[Taxacom] Centropyge (was barcode of life)
lists at curtisclark.org
Mon Jul 5 15:05:01 CDT 2010
On 7/4/2010 7:58 PM, Kenneth Kinman wrote:
> And Mayr's definition of a biological species includes both
> actually AND potentially interbreeding populations.
Ant this was his failing. I'm sure that, if he had been pressed, he
would have acknowledged that gene flow, not interbreeding, is the key.
But because of his limited point of view with respect to actual
organisms, he equated the two. Sadly for the BSC, this argument fails at
1. Interbreeding is not always equivalent to gene flow. Both I and Rich
have given examples from disparate groups, and there are many other
examples as well. What makes this such a problem for the BSC is that the
two are correlated enough of the time that it seems like a reasonable
general principle...until it fails.
2. "Potentially inbreeding populations" is impossible to measure, and
thus not scientific. The sort of experiment necessary to show that
populations (*not* individuals) are potentially capable of inbreeding
would disrupt the populations so much that the answer would be
meaningless. The alternative of assuming eventual interbreeding for any
two populations for which interbreeding hasn't been explicitly ruled out
is a rule-of-thumb, but has no empirical basis.
3. It's difficult to fully explore inbreeding even among individuals.
Prezygotic isolating mechanisms are much more often the result of active
selection against gamete loss, and so tend to occur in species with
specific classes of life history (they are much more common in annual
plants than in perennials, for example). Postzygotic mechanisms go
beyond F1 inviability or sterility to include lack of mate recognition
of the hybrids by individuals of either parent species, and various
kinds of hybrid breakdown (Mayr seems to discount these).
We can't measure potential interbreeding. What we *can* do is measure
actual gene flow (it's non-trivial, but generations of population
geneticists have done it successfully). If gene flow tells a different
story from artificial hybridization (and indeed if it tells a different
story from morphological differences or similarities) we ignore it at
the risk of separating systematics even further from evolution.
Curtis Clark http://www.csupomona.edu/~jcclark/
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University Web Coordinator, Cal Poly Pomona
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