More on Biogeographic memory

John R. Grehan jrg13 at PSU.EDU
Thu Oct 11 16:29:06 CDT 2001

>>I think we've been over this earlier in a thread on biogeographic
>>homology... My view is that the term homology is inapplicable outside the
>>context of phylogenetic analysis strictly.
>>Craw draws heavily on an analogy between phylogenetic and biogeographic
>>analysis. Granting the analogy, what would be the equivalent of character
>>analysis? In phylogenetic analysis, the purpose of a character analysis is
>>to draw up a (putative) character state tree, which is then coded and
>>inserted in a datamatrix. Would not the equivalent of that be to find a
>>taxon-tree and use that as basis for the biogeographic analysis?

Since a taxon tree contains no spatial homology information it is not
in its own right. The character analysis Craw was referring to is the
designation of distributions
as characters and finding those distributions (characters) that are
homologous (i.e. share the
same spatial synapomorphy. I admit my lack of credentials in graph theory
so I could be
wrong about this. If my primitive understanding is correct the character
analysis has been attempted
in quantitative terms using connectivity associations between localities as
proposed, for example, by
Craw and Page, and nodal analysis such as that by Henderson. I would be the
first to admit there is
not a great deal of this kind of analysis for panbiogeography and probably
that will remain the case
until some graph theoretician takes further interest.

>>>>Accepting this usage, then, in vicariance biogeography we can observe
>>>>commonality when distributions (partly) overlap, in CGH Panbiogeography
>>>>when they overlap _and_ share the same baseline. Is that it?

No as it is possible for two taxa to partially overlap without sharing the same
baseline. For example, the ratites and Nothofagus overlap, but the former has
Indian and Atlantic Ocean baselines, the later a Pacific baseline.

>>The main question in my mind is why CGH Panbiogeography offers such a
>>limited choice of baseline features? Why not let the distribution patterns
>>speak more for themselves? If several patterns agree on a
>>Madagascar-Australia relation (e.g., fig. 1-9 A, 3-3, 3-5), - why on earth
>>"homologize" these with patterns involving India or Borneo (fig. 3-11)?

The baseline offers the hypothesis that the historical origin of all these
is more likely associated with the geographic sector referred to as the
Indian and/or
Atlantic Ocean. In this respect the distributions, even though different in
detail, are
hypothesized to share more in common regarding their spatiotemporal
history, than
with an organism/taxon with a Pacific, or Southern, or Northern baseline.

>>That means that according to John Grehan  such explanations are  not "all
>>that informative at all - let alone 'coherent.'", but nevertheless
>>explaining the distributions of individual taxa is "a useful enterprise"?
>>I'm baffled.

No, it means that it is not my opinion that explaining distributions of
taxa is not a useful enterprise. To put it the other way (to end confusion
- sorry I probably
should have amplified my earlier answer in the first place) it can be
useful to explain distributions
of individual taxa - I am agreeing in this respect with Peter Hovenkamp!
The point I might
differ with many bigoeographic practices is the attempt to 'explain'
individual taxa without some sort of comparative analysis or consideration
of biogeographic patterns in general.

>>It must be again an effect of the CGH Panbiogeography book concentrating on
>>only some parts of Panbiogeography as currently being performed - all the
>>examples in the book show ocean baselines, with the exception, perhaps, of
>>fig. 6-12, which shows global biogeographic track relationships based on
>>Croizat (1958). But then, I may be confused about the difference between a
>>generalized track and a baseline.

No, I don' think you are confused. And yes, the book does emphasize ocean

>>But Croizat had no problems in drawing different lines across the same
>>ocean (again, see fig. 6-12).

And neither do we.

>>In contrast, the comparable results of the
>>CGH analysis (fig. 6-13) show far less detail - with completely
>>undifferentiated ocean basins.

It is a formalization of ocean basin homology, not a representation of
track structure. It
will be useful to some I think in helping to conceptualize the method, for
others it may
well (very likely) be unsatisfactory. If biogeographers such as Peter
Hovenkamp prefer
Croizat's global map as a superior conceptual basis that's fine with me too.

Looking out for tree roots in the woods...John Grehan

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