[Taxacom] Panbiogeography

[John Grehan]

I for one do not recall if Jason or Ken have been labeled dispersalist, but
I do recall identifying particular approaches that were cited by them (or
one or the other) as dispersalist because those approaches invoked
dispersal from centers of origin.



Evidence of organisms being sent across the ocean are indeed factual. I
would not attempt to label their probabilities as to their effect on
biogeographic patterns. It is dispersalist accounts for the origin of
trans-oceanic distributions that have often identified the dispersal as
mysterious etc. But back to the observed events. The indeed occur. The
question that they raise is to what extent they are responsible for the
observed vicariant patterns of distribution? They are often invoked as
evidence that overwater dispersal was responsible for these distributions
in relation to current ocean geography, but are they?  Panbiogeography (as
I understand it) in some ways reverses the argument to look at patterns and
from analysis of those patterns make inferences about the role of dispersal
capability and observed dispersal events in the present.



There is nothing wrong with dispersal hypotheses being generated, but it is
how they are generated that is often problematic. In most cases where
molecular similarity is used to construct phylogeny the authors also
misrepresent minimal divergence estimates as actual or maximal and from
that misrepresentation (which is sometimes deliberate as the authors begin
by acknowledging that the estimates are minimal but 'forget' about that
when constructing the historical events) they generate fictional evidence
of dispersal. I understand that Ken and perhaps others consider the gap
between the oldest fossil and the age of a vicariance divergence event is
too great to be acceptable, but such judgment seems to rest largely on a
personal belief rather than any empirical requirement. Some, for example,
may consider it too incredible for Old and New World monkeys to have
already diverged in the Cretaceous or earlier in the absence of fossils.
And maybe it is too incredible, but whether the fossil record is accurate
about these groups or not is another matter - given that so many taxa lack
fossils at all, or purportedly old taxa have a relatively recent fossil
record (I think tuatara only go back a few million years for example).



When it comes to considering dispersal or vicariance, it is not whether one
stands for either or both in general, but how one reaches those conclusions
for particular cases. In the killfish example one has a geographic pattern
that is consistent with a vicariant history (and is congruent with other
groups of different dispersal capabilities). On the other hand, Briggs just
says that the fish are too young and take his word for it. Other approaches
might use divergence estimates to predict overwater dispersal based on
misrepresenting minimal divergences as maximal. In some cases of freshwater
patterns it has been argued that some members or some related group shows
saltwater tolerance and so marine migration is possible. In theory yes, but
again what might constitute evidence as to whether or not that was
significant in the origin of particular patterns?



When a taxon located either side of an ocean basin comprises one sub clade
that is also on both sides, but vicariates with another subclade on the
other side (this is a very common biogeographic pattern as I presume Jason,
Ken and most others on this list interested in biogeography are aware so
apologies in advance for stating the obvious), one has a primary divergence
that is not between the ocean 'barrier'. A vicariance explanation would be
that the divergence predated the 'barrier' and so the 'barrier' was not a
'barrier' as such in the context of the origin of the taxon. I suppose a
dispersal hypothesis could be generated for this pattern in some way
allowed the taxon to be so highly dispersible as to cross an ocean, but
also not sufficiently dispersible to prevent vicariant divergence in the
first place, or maintain the subsequent vicariism.



Overall, for modern life, spatial analysis of differentiated taxa indicates
that dispersal is critical for the origin of ancestral ranges within a
former geography predating the origin of modern ocean basins and many other
modern geomorphological features both on land and in the sea.



I look forward to reading a specific analysis of Heads' book by Jason and
Ken.


John Grehan


On Sun, Apr 6, 2014 at 3:08 AM, JF Mate <aphodiinaemate at gmail.com> wrote:

> Hi Ken. Sorry for replying so late but i am currently travelling around.
> Your optimism is refreshing but ultimately a waste of effort. There is no
> evidence that you can offer them that will satisfy them because you don't
> know (or them I suspect) what they are arguing against. Whereas you, I or
> most others can accept vicariance and dispersal as mechanisms (and have
> said so repeatedly) we are labelled dispersalists. Go figure.
> When you show them clear, factual evidence of organisms being sent across
> the ocean, these events are either "too frequent" or so infrequent that the
> probabilities cannot be calculated and therefore no different to astrology
> (so much for only being interested in data). What you will never be shown
> are the definitions under which they operate. Funny since they are
> interested in patterns and this requires some sort of operational
> definitions. But when a man's livelihood depends on not understanding
> something, then they won't, no matter what you say.
> "It doesn't matter how beautiful your theory is, it doesn't matter how
> smart you are. If it doesn't agred with experiment, it's wrong." Stuff can
> cross oceans, let them fight facts if they want. I have critters to catch.
>
> Best
>
> Jason
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