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Date: Wed, 9 Mar 1994 08:41:07 -0800 (PST)
From: Dan Victor <dvictor at u.washington.edu>
Subject: Re: Crow Taxonomy
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Date: Tue, 8 Mar 94 18:59:02 -0800
From:wrightdb at pigsty.dental.washington.edu

Burton Guttman <guttmanb at elwha.evergreen.edu> wrote:
>After following the interesting crow taxonomy discussion from a distance,
>let me add a couple of comments. I don't have specific information about
>the crows, but, as Dennis Paulson points out, Johnston's analysis seems to
>be quite clear: that there is a cline between the main crow population
>and a smaller northwestern population that was evidently isolated for a
>time and was on its way to becoming a distinct species. Some contributors
>to the discussion have wondered that no one is doing a molecular genetic
>analysis. The question is, what would we look for? [stuff deleted]

The scenario outlined in previous posts has a coastal population of Amer
Crow becoming isolated during the last glaciation, acquiring smaller avg
size and distinctive caw, followed by merging of the NW Crow and parent
Amer. Crow populations after glaciation. This assumes that NW Crow is
most closely related to Amer. Crow, in particular to PNW populations of
Amer. Crow. This assumption seems reasonable on biogeographic grounds,
but it is not based on phylogenetic evidence (at least none has been
adduced in this discussion). Making a cladogram of all crow species,
including several populations of American Crow, would allow testing of the
validity of this assumption. Finding that the NW Crow specimens [assuming
it is possible to find "pure stock"...] are closest relatives of PNW
populations of Amer Crow would validate the assumption and thus support
the plausibility of the explanation. Finding that the Fish Crow, for
example, is the sister group of the NW Crow would demolish that
assumption, and the explanation that is based on it. Finding that (e.g.)
the NW Crow is the sister group of a clade comprising all populations of
Amer. Crow (and not just a PNW or western subgroup) would not be fatal to
the explanation, but it would suggest that things are a bit more
complicated. Etc., etc. If the NW Crow does fall out with PNW
populations of Amer Crow, it would be possible to estimate the time since
they were separated, assuming "clock-like" behavior of DNA; a short
estimated divergence time would be good support for the explanation.

>My second point refers to the issue about species definition and what is
>countable. I feel a need to defend the ideal of the biological species
>concept and the idea that a species is not just a taxon like all the
>others--that a species is not just an arbitrary category of the same type
>as genus, tribe, family, etc. The ideal species--and they're certainly
>the majority in the animal world, at least--is clearly definable and
>identifiable as a set of organisms that share a common gene pool. When a
>population reaches the point where it no longer interbreeds with its
>parent population, it has become something quite special, a node on the
>phylogenetic tree that can live its own evolutionary and ecological
>life and start to generate other nodes independently.

Mayr's Biological Species Concept has several problems. It can't
accommodate asexual organisms, and it is next to useless for plants. More
to the point, hybridization is taken as evidence of incomplete separation
of populations; i.e. that "speciation" didn't quite happen. All this
really tells us is that the organisms in question have a common ancestor
(i.e., it is a "primitive" or plesiomorphic characteristic). The AOU
lumped Baltimore and Bullock's orioles because they hybridize. But recent
mtDNA work indicates that the Altamira Oriole, *not* Bullock's, is the
closest relative of Baltimore (and Bullock's is apparently no closer to
the Altamira+Baltimore lineage than is Streak-backed). Thus the ability
to produce fertile offspring persisted in these descendants of a common
ancestor despite repeated lineage-splitting -- i.e., speciation.
Chickadees provide a similar example. Outside of Aves, White-lipped and
Collared peccaries hybridize in zoos despite more than 8 million years of
existence as separate lineages, and numerous lineage splits. Botanists
have recognized this shortcoming problem for years (decades).

To put it another way, all reproductively isolated populations represent
distinct species, but not all distinct species are reproductively isolated from
one another.

If hybridizing populations in fact represent sister groups, then "incomplete
speciation" is arguable. But if they aren't sister groups, then it's simply
wrong.

-- David Wright
dwright at u.washington.edu

P.S. Thanks for your detailed post on location of the White-tailed kites last
summer.