EvoDevo: Was Re: Who says dromaeosaurs can't fly?

["Matthew Bonnan" <mbonnan@hotmail.com>]

Jeff Hecht said:

"It's a problem because developmental genetics is showing that a lot of 
major traits are controlled by two genetic switches. One gene turns on a 
second gene -- that is, one gene controls whether or not a second gene will 
be expressed, and that second gene controls (for example) limb development. 
We have only a very primitive understanding of that process at present, but 
it's central the emerging discipline of Evo-Devo -- evolutionary 
development."


I teach a vertebrate embryology course, so I have been exposed to some of 
the EvoDevo concepts -- very interesting.  Many people seem to think that 
the genes contain a sort of master blueprint for the body -- not so.  Genes, 
if activated, will express certain proteins, but the environment in which 
those proteins are formed is just as critical as the proteins themselves.  
We develop our middle layer (mesoderm, which gives rise to notochord, heart, 
vertebrae, muscles, etc.), for example, when cells that would normally 
become skin or nerves receive certain chemical signals.  In other words, 
there is no specified heart or bones or kidneys when vertebrate embryos 
begin development, and this occurs only after certain conditions are met.

A recent paper in the Journal of Experimental Zoology has even gone so far 
as to suggest that epigenetic mechanisms (i.e., non-genetic mechanisms) have 
played a major, if not the major, role in the origin of new morphological 
characters (Newman and Muller, 2000: Epigenetic mechanisms of character 
origination, Journal of Experimental Zoology, 288:304-317).  In their words, 
"epigenetic mechanisms are the generative agents of morphological character 
origination."  They suggest that the burst of body plans during the Cambrian 
explosion and the origins of morphological innovations can best be explained 
through epigenetic mechanisms and not a one-to-one correspondence with 
genes.  In other words, the genes set up the initial conditions, but the 
environment molds the raw material.

They are not arguing too simplistically, of course -- the fact that we find 
a humerus and femur bone in all sarcopterygians, for example, shows that 
there is a genetic underpinning for those structures.  What Norman and 
Muller (2000) seem to be getting at is that the morphology of these 
features, as well as the appearance of novel morphologies, are effected 
strongly by epigenetic mechanisms -- genes can't explain it all.

And this does not mean that there is no such thing as morphological homology 
-- a homology is always defined at a particular level.  In other words, the 
wings of bats and birds are not homologous, but the bones underlying those 
wings are.  Therefore, whereas we may not yet understand or appreciate the 
complex interactions between genes and the envrionment in scuplting bone, we 
can still identify and trace homologous features (synapomorphies) at the 
level of the organism, which is what we are doing for dinosaurs.  Obviously, 
knowing what we know now, we must exercise caution in assigning homologies 
and understand their function.  And it is still most parsimonious to assume 
that my femur, the femur of a dog, of a lizard, and of an amphibian are 
probably inherited from a common ancestor who had a femur, rather than as a 
new morphological development in each us that just happens to look similar.

Epigenetically,

Matt

Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
(309) 298-2155
mbonnan@hotmail.com
MF-Bonnan@wiu.edu
http://www.wiu.edu/users/mfb100/

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