"Northwestern Crows" have already merged with "American Crows." It took=20
maybe 150 years.

Gene Hunn.

On Tue, 27 Jun 1995, David B. Wright wrote:

> Re the issues Joe Morlan raised in his last installment (25 June) in=20
> the BSC discussion/saga, I think there really are a few distinct=20
> questions here that we should recognize as such:
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> 1) Is reproductive isolation (with interbreeding as the other side of=20
> this coin) a *valid* basis for a species concept?
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> 2) Is reproductive isolation the *best* basis for a species=20
> concept? =20
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> 3) Are the hybridization and potential interbreeding criteria=20
> of the classic BSC the best ways to diagnose reproductive isolation=20
> and, more to the point, lack thereof? =20
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> The answer to #1 is "yes." I would answer "no" to #2 and #3. =20
> Number 2 is a matter of preference: species-taxa are delimited=20
> arbitrarily (but *objectively*, we hope). For phenotypically=20
> homogeneous populations regarded under the BSC as=20
> reproductively isolated, there will not be much argument; BSC,=20
> PSC, ESC, etc. would agree these are distinct "species." =20
> Differences arise when phenotypically distinct populations are=20
> lumped under the BSC. I think species-taxa based on=20
> demonstrated divergence are more useful than taxa based on=20
> lack of repro. isolation. But again, that is my preference. =20
> As to #3, I think that the BSC could do a better job of=20
> diagnosing reproductive isolation/lack thereof, and hence species-taxa,=
=20
> by discarding the hybridization and (especially) "potential=20
> interbreeding" criteria. =20
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> When two recognizably different populations are lumped under the aegis of=
=20
> the BSC on the basis of hybridization evidence, that does not indicate=20
> that interbreeding between the two populations is so extensive that the=
=20
> two populations will merge/become thoroughly mixed -- thus losing=20
> distinctions now apparent -- and conversely, lack of observed=20
> interbreeding does not mean two populations cannot/will not merge. =20
> Hybridization simply does not, and cannot, predict whether or not=20
> two populations will merge, nor even how likely that event is to occur. =
=20
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> But again, given effective population size and observed rates=20
> of gene flow, it is possible to estimate how long it=20
> would take for two populations to merge [maybe someone could=20
> do this to estimate how long it would take for NW Crow to be=20
> "assimilated" by Amer. Crow]. Why not simply use this kind of=20
> estimate to assess *significance* of gene flow between=20
> populations in question? I have to agree with Zink that if=20
> populations that freely hybridize would nevertheless require=20
> millions of years to become thoroughly mixed, they are effectively=20
> isolated reproductively, regardless of the degree of=20
> hybridization/interbreeding observed. Joe countered that this=20
> kind of evidence merely indicates an error in the original assessment of=
=20
> hybridization evidence. It will be interesting to see reassessments=20
> based on comparing these two kinds of evidence. But why can't =92BSC
> proponents use Zink'sapproach to argue that if two populations in=20
> question could merge in (for example) a thousand years given observed=20
> gene flow, they should be regarded as a single species despite their=20
> being phenotypically dissimilar? That would be a lot more persuasive=20
> than descriptions of hybrid zones. I don't have references to these=20
> methods at hand, but you can probably find them in some of R. Zink's=92
> recent work. =20
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> In addition to that approach, investigating cladistic/=20
> genealogic relationships among populations in question=20
> would be very useful for evaluating their effective independence,=20
> or potential lack thereof. And cladistic "tree-thinking" methods=20
> are now being brought to bear on questions of gene flow (e.g.=20
> Edwards 1993, Evolution 47:1118-1137). Cladistic approaches=20
> will also prove useful for the study of hybridization and=20
> introgression in the context of divergence of populations,=20
> and they will undoubtedly shed much light on the evolution of=20
> prezygotic and postzygotic isolating mechanisms. BSC types should=20
> embrace cladistic methods, not shun them simply because PSC types=20
> use them. See DeQueiroz & Donoghue 1988 (Cladistics 4:317-338)=20
> for a sympathetic treatment of reproductive isolation as a basis=20
> for a species concept by a couple of dyed-in-the-wool cladists. =20
> Ed Wiley's _The compleat cladist_ is a decent primer of cladistic=20
> methods, but one can also get a good feel for cladistics simply by=20
> exploring Maddison & Maddison's _MacClade_ application.
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> David Wright=20
> dwright at u.washington.edu
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