters must be named as a
species.

> And speaking of unanswered questions from our
> previous discussion, you still have not addressed the charge
> that the hybridization criterion of the BSC diagnoses
> genealogically incoherent species (oriole and chickadee
> examples), and that its potential interbreeding criterion
> lumps populations on the flawed basis of overall phenotypic
> similarity.

Life is messy. Sometimes species hybridize occasionally and sometimes
they don't. It is not overly surprising that populations which
occasionally hybridize may be more distant than some populations which
don't. The point is that there is significant reproductive isolation
which constricts gene flow in all these examples. These taxa are all
recognized as biological species. I have yet to see an example of two
biological subspecies (with a known hybrid swarm) which turn out to be
more distantly related than two taxa which hybridize only occasionally or
not at all and which are diagnosed as biological species.

I.e, show me two subspecies which are more distant than two species. If
you can, I'll concede the BSC needs serious revamping. Until then, it
should be understood that the BSC is just a biological concept which maps
current biological reality better than any competing concept. It does
not diagnose phylogenetic trees or phylogeny. It just tells us what is
happening today.

> Recall that I am not arguing that an extreme version
> of the PSC is the best way to diagnose species, but rather
> that the BSC is demonstrably flawed and needs either major
> overhaul or replacement.

My position is that there is no need to throw out the baby with the
bathwater. The BSC is a very robust concept which can handle most any
situation one can throw at it. It has served us well for a long time and
makes more sense out of biogeography and evolutionary processes than the
PSC or competing concepts.

> The PSC is not the only
> alternative to the BSC. You cannot save the BSC by poking holes
> in extreme versions of the PSC.

The only version of the PSC I know is the one I read about in McKittrick
and Zink. I'm not aware that they have backed off from their position.
One thing in favor of their view is that it is internally consistent. It
takes no hostages. If it is to be modified to look more like the BSC,
then it will require judgment about which of the trivial populations are
not worthy of recognition. Personally I admire Zink and McKittrick's
stand. It takes all the human judgment out of the diagnoses of species
and leaves it to computers running discriminant function analysis. They
correctly point out that the BSC requires human judgment. This is indeed
a weakness of the BSC, but it's a weakness I can live with.

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