[Taxacom] Panbiogeography

[John Grehan]

Jason,



Mike has pretty much addressed the dispersal matter so no need for me to
comment further.



"Furthermore the lack of genetic structure in many insect species, even
in discontinuous ranges spanning thousands of kilometers indicates a
fair degree of dispersal." Even if there were a genetic structure there
would be dispersal in order to obtain the ancestral range/





"So what happens when a biogeographic break and a tectonic [or
geomorphological] one do not
coincide?" Then one does not have a basis to link them.



"Furthermore, correlation is not necessarily causation, so don't
panbiogeographers also speculate?" Agreed, so one makes a predication
(speculation if you like). At least there is an empirical basis for the
predication (speculation).


"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?



My reading of the molecular literature indicates a preponderance of center
of origin and dispersal. Various review articles have also claimed that
chance dispersal has now been vindicated by most studies. If you read
Mike's book you will see that also. If you have data to the contrary it
would be interesting.

John Grehan




On Fri, Mar 28, 2014 at 4:43 PM, JF Mate <aphodiinaemate at gmail.com> wrote:

> 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
>
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