Re: Reevolving bones?

[Adam Yates <zooamy@zoo.latrobe.edu.au>]

I normally would shy away from these acrimonious cladistic arguments but 
I feel I have to respond this comment of Georges

> 
> As neither John nor you seems to understand, reversals >are< a problem, and a
> >big< problem.
>  

I still can't see why you feel that reversals are so unlikely. We know 
that some sort of homoplasy must have occured, based on the observation 
of character incongruence. Now what evidence is there that most of this 
character evidence is caused by convergence? Indeed my gut feeling is 
that reversal is more likely in the case of complex structures, simply 
because it can be brought about by re-expression of genetic information 
that was always there but simply not turned on. We know that modern 
genomes carry un-expressed instructions, for occasionally a mutation will 
bring them back into use. These atavisms include whales with hind feet, 
humans with small tails (I even had a friend in high school who claimed 
his father had tiny holes in his neck that were supposed to be remnants 
of the pharangeal gill slits that in his case failed to close over - 
somehow I doubt this example). The hoatzin with its clawed fingers 
represents such a reversal that was beneficial and thus become "fixed" in 
the population. Why is it so impossible that the instructions for 
building a full length MT I to have been present in the genome of 
Mesozoic Theropoda? (especially since we know nothing of their genetics 
and embryology) For all I know they might still be present in modern 
Theropoda.


Cheers 

Adam Yates.