[Taxacom] Underlying synapomorphies

Bob Mesibov mesibov at southcom.com.au
Fri Mar 6 16:27:59 CST 2009

Many thanks to Fred, Ken and an off-lister for clearly explaining the
context in which 'underlying synapomorphy' could be useful as an

Any thoughts on how this explanation could be tested? Take my first
example, where 2 out of 50 species in a genus share an apomorphy. Given
what we now know from evo-devo, there could be a single regulatory gene
controlling this apomorphy, and the apomorphy could be fairly complex,
too. This gene and the others needed for activating the apomorphy might
be present in every species in the genus. In this sense, the apomorphy
could be truly 'underlying'.

Ah, but which way does the apomorphy work? Which is the plesiomorphic
state for the genus? Is it 'expressed', or 'not expressed'? Do we have 2
species going from 'not expressed' to 'expressed' in evolution, or 48
going from 'expressed' to 'not expressed'?

Notice that this question wouldn't arise if the apomorphy were truly an
evolutionary novelty, generated de novo, which is the mechanism
evolutionary biologists used to believe was almost universal. If it's
new and complex, it's either a new invention in a lineage represented by
the species sharing it, or it arose independently in different lineages.
These are parsimonious explanations.

As I see evo-devo, parsimony takes a back seat and we need to dig
deeper. Complex structures can appear suddenly from a 'de-inhibited'
control locus. or disappear just as quickly. The off-list post I
received gave the example of bees, where pollen-carrying and
nest-building characters vanished independently in several lineages that
went parasitic.

So should 'underlying synapomorphy' be seen as something that muddies
the phylogenetic picture, like long branch attraction?
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
Ph (03) 64371195; 61 3 64371195
Webpage: http://www.qvmag.tas.gov.au/mesibov.html

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