[Taxacom] Recent transPacific rafting due to tsunami

Richard Pyle deepreef at bishopmuseum.org
Mon Dec 21 13:20:46 CST 2020

OK, so our disagreement is purely semantic, then.  I can live with that.


Richard L. Pyle, PhD
Senior Curator of Ichthyology | Director of XCoRE

Bernice Pauahi Bishop Museum

1525 Bernice Street, Honolulu, HI 96817-2704

Office: (808) 848-4115;  Fax: (808) 847-8252

eMail: deepreef at bishopmuseum.org

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From: John Grehan <calabar.john at gmail.com> 
Sent: Monday, December 21, 2020 9:04 AM
To: Richard Pyle <deepreef at bishopmuseum.org>
Cc: Taxacom <taxacom at mailman.nhm.ku.edu>
Subject: Re: [Taxacom] Recent transPacific rafting due to tsunami



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 tectonics". 

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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