[Taxacom] Susan Renner (2004) on dispersal

[John Grehan]

Dear Colleagues

Since Ken brought Renner's (20014) article to attention and given my
dismissal of the content as nonsense (since Ken, as usual, declined to
specify the nature of the evidence by which this article deserved
attention) it may be useful to some on this list to think further about the
veracity of this paper should they have the opportunity to consider.

Given that the title is 'Plant dispersal across the tropical Atlantic by
wind and sea currents” one would anticipate some empirical foundation.

This review article is said to present evidence on the monophyly and ages
of angiosperm lineages ranging across the tropical Atlantic” with a focus
on genera that seem “more likely more likely to be

attributable to long-distance dispersal than those involving families or
species”. The basis for this contention is based on “Molecular phylogenies
and age estimates from molecular clocks are available for 11 disjunct
genera, tribes,nand species.”

The Introduction asserts that “Flowering plant species and genera often
will be younger than 95 million years, making it likely that their presence
on both sides of the Atlantic results from dispersal across that ocean.”
This is followed by the question “Can we begin to infer the timing and
direction of dispersal events in the past?” The answer is based on the
contention that based on “estimates (based on calibrated genetic distances,
i.e., molecular clocks) we expect to see patterns in time and direction of
transoceanic dispersal events”.

OK -so this is all there is in a nutshell. Renner just reviews the
literature and accepts molecular divergence dates as actual or maximal and
“To augment the limited data from flowering plants, the discussion below
therefore includes three particularly well-studied trans-Atlantic animal
disjunctions

to illustrate the kinds of data that need to be evaluated and as a reminder
that plants are not the only organisms that crossed the Atlantic; so did
monkeys, rodents, and lizards.”

So this is why I characterized the paper as nonsense. It generates data
that does not actually exist – that the molecular divergence dates are
actual or maximal and falsify trans-Atlantic vicariance. Renner does not
bother to characterize the calibration criteria used in the various papers,
but I would doubt that they were not mostly fossil calibrations and perhaps
some island age estimates. What we have here is a typical conflation of
molecular dispersal. Claims of chance dispersal in various papers are
compiled as if they are evidence when they are all just misrepresentations
of molecular divergence estimates as actual or maximal. This is why this
approach has been characterized as one of those impossible things one is
supposed to believe in.

So when Renner states “There is no doubt that plant diaspores are
constantly exchanged between Africa, Madagascar, India, and the Malesian
region via migratory bird flyways, sea currents, and wind, and the list of
taxa disjunct between Africa/Madagascar/Asia accordingly comprises several
hundred genera (Thorne 1973).” this is just nonsense. There has been no
such demonstration – ever. I know these are strong words about the method
used by Renner, but I believe they are justified and so far no one has
demonstrated to the contrary. The only objections I have seen are based on
assertions of belief – usually of the nature that “I can't believe that
taxa are just that much older than their oldest fossil”.

It is not, as some have suggested, pointless to keep bringing this up since
the molecular representation continues to be widely practiced. Science is
about critique and this critique is necessary – otherwise the science is
reduced to dogmatic belief.

John Grehan