[Taxacom] Panbiogeography

[JF Mate]

Dear John

after looking at you replies I have realised that we are chasing each
other in circles and that you constantly refer to Michael´s book when
I seek a more concrete answer. This makes it rather difficult and
unproductive. So, I will simply state the following definitions under
which I operate:


-Dispersal: change in location. The difference between long range and
short range is merely in the probability of each resulting in
successful (i.e. surviving and reproducing). "Short" and "long" are
taxon and site specific.

-Barrier: impediment to movement. When I defined what a barrier was I
failed to consider partial blockage but instead opted for a binary
barrier. I see this was a mistake on my part, so I hope this will
clear the matter.


Dispersal by insects >100Km from the source (mostly coastline) are
very common, even when flying a few meters above the surface.
Transoceanic flights by large, easily noticed insects (mostly
butterflies) have been documented several times in recent times.
Considering that there are observed examples of organisms dispersing
beyond barriers (usually crossing bodies of water) I think dispersal
is a valid mechanism.

Furthermore the lack of genetic structure in many insect species, even
in discontinuous ranges spanning thousands of kilometers indicates a
fair degree of dispersal. Granted that the opposite is true and
particularly striking examples are common (headwater stream insects
for example). But since examples from both mechanisms can be found, I
don´t feel this somehow disproves the other.

As to your statements, I feel a couple are inconsistent. I would be
grateful for a clearer explanation:

"... but when there is a biogeographic break correlated with a
tectonic break there is reason to consider that they are related. ...
The panbiogeographic method is about pattern analysis. In principle,
distribution data provide an empirical resource. Speculations about
imagined chance dispersal does not."
So what happens when a biogeographic break and a tectonic one do not
coincide? Furthermore, correlation is not necessarily causation, so
don't panbiogeographers also speculate?

"No, the patterns make the theory problematic as the patterns are
incongruous with the expectations of chance dispersal. ... However you
chose your words, the point is that panbiogeographic analysis shows
that distance is not a predictor of distribution range for
differentiated taxa so the 'probabilities' are contingent upon the
relevant factors of geological, climatic, or human disruption.'
If dispersal is random, the result of the interaction of numerous
factors, is it suprising that simply using one (distance) doesn't
work? An island may be upwind and therefore rarely receive flying
insects from the minland whereas a more distant, downwind island can
receive a fairly constant rain of wind-carried organisms. Gillespie et
al provide a nice review.

"When it comes down to explaining vicariant patterns the principal
choice has been to frame the origin in terms of dispersal from centers
of origin."
I feel otherwise.Can you please support this argument?

Best

Jason