Place in taxonomy (finish?)
mesibov at SOUTHCOM.COM.AU
Thu Jul 4 10:28:29 CDT 2002
Pierre Deleporte has asked how, specifically, you can extract historical
evidence from distributional data. Unfortunately I have no Method (note the
capital) and no software package (Geoclade?) to offer, but here are 3
recent papers to think about:
(1) The 3 conifers Athrotaxis cupressoides, A. laxifolia and A.
selaginoides are all endemic to Tasmania. Each has its own set of
morphological and ecological character-states, and each produces fertile
pollen. It's a straightforward exercise to devise 3-species cladograms from
character-states, and you can then fiddle with statistical tests to see
which of the cladograms is most likely. All the cladograms are wrong.
Genetic evidence (RAPD & SSCP analysis of total genomic DNA) shows clearly
that laxifolia is a stable, interspecific hybrid of cupressoides and
selaginoides, and selaginoides may well be the only pollen parent (Aust J
Bot 48:753-758, 2000). Now let's expand on that "endemic to Tasmania".
Mapping Athrotaxis distributions on a fine scale, you find that laxifolia
occurs ONLY where cupressoides and selaginoides are sympatric. Here the
best evidence for clarifying the phylogenetic relationships between 3
species comes from genetic and spatial data, not from a taxon-by-character
(2) In a fascinating study of grasshoppers in the genus Chitaura on
Sulawesi (Biol J Linn Soc 72:373-390, 2001), 4 types of data were
available: exact locations for samples from 60 sites, maximum parsimony
cladograms for colour pattern, a mtDNA cladogram and a morphometric data
set from males analysed using PCA, HANOVA and a Mantel test (with
geographic distance). There are 10 named Chitaura species and the character
data certainly support the conclusion that the Chitaura lineage has
branched repeatedly over the past few million years. However, it is far
from obvious how a conventional tree can be constructed which satisfies all
the data. Instead the authors focus on spatial relationships (especially
what happens at contact zones on the island) to illuminate tree "details".
Read the paper to see how complicated the story is.
(3) A preliminary analysis has recently been made of the connections
between species range size/position and hypothesised phylogeny (Amer Nat
155:419-434, 2000). In the groups studied, range size asymmetry was
associated with relatively recent splits, suggesting that peripatric
(allopatric) speciation was the split mechanism, and sympatry seems to have
resulted from post-speciation dispersal. The hint here is that the geometry
of a set of ranges contains phylogenetic information which would not be
evident in a simple presence/absence summary of locations.
In an earlier post I pointed out that spatial information becomes less
important (more blurred) as you go up the taxonomic hierarchy. A reckless
generalisation: spatial information is essential in studying populations,
very useful for species, important for genera and helpful for families.
This shouldn't be seen as a limitation which reduces the importance of
"place" overall, any more than mtDNA analysis should be dismissed because
it doesn't help us understand basal arthropod phylogeny.
Dr Robert Mesibov
Honorary Research Associate
Queen Victoria Museum and Art Gallery
Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
(03) 64371195; 61 3 64371195
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