Re: Tiktaalik

[Tim Williams <twilliams_alpha@hotmail.com>]

David Marjanovic wrote:

> Replace the taxon names with any taxon names and the time with any time, 
> and I think you have the story of evolution in a nutshell. :-)

I'm not sure I agree.  The kind of transformations we see in the 
fish-amphibian and theropod-bird transitions each involve a major change in 
body plan.  These macroevolutionary events only happen relatively rarely.

For the fish-amphibian and theropod-bird transitions, this change is 
associated with a major shift in locomotion: water-to-land, or land-to-air.  
Thus, taxa like _Archaeopteryx_ and _Tiktaalik_ represent snapshots of 
macroevolutionary change.  So we see an opportunity for mosaic evolution 
when a major changeover in body plan occurs - as in basal tetrapods and 
basal avians.  Certain anatomical features show a much more rapid rate of 
evolutionary change than others.  The result is a great deal of 
experimentation in transitional body plans, with mosaic evolution usually 
being the rule.  There may also be some U-turns (reversals) along the way.  
This experimentation lasts until an evolutionary "tipping point" is reached, 
when the body plan becomes committed to a given ecomorphology (locomotory 
style, in these cases), and there's no way back.

Cheers

Tim