[Taxacom] Recent transPacific rafting due to tsunami

John Grehan calabar.john at gmail.com
Mon Dec 21 13:03:53 CST 2020

Hi Rich, See below

“John -- I assume your arguments apply only to larger terrestrial
organisms, correct?”


“Those of us who work on marine organisms with planktonic larvae have seen
plenty of evidence of "rare" dispersal events in the form of singletons
that appear outside their range, often as juveniles.”

Sure, one sees the same thing with many organisms – birds, butterflies,
fish, snails etc.

“Even in my own tiny lifetime, I've personally witnessed two cases where a
species previously absent from a locality (based on extensive historical
surveys) became established in large numbers. Unless they were brought by
humans (which they weren't), then the only rational explanation is that
they arrived by chance dispersal events.”

I would see this as normal ecological dispersal where species may exist as
metapopulations encompassing different localities, coming or going from
individual localities. Ecological dispersal is an empirical and observable
phenomenon by which ranges may expand or be maintained.

“Hawaii is home to some of the world's highest rates of endemism among
coral-reef fishes. Most of these species have their closest relatives
elsewhere in the Pacific. I guess you could argue that all these species
were at one time widely distributed with regular/frequent gene flow, and
then only later became isolated from each other and diverged.  But that
scenario differs from "chance dispersal" only in degree, not in kind.”

The difference here is not about chance dispersal, but about ecological
dispersal over the ancestral range which explains the range, but not
divergence of descendants.

Among those of us who study coral-reef organisms, arguments that "there is
no evidence for chance dispersal being the mechanism for the origin of
allopatry" sound a bit like arguments that "there is no evidence for
evolution by natural selection", or "there is no evidence for plate

My challenge was for those who invoke unique chance dispersal to explain
allopatry to provide some evidence. Instead I just get rhetoric or
repeating of assertions.

“None of us can be certain about events that purportedly happened in the
past (gene frequencies shifting over time as a result of selection
pressure, continents moving, rare events allowing organisms to expand their
range then become cut off from source populations, etc.)  But at a certain
point, it just seems silly to assume that such things did not happen.”

I'm not assuming anything of the sort. The underlying issue here is that
people repeatedly invoke one off chance events to explain allopatry without
providing an empirical foundation. When the BM says 'we know' that monkeys
voyaged over the Atlantic it is engaging in twaddle.


John Grehan


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