[Taxacom] Centropyge (was barcode of life)

[Curtis Clark]

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 
many levels:

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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