Vicariance patterns and baselines

[pierre deleporte]

At 22:36 17/07/2002 -0400, John Grehan wrote :

>For a particular cladogram of relationships for a taxon one can map out the
>track and identify what baseline may be appropriate to classify the spatial
>homology. For example, one might have a cladogram each for two groups that
>are identical in arrangement (and thus suggesting to some a common history)
>yet one group has a track spanning the Pacific Ocean basin, the other
>spanning the Atlantic.

This seems a nice example to question possible explanatory laws
justifying  Panbiogeography as a method.

If I am correct, the situation described above would fit the following example:

- taxon ((a,b)c)  with a in Asia, b in america, c in Africa, and the same way:
- taxon ((A,B)C) with A in Asia, B in America, C in Africa
- a contains 10 species, b and c one species
- A and B contain one species, and C 10 species

For 'cladistic" vicariance biogeography, the pattern is the same in the two
cases. Under the model of vicariance processes dominating speciation, this
would indicate in each case a first vicariant event between Asia + America
(ab and AB) and Africa (c and C). Later, a second vicariance event would
have separated Asia (a and A) from America (b and B).

This approach does not take into consideration the number of terminal taxa
(species...) on each continent, provided that they form a monophyletic
taxon on each continent. The implication is that intra-contiental events
leading to more or less local species diversification are not informative
about vicariance between continents.

Now, the Panbiogeographic approach gives a same track for both taxa (Asia -
America - Africa), but with a baseline across Pacific for ((a,b)c), and a
baseline across Atlantic for ((A,B)C). Baselines connect a "main massing"
(zone with many terminal taxa) to a next zone with fewer taxa, thus it
connects a zone with 10 species to the closest zone with 1 species, and it
seems that the baseline for (abc) must be Asia --> America (Pacific
baseline), and the baseline for (ABC) must be Africa --> America (Atlantic
baseline).
In this example, the Panbiogeographic approach does not use the
phylogenetic information that taxa in Asia and America are closest
relatives in both cases, which seemed relevant for vicariant explanation.
Instead, it considers the relative diversity of more or less related taxa
on different continents.

My question : what is the theory allowing to interpret this difference of
species diversity on different, but relatively close, continents, for taxa
that are, or are not, sister-groups, as a clue for some historical
interpretation(s) ? And consequently, what historical interpretation(s) in
the present case ?

A problem, in my view : rates of speciation, or rates of survival of
species, may perfectly be linked to environmental conditions prevaling on
different continents, and thus would have no connection with the Pacific or
Atlantic "tectonic history". The more when different taxa are compared,
which may have different ecological requirements, explaining species
radiation in different continents for different taxa.
This may perhaps shed more light on wat I called the implicit "uniformity"
assumptions behind panbiogeography, necessary for geometric distance and
relative taxonomic diversity to make universal sense.
The parallell with an exploratory, kind of "data analysis" approach seems
more and more evident to me.
As an analogy, phenetics (numerical taxonomy based on overall similarity)
in general is an exploratory tool at best ("look at some aspects of overall
similarity and guess..."), but phenetics makes historical phylogenetic
sense under the molecular clock model (quite a uniform-regular process
assumption).

Pierre