[Taxacom] Old taxa on young islands

John Grehan jgrehan at sciencebuff.org
Fri Feb 4 06:15:29 CST 2011

For those of you interested in biogeography but not subscribers to the
SEBA list, I would highly recommend reading  "Old Taxa on Young Islands
: A Critique of the Use of Island Age to Date Island-Endemic Clades and
Calibrate Phylogenies, available at

The paper continues Head's extensive critique of the all too widespread
misrepresentation of calibrated molecular divergence dates as actual or
maximal estimates rather than the minimal estimates that they really are
and how this has led to the mistaken claim that molecular clocks have
demonstrated the reality of long-distance intercontinental and island
dispersal as widespread and pervasive. Despite these critiques, the
practice seems to be too tempting to drop and continues to be the
general state of play for biogeographic papers using molecular

 In this paper Mike tackles the many examples of inconsistent and
contradictory approaches to molecular clocks and the biogeography of
island biota and the problematic use of island age as a supposedly
alternative approach to fossils for calibrating molecular clock dates.

 One example that many will find of interest is the case of Hawaii that
is widely represented as a proven 'oceanic island' that inherited all
its biota from overwater dispersal, and that some even think all this
took place in less than five million years. Two main assumptions for
this model of Hawaiian biogeography are called into question. 

 The first assumption is that the intraplate volcanism is well
understood as a fixed mantle plume hotspot. But it is pointed out that
despite the popularity of this model the causes of intraplate volcanism
are the subject of intense debate among geologists.  One example is the
possibility that the linear age sequence of volcanism could have
developed along a propagating fissure. Heads points out that there is no
need for biologists to accept any one geological theory as the basis for
biogeography and fit a scenario into that. 

 The second assumption is there were no emergent islands at one time in
the Eocene-Oligocene. Despite admitted uncertainties of estimating
island longevity, these estimates have been accepted uncritically by
many biologists. Although the evidence has been characterized as
compelling, the method has underestimated the height of extant
volcanoes. Other problems include the presence of other seamounts and
the admission that transient low islands did exist even if high islands
did not. 

 Heads points out that if the Hawaiian chain formed along lines of prior
tectonic stress there may be recurrent volcanism and the age of an
island may not be the same as the age of the exposed rock, and that a
hotspot island may grow at one end while subsiding and eroding at the
other so that the eventual result is an old "island" with a young
stratigraphy. The biogeographic consequence would be that many
biogeographic patterns may be older than the individual islands, and
that even where phylogenetic sequence seems to match island age, others
do not - something described as "challenging to explain".

 There are many other examples covered in this paper which is perhaps
the most comprehensive evaluation, not only of molecular clock dating
for island life, but also the very fundamental question of the
relationship between island origin, tectonics, and biogeographic

 John Grehan



Dr. John R. Grehan
Director of Science and Research
Buffalo Museum of Science
1020 Humboldt Parkway
Buffalo, NY 14211-1193

email: jgrehan at sciencebuff.org
Phone: (716) 896-5200 ext 372
Fax: (716) 897-6723


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