Real biogeography problem

[P.Hovenkamp]

At 11:06 AM 30-09-99 -0700, Fred Rickson wrote:
>Maybe a real biogeographic problem would be enjoyable and helpful to the
>current discussion.  I would love to have some help/discussion/brainpower
>thrown at a current biogeographic situation I am working with
.so here is a
>REAL problem.

I'm afraid this kind of real problem is not going to help the discussion
much. So far, the discussion was concerned with Earth history - that is the
problem of reconstructing  geographical/geological events on basis of
biotic data. 

The real problem thrown at us here concerns Taxon history - that is, the
reconstruction of the history of a particular taxon. 

As Fred Rickson notes, on basis of a single datum (allopatry of
sister-groups) it is not possible to distinguish a vicariant from a
dispersal event. And we shouldn't try to. 

We can tentatively diagnose vicariance events on basis of repeated
allopatric sister-group relationships (whether these are represented as
tracks or in any other way, I don't greatly care), but we can use
vicariance events as an explanation for the occurrence of a single
sister-group relationship only if we have at least some independent
evidence that these events have really happened.

Peter Hovenkamp
>
>Humboldtia is a genus of small legume trees with 4 species distributed
>along the Western Ghats of south India (moist areas).  Individuals are
>quite gregarious often forming small area of 10s to 100s of trees.  Two
>species have special morphological structures which facilitate occupation
>by ants and other invertebrates.  Two species do not have most of these
>special traits.  Now, a fifth ant-associated species exists in Sri Lanka
>(same moist requirement) and forms dense little areas of many, many trees
>(very common).  DNA analysis shows the Sri Lankan tree sister to the most
>eastern of the Indian species, also an ant form.  On the basis of
>morphology the Sri Lankan species is also the most advanced of the five.
>No other species of this genus exist.
>
>So, it seems to me as if the south Indian area is ancestral and Sri Lanka
>is derived.  If any of the Indian species ever occurred in Sri Lanka, or
>vice versa, I would think the species would still be around given the
>gregarious and successful nature of all of the species within their home
>range.  Now, do I draw a track from the Indian sister species to Sri Lanka
>and claim vicariance across the former, but now submerged, land bridge, or
>do I just do the botanical thing and suggest a wanton seed sometime after
>the Eocene became established, and subsequently evolved into the Sri Lankan
>species?  I really don't have any idea of an answer to this distribution
>question.
>
>Now that is a tight little story and there is still no easy answer.  I may
>have some help because a proposed comparison of the invertebrates living in
>association with each species (within swollen, hollow, self-opening stems)
>might shed some light on whether whole plants and there pals marched across
>the land bridge, or seed arrived and the plants have picked up a new,
>unrelated set of friends.  Any comments appreciated, biogeographical or
>otherwise.
>
>
>Fred R. Rickson

P. Hovenkamp
Rijksherbarium, Leiden
The Netherlands
hovenkamp at rhbcml.leidenuniv.nl
http://rulrhb.leidenuniv.nl/