Re: Tiktaalik

[Tim Williams <twilliams_alpha@hotmail.com>]

Scott Hartman wrote:

> Sorry I'm a day late, but I had to jump in here; macroevolution is NOT 
> simply "large scale changes" (and microevolution is not always less 
> phenotypic change than macroevolutionary events).

Well, yes and no.  Macroevolution can be large scale phenotypic changes.  It 
doesn't *have* to be, but it *can* be.  Since macroevolution can refer to 
any evolutionary change at or above the level of species, it also can go as 
high as "classes" or "phyla".  Yes, we're back to outdated Linnean ranks and 
typology.  But the modern neo-Darwinian synthesis of evolution posits that 
the mechanism behind micro- and macroevolution is identical, and that the 
only difference between the two is time and scale (with the latter usually a 
product of the former).  That's the rationale I was using.  Body plans (and 
mosaic evolution) are therefore within the domain of macroevolution.

> The distinction arose out of the days post Darwin when many biologists 
> thought that natural selection could not alone account for speciation, let 
> alone large scale changes.  Of course, back in the day of asking questions 
> like how one "class" of vertebrates evolved from another "class", this 
> meant that large phenotypic changes were indeed the domain of 
> macroevolution.

Yes, this is the typological concept of macroevolution.  I was using a more 
modern synthetic version.

> But today biologists generally accept that large changes in body plan 
> result from the accumulation of many small changes over time; there is 
> scant evidence for even a weak version of
> Goldschmidt's "Hopeful Monster" form of phenotypic saltation.

Aside from homeotic events.  Take the different number of segments in 
centipedes, for example.  Segments appear to be added or subtracted as a 
whole, not gradually.  Same for numbers of teeth in the jaw, number of 
remiges on a wing, number of phalanges in a digit, etc.

Other such changes may involve shape, not number.  The shape of the 
articular surfaces of caudal vertebrae in titanosaurs is a good example.  
Titanosaurs across the board display opisthocoelous, procoelous, 
amphicoelous and biconvex centra.  The most parsimonious explanation behind 
this diversity in caudal morphology is that the front and back surfaces 
could switch between one and the other conditions (e.g., amphicoelous or 
opisthocoelous directly from procoelous), without always having to pass 
through intermediate morphologies (e.g., weakly procoelous, amphiplatyan 
conditions).

Cheers

Tim