Chris Hill wrote:

> I am well aware, and of course, so is Zink, that the "do they
>hybridize in the wild?" criterion has been an important part of the
>definition of a species in ornithology for a long time, although it is
>not given such weight in botany, for one example. Hybridization may
>or or may not be important, but why focus so much attention on the process
>(hybridization)? Why not instead look directly at the results (degree of
>genetic mixing across populations)?

There is no special reason to focus on hybridization as a criterion for
species limits, I entirely agree, but it is still an interesting phenomenon
and still *does* bring to bear evidence for or against species status. The
idea that hybridization is important is that of one school of systematists,
the proponents of the biological-species concept. I am attracted to the
viewpoint of the other school, the phylogenetic-species proponents, in
which it is thought that two creatures can be considered different species
if they have separate evolutionary histories, no matter if they are much
alike and can produce viable hybrids, but I'll remain with an open mind.

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. 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. I realize that the Fox Sparrows to which we are referring
don't freely interbreed, but, of course, they come in rather limited
contact anyway.

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.

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.

Dennis Paulson, Director phone: (206) 756-3798
Slater Museum of Natural History fax: (206) 756-3352
University of Puget Sound e-mail: dpaulson at ups.edu
Tacoma, WA 98416