Subject: Re: species concepts
Date: Jun 25 10:35:35 1995
From: Joe Morlan - jmorlan at slip.net


On Wed, 21 Jun 1995, David B. Wright wrote:

> As Chris Hill already pointed out in this discussion, it is possible to
> evaluate the extent of gene flow across hybrid zones and to estimate how
> long it would take for the hybridizing populations to merge. Some such
> estimates are on the order of millions of years, indicating that it is
> highly unlikely that such merging will occur, i.e. it is likely that the
> BSC will have failed to recognize an evolutionarily independent population.

The rate of gene flow across a hybrid zone cannot be measured on the
order of "millions of years." I fear that there is some confusion
between "hybrid zones" and "zones of overlap & hybridization." The two
are totally different. In a hybrid zone, there is free gene flow and the
hybrids are selectively neutral or better. In a zone of hybridization,
gene flow is restricted and the hybrids are at a selective disadvantage.
In such cases, the hybridizing taxa are termed biological species. In
the former, the hybridizing forms are subspeices.

If a true hybrid zone exists, it will expand fairly rapidly. If a hybrid
zone appears to be stable and narrow, then I question whether it is
properly a hybrid zone. I would expect some factor is preventing the
zone from expanding and thus there is some barrier to gene flow across
it. Identification of such a barrier should call into question whether
the zone was correctly characterized in the first place.


> If the keepers of the BSC flame are so concerned about gene flow, why not
> just evaluate it using modern methods instead of clinging to the misleading
> hybridization criterion?

What are these "modern methods?" Don't they distinguish degrees of
hybridization and identify isolating mechanisms which constrict gene
flow? That's exactly what the BSC already does.

> Both the BSC and the PSC work for the here and now. Neither is capable
> of predicting the future.

The PSC is the ghost of evolution past. The BSC identifies the true units
of evolution, present and future.

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Joe Morlan
Albany, CA
jmorlan at slip.net