Fwd: Re: Agreement yet?

John Grehan jrg13 at PSU.EDU
Wed Jul 17 22:36:23 CDT 2002

The first part of the following presents comments in response to Tom
DiBenedetto that I forgot to send to the entire list. The second part
presents comments in response to questions Tom raised to the first part.

>>>When constructing tracks (as per instructions on your website), one connects
>>>nearest neighbors between taxa (after having connected nearest neighbors
>>>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).
>>Agreed. Where there is no phylogenetic resolution nearest neighbor
>>connections are made. Where there is some kind of resolution one may
>>connect closest relatives - according to the criterion of minimum
>>distance. One may also compare minimum distance connections between taxa
>>to their proposed phylogenetic relationships. Where the two do not match
>>one may be in a position to consider re-evaluating the phylogeny - should
>>one wish to. Or one may taking into account possible geological
>>correlations or biological mobility etc.
>>>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
>>>others sisters - as far as gaining any insight into the evolutionary
>>>history of the group.
>>The meaning may be that the taxa in question are really each others
>>sisters, or that they are part of a complex vicariant event involving a
>>polytomy where the characters are assorted in such a manner to provide a
>>dichotomous level of biological resolution.
>>>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
>>>If so, then I wonder from where comes the assumption that closest
>>>geographical proximity is evidence of closest relationship.
>>Its not a matter that closest geographic proximity IS evidence of closest
>>relationships. It provides a geographic criterion of a possible
>>relationship according to the spatial criterion being applied. I don't
>>know how to argue the case in any definitive terms. Is it true that in
>>general for a vicariant series of related taxa the nearest neighbors have
>>a closer biological relationship than those further away? Has anyone done
>>a statistical test on this phenomenon?
>>>That strikes me as an
>>>evolutionary process assumption that would unjustifiably influence ones
>>Unjustifiable or not the method has resulted in predictions of
>>relationship that were later corroborated by new biological information.
>>> > 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.
>>I have no problem with the second sentence as it stands.
>>> > 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
>>No, in that they rejected the spatial criterion for analyzing the
>>geography of area cladograms. Morrone and others have suggested that
>>biological cladograms need to be analyzed in the context of a track,
>>while vicariance cladists such as Nelson and Platnick appear to be opposed.
>>>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.
>>Panbiogeography focuses on spatial homology, not distinguishing
>>'dispersal' and 'vicariance' which is a pseudo-problem in the larger
>>scheme of biogeography.

Thank you John for your patience with my questions - I have a few more.
John R. Grehan wrote:
 > Morrone and others have suggested that biological
 > cladograms need to be analyzed in the context of a track, while vicariance
 > cladists such as Nelson and Platnick appear to be opposed.
I don't quite picture what this means. I have a cladogram. What
track-context do I
analyze the cladogram in? A pre-existing track? A track drawn with those
What does the analysis conclude?

For a particular cladogram of relationships for a taxon one can map out the
track and identify what baseline may be appropriate to classify the spatial
homology. For example, one might have a cladogram each for two groups that
are identical in arrangement (and thus suggesting to some a common history)
yet one group has a track spanning the Pacific Ocean basin, the other
spanning the Atlantic.

 > Panbiogeography focuses on spatial homology, not distinguishing 'dispersal'
 > and 'vicariance' which is a pseudo-problem in the larger scheme of
 > biogeography.
Homology is one of those popular terms that tends to be used in different
ways by
different disciplines. To a cladist, homology means historical sameness - two
character states judged to be the same by virtue of descent from a common
What exactly does spatial homology mean?

Spatial homology means distributions share the same defining feature. The
historical implication is that two distributions are judged to be the
'same' by virtue of their having 'descended' from a common historical event
or process. Distributions sharing a Pacific baseline share a Pacific
history in the evolution of the taxa that is no shared by Atlantic or
Indian taxa for example.

Dispersal and vicariance seem to me to be two valid explanations for why a
might occupy a different space than it did at a previous time. Choosing the
appropriate explanation seems to be an interesting question for anyone
interested in
the evolution and biogeography of the species. In what sense is this a
false problem?

I view it as a false problem in that selecting vicariance or dispersal does
not really accomplish anything. Its just a binary opposition and selecting
one just requires a rejection of the other, but the other alternative does
not just disappear as a possible solution. In all the obsession with
dispersal and vicariance going back to Darwin and Wallace (and perhaps
others also) there was a general failure in biogeography to even recognize
the basic patterns of spatial geometry for life and its correlation with
geology - until Croizat.

John Grehan

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