[Taxacom] Panbiogeography

[JF Mate]

I think that Richard hit the proverbial nail on the head with his
questions, to which I would readily answer "yes" to the first two. In
my mind the argument is not if the relative contribution of dispersal
or vicariance is 50:50 or 1:99; or on the specifics of chameleon
biogeography in Soqotra. These, in themselves, are not vey interesting
to me or most others since they fall outside my/our study group or
area. My only interest is simply making it clear that panbiogeography
fails to consider a mechanism that, albeit possibly minor globally in
comparison to vicariance, is nevertheless real. The examples of
dispersal are there, plain to the eye to see. So even though
vicariance does carve out whole biotas (a mechanism that few since the
1950?s has denied) once in a while chance messes things a bit by
giving a lucky few the opportunity to colonize areas that are out of
reach to others.

As to the relative importance of one mechanism over the other, this
question will answer itself after we have accumulated a sufficient
number of well supported, taxonomically complete and widely agreed
upon phylogenies covering all groups of organisms, a task that nobody
currently reading this blog will see achieved in their lifetimes.

Richard has, however, provided more food for thought with his other questions:

Q4, Sympatric speciation: the case that always surfaces is Rhagoletis
pomonella hawthron vs apple races. It believe that this and other
"trophic" races are the closest examples we have of sympatry outside
of parasite induced (i.e. Wolbachia) assortative/disassortative
mating. I am not sure if "ring species" would count although they are
quite relevant when you consider that gene flow, even when unimpeded,
has limits.

Q5, Michael provided two examples of small, easily transported species
showing highly sructure phylogeographic patterns. I can provide two
different examples. In Daphnia pulex there is little structure
throughout the N. hemisphere
(http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0046620).
Similarly several Aphodius (Scarabaeidae) species show Palearctic
distributions with little structure (J.F.Mat? unpub).

Q6, I?m afraid I have no experience in marine environments but a
similar scenario can be seen in Europe. N and C European plains allow
for easy migration of species E-W. However the glacial periods have
resulted in periodic extinction and recolonization events that have
left a clear imprint in the genetic make-up of populations. The
constrast between N and C Europe and S. Europe in this Triops paper
(http://www.naturkunde-chemnitz.de/dateien/Thorid.Zierold.pdf) is
pretty much text book and found in countless other examples. Multiple
S European refugia (high genetic diversity) and little or no genetic
diversity in N and C Europe. Often one or two species have colonized
these areas whereas the refugia may contain several species that stay
put (from personal experience, Aphodius (Liothorax) complex but pretty
much any invert lineage shows this pattern).

Best

Jason