Explanations in biogeography

Stuart G. Poss Stuart.Poss at USM.EDU
Mon Oct 4 11:21:19 CDT 1999

John Grehan wrote:

> Since Stuart Poss does not appear to recognize spatial structure as
> informative, he would fit into one of these or another category I would
> assume (but I can be corrected on this).

I don't know any biologist who would argue that an an organism's geographic
location is not informative.  Indeed, to have knowledge of any organism one must
be able to observe it somewhere.  Likewise, the same is true of time, and hence
fossil evidence is particularly important in biogeography.  Time and space are
not solely the provenance of panbiogeographers.  Others have been known to occupy
and observe the effects of both.

In a previous post, you note that if a track is correlated with a vicariance
event, then you can make historical predictions.  I would not disagree, so long
as you have information that this is in fact true.  However, my point is that
simply making the assertion that a track drawing an association between two areas
exists does not provide an explanation for a disjunct distribution.  This is true
because a wide number of possible interpretations of how such an association, if
real, could have arisen.  One must be able to choose among them to arrive at an
explanation.  A given disjuction could have resulted from vicariance, but it is
also clear that disjunct distributions in many cases have not arisen from
vicariance.  One need only to examine the disjunct distributions of many invasive
species whose disjunctions have occurred in our own lifetimes to see that this is
true.  We often do not need to peer deep into the past nor wait for continents to
move, before the highly dynamic dispersal of individual organisms capable of
reproduction makes itself evident.

For many marine organisms it is extremely difficult to distinguish vicariance due
to moving plates from multiple episodes of dispersal, particularly when the track
geometry is the same for both kinds of events and when the episodes of dispersal
may be separated in time measured in a scale similar to that for tectonic
motions.  It is even more difficult to exclude the possiblity of dispersal
followed by extinction as accounting for disjunct distributions along such a
track, particularly for organisms that occur in highly erosional environments not
likely to leave much of a fossil record.

You speak of "spatial structure", but in many cases the relative duration of such
"structure" must be demonstrated before it can be assumed.   In my view, simply
drawing tracks between disjoint distributions and making assertions about
vicariance in the absence of information that would preclude extinction and/or
multiple episodes of dispersal along the same (geometric/spatial) track does not
provide a compeling case for discriminating between dispersal and vicariance as
explanations for the disjunction.   Indeed for a given distribution, it may be
that both phenomena are involved.  They are not exclusive.  I have yet to
appreciate what in the pangeographic method  of track drawing/discovery allows us
to infer such prior knowledge, that would prima fasciae in my view, be necessary
to make such distinctions.

I would argue that such knowlege must come from the evidence regarding the
biology of the organisms and the relative ages of the taxa and habitats involved
and not their spatial position, per se, although obviously all organisms occupy
some space and are likely to be adapted to life in that space as the result of
natural selection.  Without doubt  certain kinds of processes have a strong
spatial component, as for example two islands that are closely adjacent or in the
path of an ocean current that makes it relatively easier for dispersal between
them to occur, or in general it is warmer at the equator than at the poles and
this may impose physiological limits to dispersal, etc.   Other spaces
(environments), perhaps adjacent will vary somewhat and hence the ability of such
organisms to occupy such other spaces  will be dependent upon the probability of
their survival in that space (or their survival in reaching that space).

Your arguments will be more likely to be useful when the vagility of a given
species is low and it is so fixed or adapated to a particular habitat that the
probability of its dispersing elsewhere is extremely low.   Certainly, the
presence of caecilians on various Indian Ocean islands do provide good examples
for disjunct distributions that seem explanable only through resort to plate
tectonics.  However, one can not easily use the same tracks to explain the
presence of certain species of the lancelet genus Epigonichthys at many of these
same islands.  One can draw the tracks, but their explanation are in all
probability quite different.  Generalization is not warranted.  No "pattern"
exists, just a confounding of multiple "patterns".

Since, I believe in earlier posts you noted an unwillingness to accept natural
selection as the primary mechanism of evolution, it is not surprising that you
find it necessary to adopt explanations of geographic distributions that do not
involve natural selection or effects that do not befall upon individual
organisms.  I would argue that such an approach will always be futile even in
cases where vicariance does explain the disjunction, because the information
necessary to make the distinction among different possible "tracks" associating
disjunct parts of a geographic distribution will come from the biology of the
organisms and not the space they occupy in a purely geometric sense.   To see
that this is true one need only observe that the notion of track geometry is
insufficient to explain distributional limits.   If only spatial geometry were
required to infer the origins of distributional patterns, how could one account
for the inability of many organisms to occupy the "spot next door" even though
they have disjunct distributions that are widely separated?   Perhaps there is
another part of the panbiogeographic algorithm that still needs to be specified
to account for such important biogeographic phenomena.

Stuart G. Poss                       E-mail: Stuart.Poss at usm.edu
Senior Research Scientist & Curator  Tel: (228)872-4238
Gulf Coast Research Laboratory       FAX: (228)872-4204
P.O. Box 7000
Ocean Springs, MS  39566-7000

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