Digression within a digression

Robert Mesibov mesibov at SOUTHCOM.COM.AU
Wed Apr 6 16:49:02 CDT 2005


This post responds to Karl Magnacca's of 4 April, but I hope it might be of
more general interest.

"Mosaic" refers to organisms on the map, not habitats. For examples, go to
the hyperlinked checklist on my website,
http://www.qvmag.tas.gov.au/zoology/multipedes/mullist.html, and follow to
the pages on the millipede genera Atrophotergum, Dasystigma and
Gasterogramma for tightly-fitted parapatry, and to the Bromodesmus pages for
a strictly allopatric mosaic. Check the maps and the SEMs and drawings of
(some) diagnostic characters.

Karl: "And how do you differentiate "forms" reliably, if there isn't enough
morphological distinction to separate them as species without geographical
considerations?"

In my mind "enough morphological distinction" and "geographical
considerations" are inextricably tied together in an operational sense. I
feel uneasy if I have to describe a new species from only two or three
localities.

Here's an example. If I'd had in front of me only the three now-holotype
specimens of what I've called Dasystigma bonhami, D. huonense and D. tyleri,
I probably would have identified the lot as "geographical variants" of
Lissodesmus margaretae Jeekel, 1984, because millipedes with large ranges
are sometimes more or less continuously variable across those ranges.
Instead, I had hundreds of specimens from locations scattered around nearly
two-thirds of Tasmania. Studying these I found that the "minor" character
variations had distinct transition zones in the landscape (boundaries of the
mosaic tiles). I followed these up in the field to get more material. I
looked again at the specimens and found other characters which changed
abruptly at these places. (One of these - a synapomorphy missed by both Dr
Jeekel and myself, at first - led me to erect a new genus for the four
Dasystigma tile-species.) I observed that the variations between tiles in
what I was considering diagnostic characters were (mostly) more substantial
than those within tiles. I could discount the between-tile diagnostic
variation as merely ecotypical, because within each of the tiles the
diagnostic characters were present in high country and low, wet and dry,
forest and scrub, on a big range of different geologies and soil types.
(That's another use of geographical information in delimiting species.)

Karl: "I think you have a fairly unusual situation though, and I don't think
it takes very much mobility to keep up a scenario of interchange between the
tiles rather than what's happening with yours.."

Don't know about that "unusual". Mosaics are typical for millipedes, with an
estimated global species total of 80000. Mosaics are also well-known in
other groups (like grasshoppers, many of which fly) where fieldworkers have
sampled intensively across the map, as opposed to relying on opportunistic
collecting. Interchange across tiles? Maybe introgression of some alleles,
but not the form-making genes. Check the recent literature on parapatry and
its mechanisms to see how common and how apparently stable these boundaries
are.

Karl: "How did you do the geographical phylogeny?  By taking adjacent or
parapatric species as being most closely related?"

The phylogeny isn't done yet, just roughed out, but yes, one organising
principle is the default guess that species in parapatry are sisters. Where
morphology suggests this is nonsense, I look for other possibilities, like
the Wallace idea I cited earlier. My underlying evolutionary model says that
speciation can be allopatric, parapatric or sympatric - I'm not prejudging.

I have to stress that my goal is not, in fact, specifically to come up with
phylogenies. It's to hypothesise an evolutionary history for the many
millipede lineages in Tasmania. That's an "historical-narrative explanation"
rather than a "nomological-deductive explanation", to quote the
philosopher/zoologist Walter Bock, and earth-history narratives from my
geology mates go into the mix as well. Since I will never know the true
history, the best I can do is minimise contradictions with known facts as I
go along.

I also emphasise that I'm not hoping to come up with yet another historical
biogeography method of purportedly general applicability, to fill the
journals with hundreds more pages of debate. I study particular creatures,
and I would like to have some explanation of the particular where's and
when's of their history.
---
Dr Robert Mesibov
Honorary Research Associate, Queen Victoria Museum and Art Gallery
and School of Zoology, University of Tasmania
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 6437 1195

Tasmanian Multipedes
http://www.qvmag.tas.gov.au/zoology/multipedes/mulintro.html
Spatial data basics for Tasmania
http://www.geog.utas.edu.au/censis/locations/index.html
---




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