dispersal fantasy

John Grehan jrg13 at PSU.EDU
Mon Jun 24 22:17:27 CDT 2002

I will present some comments here on a paper that was recently drawn to my
attention and give, in my opinion, and example of an absolutely abysmal
scientific quality for a 'biogeographic' study. I say this knowing that
99.9% of systematists practice exactly the same kind of biogeography and
therefore would accept the scientific credentials of this paper as being
first class. I acknowledge, therefore, the likelihood that most on this
list will share that view. However, I make my comments for the few that may
be seriously trying to think about biogeography in a different way. For
those who believe my criticisms are misguided, I will be interested in any

The paper is by Raxworthy, C. J. et al. 2002. Chameleon radiation by
oceanic dispersal. nature 415: 784-787.

In the conclusion of the paper they claim that their analyses provide
evidence for considerable oceanic dispersal of chameleons, and support the
hypothesis that dispersal, rather than continental break-up, was the
precursor for species radiation. Far from providing 'evidence' my
contention is that this was the view the authors read into their
non-biogeographic data. This 'data' comprised preconceived ideas about the
historical significance of matching biological area cladograms to
geological narratives. According to the authors a (vicariance) geological
history is supported where biological area cladograms match a postulated
geological history, whereas the absence of a match is 'evidence' of
dispersal. This approach demonstrates a complete absence of any integration
between biology and geology. Instead biogeography is simply the narrative
invented according to a selected geological narrative.

Of course the authors use the propaganda tool of asserting the geological
narrative is "well documented", but its still a story. They also appeal to
dispersal by accepting the precedence of their 'molecular' clock
reconstructions that are also incongruent with the postulated geological
ages of the areas occupied by the lizards. They do not bother to consider
that they may have novel evidence that the geological age is wrong. Neither
do they consider the possibility that the molecular evidence may be wrong,
and the lack of congruence between biological area cladograms may be due to
differentiation independent of the postulated geological splitting events.

Another argument about dispersal also comes from geology, rather than
biogeography. They claim corroborating oceanic dispersal' by the presence
of lizards on the Comoros islands that 'never had contact with other
landmasses leaving only oceanic dispersal (note the similarity to the
Galapagos dispersal argument. Their contention may in actual fact be
correct, but notice its based on the acceptance of a geological theory. It
is not 'evidence' from any biogeographic analysis.

The claim (abstract) that the study 'further highlights the importance of
oceanic dispersal as a potential precursor for speciation' which may be a
misrepresentation of their actual work. Far from highlighting the
importance of oceanic dispersal, their study seems to highlight the
continued acceptability and credibility of dispersal fantasies based on
geological and genetic narratives - in a supposedly leading scientific journal.

Food for thought and rebuttal.

John Grehan

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