Agreement yet?

Tom DiBenedetto tdib at OCEANCONSERVANCY.ORG
Fri Jul 12 12:27:38 CDT 2002

 John R. Grehan wrote:

> I don't intend to speak on behalf of anyone but myself. In general one has
> a biological hypothesis of relationship, and biogeography may generally
> proceed from that point. This of itself does not preclude geographic
> relationships being considered (should one wish to) to evaluate the
> phylogeny in question or to even offer a hypothesis of relationship before
> the biological analysis is carried out.

When constructing tracks (as per instructions on your website), one connects
nearest neighbors between taxa (after having connected nearest neighbors within
taxa). A "phylogeny-first" approach would, it seems to me, demand that one connect
closest relatives rather than nearest neighbors. (If one had a fully resolved, fully
bifurcated cladogram in which each node represents a taxon, then perhaps the two
approaches would be identical since you do indicate that one must do "within taxon"
linking before "between taxon" linking - in other words, one could work ones way
down the tree).
I think that a basic cladistic critique of your method would focus around the question
of what the meaning might be of a link between  two taxa that may not be each
others sisters - as far as gaining any insight into the evolutionary history of the group.
You mention the possibility of offering relationship hypotheses on the basis of
geographic information - does this mean that you envision making these track links
between nearest neighbors and proposing that these might indicate closest
relatives? If so, then I wonder from where comes the assumption that closest
geographical proximity is evidence of closest relationship. That strikes me as an
evolutionary process assumption that would unjustifiably influence ones phylogenetic

> My view is the 'integration' between Hennigian principles and biogeography
> and Croizatian biogeography never occurred. ...

I think that N&P raised the concerns I outline above. I find them to be valid concerns
such that I feel that any failure of integration detracts from the utility of the
biogeographic method.

> Vicariance biogeographers first claimed tracks to simply be area  cladograms
> mapped out geographically, but they never applied the minimum
> spanning criterion that would make that relationship effective.

As I recall, their point was that repeated discovery of the same track, when analyzing
many different taxa, would indicate a "general" track, and that this would be evidence
that a particular geological phenomenon might be influencing the distribution of these
many different taxa - thus the clade distributions would be informing our
understanding of geological history as well as providing a common explanation for
the taxic distributions. (Isn't this Croizatianism with a Hennigian emphaisis on
clades?). Dispersal explanations would be seen as plausible or necessary only if the
taxic distributions were not fully explained by the common phenomenon. I am not
sure where the "minimum spanning criterion" fits into this conceptual scheme, nor do
I fully understand where panbiogeographers part company with this approach.

Tom diBenedetto

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