[Taxacom] More on 'chance' dispersal

[John Grehan]

This is a bit of a late catch up with a conversation that started on the
list and went off the list accidentally with Richard Pyle who agreed to
have it continue back on the list.

Richard stated the following:

"see, so you’re saying that chance dispersal happens all the time, except
not when it creates “allopatric situations”?



My point is this:  If we acknowledge that chance dispersal **can** happen,
whereby a population representing one species can arrive at a novel
location where that species hadn’t previously existed, then why are we
resistant to the idea that doing so can lead to a founder population that,
over time, leads to a distinct allopatric species?



Maybe it’s that I’m having a hard time understanding what, exactly, you
think requires extraordinary evidence to support."


I can see how this matter may sometimes seem like splitting hairs, but
there is a distinction to be made between ordinary ecological dispersal
that is observable and observable responsible for range expansion and range
maintenance, and usually unique one-off events that are imagined to explain
disjunct allopatry (or even adjacent allopatry). The only "resistance" have
have to the latter is that it is not based on anything empirical, and ends
up requiring correlated patterns of allopatry involving different taxa with
different means of dispersal to all have somehow ended up in different
allopatric places by the same 'chance' process. it's like throwing dice and
again and again always coming up with sizes. Perhaps it could happen, but
it's necessary when taxa are found to share common tectonic correlations
that support a shared vicariance origin. Yes there could be exceptions to
the rule, but they are exceptions and pretty much unverifiable in any
meaningful way. For example, various Galapagos taxa have allopatric
relationships that can be correlated with a range of tectonic events
supporting vicariance. It is possible that one or more Galapagos organisms
have originated as a unique stray, but demonstrating that is so far beyond
empirical capability. But with evidence for tectonicaly mediated mechanisms
for vicariance, why worry about the origin of something that is imagined?
But as Heads keeps on pointing out, everything dispersed to where it is,
but that does not mean that that dispersal is automatically to be assumed
to be responsible for the origin of allopatry.


Cheers, John Grehan