On Fri, 16 Jun 1995, Dennis Paulson wrote:

[contrasting views of Phylogeneticists and Biological Species Concept
proponents contrasted]

> I don't think the most fervid phylogenetic-species proponent would argue
> very strongly against considering two parapatric (occurring adjacent to one
> another) populations as the same species, no matter how distinct their
> genes, if they freely hybridized all along their contact zone.

Well, I'd say that Bob Zink is a pretty well described by Dennis's phrase
"the most fervid phylogenetic-species proponent" ;-)

And I have seen him argue "very strongly" that flickers, yellow-rumps,
sapsuckers et al should be considered separate species, even if they
freely hybridize all along their contact zone. It is quite possible,
apparently, to have stable hybrid zones persist for thousands of
generations without any significant gene flow across the zone. If the
hybrid zone maintains itself indefinitely without affecting the different
forms on the two sides of the zone, why should we not recognize the two
forms as distinct species?


> For example, Myrtle and Audubon's Yellow-rumped Warblers are quite distinct
> birds, yet their profligate interbreeding (hybridization) in the Rockies
> has apparently convinced the ornithological systematic community that they
> are conspecific.

Well, some of the community, anyway ;-). I don't think it is accurate to
characterize the views of systematists on *any* subject as monolithic.

> Red-naped and Red-breasted Sapsuckers have been long (but not always)
> considered distinct species, yet in collections of them from all along
> their contact points in the Cascades and Sierras, a surprisingly large
> number show hybrid origin. Knowing that, I would certainly look again at
> this pair of species.

With the sapsuckers and the warblers, you are basically arguing the
Biological Species Concept (BSC) point of view, Dennis: they hybridize
extensively, therefore they are not separate species. I thought you
hadn't made up your mind yet on BSC vs PSC. ;-)

> Carried to its extreme, the phylogenetic-species concept would have every
> distinct population (typically recognized as named subspecies), and there
> are *lots* of them, of a species considered as separate species. A
> lister's dream that may be realized with each new AOU Check-list
> Supplement, if this concept drives bird systematics for as long in the
> future as the biological-species concept has in the past.

Fair summary. Zink might point out that no matter how iconoclastic it
might seem to have, for example, six species of song sparrows, or twenty,
the PSC would bring ornithological systematics more in line with the way
species are recognized in entomology, botany, and most other disciplines.
Also, if yellow-shafted and gilded flickers are steadily diverging despite
hybrid zones, why not recognize them as species? Because you think they
*could* merge again? Because you think they *might*? That's what the
hybridizing criterion is supposed to evaluate, isn't it? A model I saw
that took into account the rate of gene flow across the flicker or oriole
hybrid zone, I forget which, estimated that despite all the hybrids, it
would take 9 million years, I think, for the two species to mix
completely because of hybridization. They are diverging a lot faster
than that, so the mixing is basically irrelevant. If you can put a
date on the fork in the road when the gilded gene pool went right and
yellow-shafted went left, and that date is 10,000 years ago, they should
be two species, regardless of how they comport themselves when in contact
with each other.

My it's fun to get all worked up about this stuff! Sorry I don't have
data to present. I am, however, convinced that you *can* determine if,
and to some extent when, two forms diverged, more reliably than you can
decide whether they will rejoin. So I have to say that I'm pretty firmly
in the PSC camp. Hybrids don't mean squat unless you can show that the
effect of the hybridization is to mix the two forms pretty quickly.

Chris Hill
Seattle, WA
cehill at u.washington.edu