Centre of origin digression

Ken Kinman kinman2 at YAHOO.COM
Tue Apr 5 11:58:33 CDT 2005


John,
     I guess what bothers me about calling them "trans-Pacific distributions" (and especially connecting them with a minimum distance tract) is the possible inference that most of them are the result of only one or two "stories".  Croizat's former land in the Pacific (and terranes making the Americas a geological composite) would only explain some cases where the trans-Pacific jump was made a long time ago.  Many other trans-Pacific jumps were probably more recent, whether the species was carried by rafting, migrating birds, or even very recently by humans.  Add to those other cases of circum-Pacific dispersal (north via the Bering Strait, or south via Antarctica), and you have a lot of possible stories that are not well represented by drawing the shortest possible line across the Pacific and invoking island-hopping.  To me "pan"biogeography seems like a bit of a misnomer, and I can certainly understand why some workers would suggest that Croizat's prediction (of geologically composite Americas) proving true was perhaps a lucky coincidence.

     As for the bibionid genus Enicoscolus, I'm not sure we have enough morphological information to explain how it got to New Guinea.  Only females were known until recently (a 2004 paper finally reports finding males of one of the species).  Once males of other species are found (especially the one in New Guinea and Queensland), perhaps a robust phylogeny can shed light on the biogeography, but it would be premature to narrow the list of possible "stories" at this point.  As usual, I think morphology and life history will trump anything we might guess from widely disjunct geographical distributions.
   --- Ken

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John Grehan wrote:
Thus, for the trans-Pacific distributions (which are identified by everyone (or virtually so whether they profess to be dispersalists or vicariance/cladistic biogeographers) using panbiogeographic criteria of minimum distance and main massing) the story concocted by Croizat was that the general pattern was the result of former land in the Pacific. This story led to the testable prediction that the Americas are a geological composite.  When he made this prediction there was no 'available evidence' from geology, so using Don's criterion this story wasn't credible, yet later geologists corroborated Croizat's prediction.
I would argue that Croizat's story was better than that of the dispersalist/center of origin biogeographers as it was able to predict facts beyond current knowledge. Some time back one dispersalist dismissed this achievement as being the product of 'coincidience' so it seems that even if a method works its not sufficient for people to take it seriously.




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