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EvoDevo: Was Re: Who says dromaeosaurs can't fly?
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
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.
Matthew F. Bonnan, Ph.D.
Department of Biological Sciences
Western Illinois University
Macomb, IL 61455
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