Biogeographic memory

[STEPHEN MANNING]

At 02:17 PM 8/24/01 -0400, John R. Grehan wrote:
>>May I interject something here from left field?  I agree that
>>biogeographic analysis probably is not to be informed by phylogenetic
>>analysis based on other characters, but what about the opposite?  I do
>>believe that phylogenetic analysis can be informed by biogeographic
>>considerations, if paleoclimates as well as present climates can be
>>determined with reasonable certainty.  What do you think about this?
>>
>>Steve
>
>Thanks for the pertinent observations. I don't know if you have seen the
>panbiogeography book, but it more or less outlines my position on the
>relationship between phylogeny and biogeography. Phylogenetic analysis is
>necessary to postulate a particular biological relationship that can be
>submitted to biogeographic analysis. The latter is independent if there is
>an independent criterion for relationship. In panbiogeography that
>criterion is spatial homology. One may reassess the biological
>relationships according to their spatial relationships through criteria
>such as common baseline, spatial geometry, main massings etc. Any of these
>spatial characters may lead one to reassess the biological hypothesis, just
>as the biological hypothesis itself introduces constraints on the spatial
>analysis in the first place. Some such as some cladists would view this as
>circular whereas I look at it as reciprocal illumination.
>
>If you introduce other issues such as paleoclimatic considerations what one
>is doing is taking a biological pattern of relationships and assessing it
>in some way by its relationship to a historical model which is only a
>theory about the past. The critical issue is how does one link the
>biological pattern to a particular historical event hypothesized to occur?
>In most cases biogeographers just assert that there is a link.

Yes, this is helpful.  There are non-biological analyses that can inform us
regarding paleoclimates.  So we learn that certain layers of rock were
previously under water, etc.  We also can analyze the rock strata
geochemically and do other things that could have predictive value as to
what kinds of mutations would be selected for or against in the environment
at the time the stratum was at the surface or under water etc.  Then if
fossils are described from a stratum with certain characteristics which
tend to have selective significance (e.g., xerophytic adaptations in
plants) and it is known independently that this was a desert at the time
the fossil was deposited, one would assign less phylogenetic significance
to this than one would if the character were not so obviously an adaptation
to a particular environment.

In simpler terms, we don't put succulent Euphorbiaceae phylogenetically
close to Cactaceae (at least I don't think we do) even though many
character states may be scored similarly, because of our perception that
both have adapted independently and in parallel to desert conditions in
different parts of the world.  This is based on easily observed
ecological/biogeographic data.  My point is that if we extrapolate back in
time, similar patterns may emerge.  Two fossils that appear quite similar
may be the product of similar selective pressures operating in parallel
rather than close phylogenetically, and knowledge of past climates and
biogeography can help decide when this is the case.

Steve

>I'm not sure if that sheds much light on the matter.
>
>
>John Grehan
>Frost Entomological Museum
>Pennsylvania State University
>Department of Entomology
>501 ASI Building
>University Park, PA 16802. USA.
>
>Phone: (814) 863-2865
>Fax: (814) 865-3048
>
>Frost Museum
>http://www.ento.psu.edu/home/Frost/index.html