More on Biogeographic memory

Pierre Deleporte Pierre.Deleporte at UNIV-RENNES1.FR
Wed Oct 17 13:49:07 CDT 2001


At 15:53 16/10/2001 -0400, John Grehan wrote:

 >A baseline is a hypothesis of historical relationship between the origin
 >and evolution of a track (individual or standard) and the baseline feature
 >crossed by the track.

Many thanks for this statement.
If I understand well, in Panbiogeography when straight lines (tracks)
connecting neighbouring localities harbouring identical taxa cross a
geological feature, and the more if they do this redundantly for different
taxa, this means that the history of the distribution of taxa (origin and
evolution of this neighbouring distribution = track) has something to do
with the considered geological feature?

If we keep at this level of generality (I translate "hypothesis of
historical relationships" by: "have something to do with, historically"),
and if I cross this with your specification further in the message that:

 > I don' think ecological screening is an assumption in panbiogeography.

then I am reinforced in my view that Panb. as it stands is largely
conceived as an "exploratory" approach (rather than "descriptive",
finally), trying to identify "patterns of spatial distribution" some
specific way (minimal spatial spanning networks) without assumptions (or
restrictions) concerning ecological requirements of the organisms involved
in the analysis (be they marine, freshwater, flying or subterraneous
organisms, everything would go without discrimination).

Hope I don't distort your thought, I'm just trying to put your
Panbiogeography in my own words to be sure I understand well.

The pretended lack of ecological discrimination between organisms you deal
with puzzles me the most.

May I just suggest that you recently stated another way, and quite
explicitly, treating organisms with coastal habits not in straight line
connections but along the coasts? (see quotation further below for memory).
Hence ecological screening would, not surprisingly fo me, also be a common
(even if generally implicit) assumption in Panbiogeography.

In a large-scale approach supposed to deal with continental drift (be it
"just exploratory" or attempting to test specific models) I would admit
that you treat freshwater organisms like terrestrial ones (all being
"continental" organisms some way), but even in this case I really don't
think you would treat marine organisms (or interpret their tracks) just
like continental ones. Would you?

Relevant quotation, including an explicit consideration and extrapolation
to the past of the lineage, of "coastal habits":

At 13:11 12/10/2001 -0400, John Grehan wrote:

 >> (P. Deleporte:) I just want to emphasize the possibility of practising
an >> "ecological screening" of the taxa we use, according to different
 >> biogeographic problems at stake.
 >
 > (J. Grehan:) In panbiogeographic methodology the shortest line length
 > provides a criterion for track construction IN THE ABSENCE OF
 > OTHER INFORMATION (sorry - caps just for emphasis). So I concur
 > with Deleporte that one may draw this minimum distance in other ways
 > than by a simple straight line.
 > As long as the criteria are explicit the choice is open to critique. In a
 > forthcoming paper on the Galapagos I drew a track between
 > southeastern Brazil and the Caribbean with a line around the coast of
 > S America rather than across the interior to take into account the
 > coastal habit of the organism in question with the supposition (that
 > might be wrong!) the ancestral distribution of that organism did not
 > include the interior.
 > When one looks at Croizat's tracks one can see that they are
 > sometimes drawn other than a strictly straight line - sometimes they
 > follow geomorphology for example. There are many possibilities for track
 > construction and even though our book emphasized the strict minimum
 > spanning tree this is not to preclude those alternatives.

Q.E.D.

By the way, it appears that Croizat seems to have drawn lines "following
geomorphology", just as I suggested Peter Hovenkamp to do with freshwater
fishes / mountain ridge:

 >> (Peter Hovenkamp:) To Pierre: The way across a mountain range may
 >> not be the "shortest" way to connect two groups of fish - but if it
 >> repeatedly turns up between different pairs of sister-groups of fishes -
 >> then maybe the mountain is a relevant feature after all...
 >
 > (John Grehan:) I would agree also.

Thus according to John Grehan, looks like Croizat could not have agreed,
and could have considered that fishes would gain to be treated a specific
way regarding mountain ridges, and thus "following geomorphology" some way...

Please don't misinterpret my goals in emphasizing such apparent
contradictions: I'm not teasing for fun, I just want to illustrate the
burden of implicit assumptions I think we are (generally) carrying in
biogeographic approaches. Maybe we can agree on this and consider the
necessary task of clarification.


On a more general ground:

 >> (Peter Hovenkamp:) Perhaps the confusion is in the use of the word
 >> "synapomorphy"? To me, the use of the word synapomorphy suggests
 >> going all the way from primary homology assessment via phylogeny
 >> reconstruction to recognizing synapomorphies on the preferred tree. I
 >> realize that with a more restrictive view of synapomorphy ("derived
 >> similarity") the whole discussion may seem to be pointless...
 >
 > (John Grehan:) I have not delved deeply into the
 > philosophical/methodological issues of synapomorphy in biogeography
 > and systematics so I cannot offer more than my current understanding
 > of the baseline as a synapomorphic character for tracks. I would agree
 > that its a subject that could warrant further
 > evaluation and investigation by those so interested.

Guess we can look forward for the coming paper by Juan Morrone, which seems
to point exactly at this question:

At 09:38 09/10/2001 -0500, Juan Morrone wrote:
 > Dear Curtis (an all),
 > as I conceive it (although panbiogeographers as well as cladistic
 > biogeographers will disagree) main differences between
 > panbiogeography and cladistic biogeography are not only
 > methodological, but their practice seeks to answer different questions.
 > I have began to explore these issues in an article with J. Crisci
 > (1995, in Annual Review of Ecology and Systematics), and posteriorly
 > elaborated it further. An article in Diversity and Distributions, which
 > will appear very soon, will present this in terms of primary geographic
 > homology (panbiogeography) and secondary biogeographic homology
 > (cladistic biogeography).


In my view, "primary homology" (a priori statement of similarity) does not
carry any historical dimension in itself in phylogenetic systematics. Only
the model of generalized descent of characters in direct lineage with
possible modification (and variants around this assumed model of
evolutionary process, or "transformation law") provides this historical
dimension, leading to the possible "secondary" identification of the best
fitting synapomorphies via phylogeny reconstruction.
The same way a neighbouring occurrence of two localities (track) may
possibly be viewed as "geographical similarity", without carrying a
historical dimension in itself. And the same way a so-called
"biogeographical synapomorphy" would require a statement of "similarity by
descent" (or at least "similarity through a history of transformation")
some way.

The specific historical dimension of "a track in general" (geographic
proximity) still escapes my understanding, although I noted well that in
the method it is intended to have "something to do" with a geological
feature crossed by the track.
Hope this helps.


I thus agree with John Grehan on the following general point:

 > (J. G. to P. Hovenkamp:) I guess I take the view that nothing 'speaks for
 > itself' in a pure unmodified ideal sense. Patterns speak only in the
 > context of meaning we give to them.

hence my suggestion to dig deeper into the question of which way does
Panbiogeography is (or could) be "giving meaning to a distribution
pattern", to be contrasted with the way other methods do (explicitly, or -
more difficult -implicitly).

Seems quite clear that some consideration of e.g. geomorphology (Croizat
above) and ecological requirements (John Grehan above) CAN come into the
Panb. reasoning some way.
This is not a charge from me, to the contrary, in my view the extreme
reverse position would not make sense in historical biogeography (just-so
patterns of neighbourhood for patterns' sake).

Pierre




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