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Re: a rose by any other name(was fish & dogs)



Dinogeorge

I am dealing with just this issue at the moment. Please permit a fishy 
digression to make a general point. One of my current projects involves 
working on a group of small blennioid fishes called triplefins. This 
group is unusually diverse in New Zealand. The largest genus here is 
Forsterygion. Our phylogenetic work (based on both morphological and 
molecular characters) has shown that this genus as currently defined is 
both polyphyletic and paraphyletic. The monotypic genus Obliquichthys 
is the sister group to two species of Forsterygion, although 
superficially it looks quite different to Forsterygion species.

Our work suggests that Obliquichthys is paedomorphic, and certainly 
this fish resembles juvenile Forsterygion in many respects. The 
question is, do we retain Obliquichthys and a paraphyletic Forsterygion 
to preserve the semblance of superficial similarity, or do we revise 
the taxonomy so that it reflects what we think is the actual pattern of 
evolutionary relationships? If we go with the former, how different 
would Obliquichthys have to be to make us come to this decision? I 
don't know how to answer such a question.

Coming back to the general point, the evolutionary information that 
paraphyletic groups preserve is in the form of shared plesiomorphic 
characters. Of course I agree that the characters that "fish" share are 
characters they inherited from a common ancestor. But your decision to 
"exclude" some of the descendants of this common ancestor (i.e. 
tetrapods) is subjective, as is your argument for "morphological 
consistency." Presumably you think that limbs are special. But 
limb-like appendages with digits evolved independently in several 
groups of sarcopterygians, only one of which led to tetrapods. The 
oldest known tetrapod (Acanthostega) had gills and lived in water, so 
what made it so special? You might say tetrapods are special because 
they are diverse and successful. Teleosts are also highly derived and 
successful animals. How would you feel about a group that contains all 
gnathstomes excluding teleosts? Such a group could be easily defined on 
morphological grounds as vertebrates with the premaxilla attached to 
the cranium. Do you think this group preserves useful evolutionary 
information?

Cheers

Kendall

----------------------
Kendall Clements
k.clements@auckland.ac.nz