[Taxacom] Dispersal paradigm (was: Antarctic dispersal)

[John Grehan]

These are some interesting points. Comments inserted below:


?It's also important to remember that speciation also happens across a
range of time.  Even in cases where populations were instantaneously split,
the sub-populations on either side of the barrier don't themselves
instantly become distinct species. Time has to pass, populations diverge,
biological barriers to reproduction happen.  The scale of time represented
by this range of speciation time no-doubt varies according to the biology
of the particular organism, and the magnitude of selection pressure or
drift or environmental mutagens or whatever -- but I imagine that it may
well overlap with the scale of time across which vicariant events occur.?


I see nothing problematic with these observations. It has never been
asserted (in panbiogeography) that the estimated date of a vicariant event
is any more precise than the geological estimate for that event. One aspect
that is different from molecular divergence is that fossil calibration
gives only a minimum date whereas a tectonic correlation offers an absolute
date, albeit with varying degrees of uncertainty (so one might say, for
example, that a tectonic correlation indicates separation at 100 Ma plus or
minus 20 Ma). It is indeed true that biological divergence may not occur at
the precise time of the vicariance event, but nevertheless the spatial
isolation leading to that divergence begins at that time (or one would not
see the biogeographic break coinciding with the tectonic break).

?Which brings me back to the question I asked some months ago.... What,
exactly are we debating?  Has someone claimed that vicariance plays
absolutely no role in the history of speciation?  Has someone claimed that
dispersal plays absolutely no role in the history of speciation?  Has
someone claimed that allopatry is always a necessary component of
speciation (and what definition of "allopatry")?  Are we really just
arguing about what has played the *most important* role in the overall
history of evolution on this planet?  If so, then do we really make any
progress in addressing  that question by extolling on this particular bird
or that particular plant?  What about fishes? (Freshwater? Marine?)  What
about insects?  What about fungi?  What about algae? What about plankton?
What about bacteria??


Great questions! (rhetorical or not). Panbiogeography does go beyond
extolling particular organisms. Indeed, that made Croizat?s approach such a
contrast to many previous studies, and now with Heads? two books one is
afforded the opportunity to see the biogeography of many different types of
organisms. But as to the main questions, I see the fundamental question as
whether or not spatial analysis offers a credible approach to biogeographic
reconstruction, and an independent test of the claims for molecular
divergence timing based on misrepresentation of minimal divergence
estimates as actual or maximal.

?Perhaps the only generality one can make concerning broad patterns in the
history of evolution of life on Earth is that there are very, very few
generalities we can make concerning broad patterns in the history of
evolution of life on Earth.?


Perhaps there are only a few generalities, but that does not mean that
those generalities are not important. For example, one generality that can
be made about spatial analysis is that it can (and has) predict geological
relationships that are unknown to geologists.


John Grehan


On Tue, Jun 17, 2014 at 8:02 PM, Richard Pyle <deepreef at bishopmuseum.org>
wrote:

> Just a quick addition to Jason's comments (I've been loosely following
> this thread, and generally find myself nodding in agreement with Jason):
>
> > Firstly, that geological dates are a range,
> > from first opening to last connection (and I am ignoring the uncertainty
> in
> > determining each of those dates, so there are ranges to each of your
> > endpoints). Secondly, that taxa will vary in their response to the
> barrier
> > emerging. Some will be cut off immediately, others will be able to cross
> it for
> > a while, while others may have populations right around the barrier. With
> > such complex scenarios the idea that you can collapse multiple splitting
> dates
> > into a single common one is just not possible. The current iterative
> method,
> > albeit imperfect, serves better the purpose of crosschecking (our
> differing
> > opinions on dispersal not withstanding).
>
> It's also important to remember that speciation also happens across a
> range of time.  Even in cases where populations were instantaneously split,
> the sub-populations on either side of the barrier don't themselves
> instantly become distinct species. Time has to pass, populations diverge,
> biological barriers to reproduction happen.  The scale of time represented
> by this range of speciation time no-doubt varies according to the biology
> of the particular organism, and the magnitude of selection pressure or
> drift or environmental mutagens or whatever -- but I imagine that it may
> well overlap with the scale of time across which vicariant events occur.
>
> Which brings me back to the question I asked some months ago.... What,
> exactly are we debating?  Has someone claimed that vicariance plays
> absolutely no role in the history of speciation?  Has someone claimed that
> dispersal plays absolutely no role in the history of speciation?  Has
> someone claimed that allopatry is always a necessary component of
> speciation (and what definition of "allopatry")?  Are we really just
> arguing about what has played the *most important* role in the overall
> history of evolution on this planet?  If so, then do we really make any
> progress in addressing  that question by extolling on this particular bird
> or that particular plant?  What about fishes? (Freshwater? Marine?)  What
> about insects?  What about fungi?  What about algae? What about plankton?
> What about bacteria?
>
> Perhaps the only generality one can make concerning broad patterns in the
> history of evolution of life on Earth is that there are very, very few
> generalities we can make concerning broad patterns in the history of
> evolution of life on Earth.
>
> Aloha,
> Rich
>
>
> Richard L. Pyle, PhD
> Database Coordinator for Natural Sciences
> Associate Zoologist in Ichthyology
> Dive Safety Officer
> Department of Natural Sciences, Bishop Museum
> 1525 Bernice St., Honolulu, HI 96817
> Ph: (808)848-4115, Fax: (808)847-8252
> email: deepreef at bishopmuseum.org
> http://hbs.bishopmuseum.org/staff/pylerichard.html
>
>
>
>
>
>
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