[Taxacom] Dispersal paradigm (was: Antarctic dispersal)

[John Grehan]

I think Ken is quite right to attribute the allure of dispersal
explanations for evolution as a product of personal experience. Dispersal
as an ecological phenomenon (the movement of animals and plants (seeds etc)
is a highly visible phenomenon so it is not suprising that it has such a
hold over explanations for vicariant (allopatric) differentiation. And
certainly any species may spread to occupy a much larger areas, but this
has to do with range expansion (or contraction), not differentiation.
Obviously a widespread ancestor established its range in the first place
through means of survival becoming means of dispersal.


Ken may feel that ?the most vocal proponents of vicariance (since the
acceptance of continental drift) have often overdone their backlash against
the predominant dispersal paradigm? has not given any empirical foundation
for such a view. This is not about a pendulum of popularity of an idea, but
whether the idea (method, model, theory etc) works for the facts at hand.
Dispersal models have to ignore or explain way allopatry, vicariance does
not. Science is not about ?balanced? views for such a concept is purely
political, not scientific. Einstein?s theory of relativity may have been an
entirely ?unbalanced?, but it matters not a wit.


It is not so much about disagreeing with Ken over panbiogeographic
approaches that is of interest, but the nature of that disagreement. Ken?s
view is based on assumptions about what is correct or not rather than on
empirical evidence. Ken believes that fossil calibrated (or any historical
calibration) of molecular divergence ages not minimal even though that goes
against the facts. Panbiogeographic reconstructions are based on spatial
analysis that recognizes the existence of allopatry being widespread among
taxa, and that this allopatry can be correlated with tectonics that
provides a hypothesis of an actual biogeographic age of origin. Some
analyses show correlations with pre-drift tectonic formations, thus
providing evidence of differentiation prior to drift taking place in
particular regions.



John Grehan


On Mon, Jun 9, 2014 at 8:44 PM, Ken Kinman <kinman at hotmail.com> wrote:

>
>
>
> Hi Michael,                                                           I
> suppose the centre of origin/dispersal view has been dominant for thousands
> of years because humans have been able to recognize that their own species
> is so prone to such dispersal.  And today biologists can see the evidence
> that our species (and related species like Homo erectus) dispersed from
> smaller areas to occupy much larger areas (eventually including the
> Americas for Homo sapiens).  How widely we should extrapolate that to other
> taxa is obviously debatable, but I still believe that the most vocal
> proponents of vicariance (since the acceptance of continental drift) have
> often overdone their backlash against the predominant dispersal paradigm.
>       It really does far more harm than good to swing any pendulum too
> far, and more recently we have the resulting opposing backlash of
> dispersalists against too much vicariance.  Of course, this is not as
> extreme as the pendulum swing which resulted in a total rejection of all
> paraphyletic taxa (and throwing all the babies out with the bathwater), but
> it is still problematic.  In both cases, I believe a middle ground will be
> found that results in more balanced approaches.  In my view that means
> pulling back a bit on the vicariance front that went too far (and easing up
> on attacks against dispersalists), as well as accepting formal paraphyletic
> taxa where they are most useful (thus ending a decades-long backlash
> against all paraphyletic taxa whether they were good or bad).   In any
> case, I disagree with your panbiogeographic approach to both ratites and
> Order Crossosomatales, so I guess it is best to just agree to disagree on
> how much dispersal occurred during their evolution.  I also disagree that
> the age of the eudicot clade is nearly as problematic as you make it out to
> be.   So it goes.
> ---------------------------Ken
>
> Michael wrote:
> Whether or not ideas are accepted or popular is interesting for sociology,
> but is not relevant to logical, scientific argument. The centre of
> origin/dispersal view is the dominant paradigm, and has been for thousands
> of years, but that does not mean it's right. In any case, the eudicot age
> is problematic (there is a whole section on it in my 2014 book (p. 67).
>
> ----------------------------------------------------------------------------------------------------------------------------------------------------
>
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>
>
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