Biogeographic memory

[John R. Grehan]

>
>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.

These remain speculations (hypotheses) that in traditional biogeography
have been used to delineate
biogeographic events (e.g. the possibility of terrestrial connections being
ruled out, or climate being unsuitable
for survival). In panbiogeography such historical speculations may be
evaluated against the spatial patterns
of distribution and it may be possible that the historical
paleorecontructions conflict with the biogeographic
inferences. In such cases it may be possible that the paleoreconstructions
are wrong as much as one
might also consider the possibility that the biogeographic inference is wrong.

>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.

I'm not sure how effective this would be. It seems to be based on a whole
series of speculations.


>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.

It would be interesting to see an example of such a case. For the
Euphorbiaceae/Cactaceae relationship I don't
think its simply a matter of perception about independent origin, but
rather the delineation of characters that
lead to that conclusion. I'm not sure that this is simply a matter of
ecological/biogeographic data leading to
such a conclusions.

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