TG
We are a bit behind where I'd like to be, so instead of continuing the
evolution discussion tomorrow, I'm going to begin the discussion principles
of protein structure. However, I'd like to summarize a few additional
points that I would have added in the discussion in class. These are also
discussed in your text.
If you accept the evolutionary interpretation of the sequence data as
presented Wednesday in class, the following additional ideas flow out of
it:
1. Sequence comparisons become another means (in addition to fossil record
data and traditional taxonomy) to establish evolutionary relatedness. [In
other words, once it is established (or accepted) that sequence
similarities are due to evolutionary relatedness, they can be used to
elucidate evolutionary relatedness. This is not circular reasoning,
because the acceptance of the premise depends not just on sequence
similarity alone, but sequence similarity coupled with what is known about
DNA replication, mutation, neutral drift, etc.)
2. If sequence comparison data is combined with the fossil record time
line, then sequence differences can be used determine time since the two
branches of the phylogenetic tree diverged. The observation is that the
rates of sequence change is relatively constant in time. This idea is known
as the molecular clock. Each protein has its own characteristic rate
however, i.e. each protein runs on a different clock. The rate of sequence
change appears to be related to what fraction of the protein that consists
of invariant residues: the more invariant residues the slower the rate of
mutation.
3. Because the rate of mutation is constant in time (vs. number of
generations), mutations must be caused primarily by random chemical changes
or radiation damage than be replication error. If replication error were
the chief source of mutation then organisms with short generation times
(insects, bacteria, yeast, etc) would show faster change and this would
disrupt the linearity of the molecular clock.
4. Both branches of a lineage following a divergence event continue to
accumulate mutations at the same rate. Thus fish continue to accumulate
amino acid substitutions along with new groups (mammals, reptiles) that
descended from ancestral fish. Consequently, the present day members of
more primitive groups are equidistant from another organism on an outlying
branch. Some people find this surprising and claim that because it is so
that the sequence data do not support evolutionary theory (see Michael
Denton, _Evolution: A Theory in Crisis_ and _Pandas and People_, a high
school biology textbook supplement that criticizes evolutionary theory and
argues that the failure of evolutionary theory points to a Creator God).
This view commits the error that ancestral groups stopped evolving (at the
molecular level) following a divergence event.
5. Structural protein evolution is not the basis of
morphological/organismic evolution. Humans and chimps are basically
identical when their structural proteins are compared. Apparently, a few
mutations in key developmental regulators can produce the sorts of large
scale morphological differences that we observe between distinct but
closely related groups. [Some have claimed that our taxonomy is just due
to an anthropocentric orientiation. They claim that if objective observers
were to come from outer space that chimps and humans would be classified
into the same genus and perhaps a subspecies of a common species. Perhaps
this case could be made on the molecular evidence alone, but there is more
more to biology than DNA sequences and structural protein identity (back to
our reductionism discussion!). Small changes in developmental patterns and
the ways the very similar structural proteins are arranged can give rise to
large changes in final organismic morphology. These large differences can
become, in my opinion, a justifiable basis for taxonomic differentiation.]
Please feel free via email to discuss these points and the
creation/evolution papers that I handed out in class. Also, don't feel
like the discussion has to be teacher/class; I'd be very encouraged to so
class/class interchange occur.
Terry G.
_____________________________________________________________
Terry M. Gray, Ph.D. Department of Chemistry and Biochemistry
Calvin College 3201 Burton SE Grand Rapids, MI 40546
Office: (616) 957-7187 FAX: (616) 957-6501
Email: grayt@calvin.edu http://www.calvin.edu/~grayt