More on Biogeographic memory

John Grehan jrg13 at PSU.EDU
Tue Oct 9 09:02:05 CDT 2001

>Readers interested in Croizat's methodology will need to read Croizat then
>- there seems to be no "friendly" summary then that does not go
>significantly beyond Croizat's approach.

One can still learn about Croizat's methodology from panbiogeography since
Croizat, but if one is interested in Croizat's methodology as it was
applied by Croizat alone then yes, one would need to read Croizat. Even for
post-Croizat applications and developments Croizat's works remain necessary
reading (in my opinion at least) as opposed to being rendered a historical
curiosity in vicariance biogeography.

>And I would call these starting points - hardly conclusions.

Each to their own opinion.

>If they are
>conclusions - they are often curiously vague: "attempts to", "emphasizes",
>"can lead".

Again, each to their own.

>But I admit it might have been clearer if earlier I had asked for the major
>*result*  of Panbiogeographic analysis.

I have a feeling that Hovenkamp already knows in his own mind what "the"
major result is. I have my opinions about this, and others can form theirs.

>Then why the remark that "vicariance biogeography seems to simply be a
>version of Darwinian biogeography" you made earlier?

Because most, if not all, applications of vicariance biogeography utilize
the Darwinian concepts of centers of origin and dispersal (with vicariance
simply being part of this framework as correctly pointed out by Mayr) and
utilize Darwinian concepts of spatial structure (such as areas of endemism
as natural units of biogeographic classification) and homology.

>Agreed. In vicariance biogeography (and in panbiogeography as well) we can
>observe commonality in geographic overlap

Except the homology criteria for each is different. Homology in
panbiogeography is a spatial character.

>Grehan's "Darwinian biogeography" seems to be virtually synonymous with
>"Taxon history", and thus does not aim at making novel predictions about
>geology. T

I agree with Hovenkamp on the inability of Darwinian bigoeography to inform
about geology. I don't see any such Darwinian "taxon history" being all
that informative at all - let alone 'coherent.'

>he criticism is invalid. It may be John Grehan's view that
>explaining the distributions of individual taxa is not a useful enterprise

No it is not.

>It would also run counter to the general trend in the CGH
>Panbiogeography-book, where most examples (and many of Croizat's examples
>also) are concerned not with finding common patterns (generalized tracks)
>but with explaining single distribution patterns.

So panbigoeography can accomodate both "earth history" and "taxon history"
in Hovenkamps dichotomy.

>But which I applied deliberately - I see no better term to describe the way
>in which different biological patterns are assigned the same baseline
>(examples cited in earlier posts).

Forcing implies no necessary methodological relationship whereas there is a
methodological requirement for baseline assignment. However, if one defines
'forcing' as necessary assignment then I would agree just as one may
'force' a character to be a synapomorphy by virtue of it conforming to the
crtirion of synapomorphy.

>This makes the distinction one of degree: CGH Panbiogeography simple makes
>use of less detailed geological data.

No, the difference is Darwinian biogeography simply taylors biogeographic
'explanation' onto selected geological narratives about the past, whereas
panbiogeography assigns baseline relationships with respect to geological,
geomorphological, tectonic structures to generate a homology link between
biology and geology. For panbiogeography the geology can be as detailed as
available. I get the impression that Hovenkamp views the panbiogeography
book as the be all and end all of post-Croizat panbiogeography.

>>>So there is
>>>*no* independence.
>>There is independence of method with respect to constructing tracks, nodes,
>>and main massings.
>Baselines significantly left out here. Q.E.D.

Obviously it is necessary to make a methodological connection with earth
history or biogeography has no necessary relationship with earth history
hypotheses other than speculation in light of current knowledge. The
baseline provides that link. The independence of panbiogeography is
independence of its methodology - including baselines - from hypotheses of
geological history. By having a pattern connection with geology Croizat was
able to use baselines to generate novel geological predictions -
predictions that have since received independent geological corroboration.

>Sorry - I  was were dealing with the question whether "derived from" or
>"inspired by" applies best - so the different concepts Grehan mentions are
>irrelevant here.

So was I. If there is no commonality of homology concept between
panbiogeography and vicariance biogeography then 'derived from' would not

>1. Baselines are not independent of geology (admitted by Grehan, see above)

Of course baselines are not independent of 'geology' any more for post
Croizat panbigoeography than for Croizat.However 'geology' is not the
issue. The independence of panbiogeographic methodology (Croizat and post
Croizat) is independence from hypotheses of  'geological history.'

>Making John Grehan's claim that "Panbiogeography can incorporate as much
>detail that resolved (biological) phylogenies provide in track
>construction" a rather unsubstantiated one.

No it does not. It simply means that the book was not focused on details of
phylogeny. It is obvious that one may draw tracks for any phylogeny. The
more detailed the phylogeny the more track connections between related taxa
one may draw. Simple as that.

>And thus relinquishing the hard-fought independence from geology?

Since vicariance biogeographers resort to historical geology/climate to
"explain" cladistic patterns of biological relationship there is no
necessary independence from geology in vicariance cladistics.

John Grehan

More information about the Taxacom mailing list