>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.)
Sorry, its circularity. Similar assumptions have to be made in any other
line of reasoning you wish to invoke.
>
>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.
Another assumption added to the ones above. Adding assumptions to
assumptions is ok in philosophy; not very safe in science.
>
>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 [rather?]than be[ing?] 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.
Hmmm.. maybe something is wrong with the assumptions...remember the
linearity of the clock is an assumption. If you make this assumption, then
you can assume that replication error is not the major source of variation,
if you assume that the variation that you are seeing has all arisen from
phylogeny....
>5. Structural protein evolution is not the basis of
>morphological/organismic evolution. Humans and chimps are basically
>identical when their structural proteins are compared.
As you are aware, fruit flies and humans are essentially identical in the
homeotic proteins as well. So where do you account the differences to be???
Art
http://chadwicka.swac.edu