On Tue, 20 Jun 1995, Joe Morlan wrote:
> Evolution basically works at the level of the population. Selection
> works on the individual members of the population. Evolution is a change
> in allele frequency in a population over time. The BSC has the advantage
> in that it correctly diagnoses those populations which constitute
> independent gene pools and thus independent evolutionary units. No other
> species concept does this. No other species concept diagnoses species in
> terms of isolated gene pools.
>
> This is an important and perhaps unappreciated point. If there is gene
> flow across hybrid zones then the taxa at the ends of the hybrid zone are
> not independent evolutionary units. Natural selection of individuals at
> one end will affect allele frequency at the other. If there is
> constriction of gene flow between populations (reproductive isolation),
> then selection of individuals at one end does not affect the other.

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.

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?

> Ok. Then I'm afraid I'll stand by my position. Past genealogic
> relationships do not necessarily predict the units of independent
> evolutionary futures. The BSC is a concept for the here and now and for
> the future. Hybridization does not mislead. It identifies the current
> boundaries of the gene pools which are, in fact, the true measure of
> biological diversity. What more can one expect from any species concept?

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

David Wright
dwright at u.washington.edu