Mutation rate differs from the board measurement in that it varies
not only from organism to organism but also within the genome of a
single individual and also over time, especially over geologic time.
This makes it difficult to get a good measurement. Organisms with
relatively short lifespans or with well-documented genealogies are
amenable to studies of mutation rate based on known starting and
ending points. Extrapolating them back farther into the past runs
into two problems. First, the starting point is imprecisely known.
It is very unlikely that we will have and be able to recognize the
fossil of the very last common ancestor of the two lineages. For
example, in the case of the chimp-human split, primates tend to live
in tropical forest settings, which is a lousy place to get
fossilized. Thus, our data on the occurrence of the fossils is
somewhat patchy. Furthermore, what we want in order to date the
split is the very last common ancestor of chimps and humans.!
Thus, we are looking for something with some features of both, but
probably with a few distinctive features of its own, and lacking some
of the distinctive features of either group. Such an individual may
be difficult to distinguish from a generic ape. On the other hand,
if one branch (in this case, the chimp) has changed relatively
little, then the fossil of the common ancestor might look like a
fossil chimp. Furthermore, it is possible that a population with the
ancestral form survived after the ancestors of chimps and humans
split, so that the last occurrence of an intermediate-looking form
may postdate the split. On the other hand, the oldest definite
ancestor of only one (e.g., the first definite hominid fossil) is
probably not the very oldest individual, because the odds that the
very first one got fossilized and found are very low. From a
geologic point of view, the uncertainties may still not be too large,
but they introduce a margin of error that is not alway!
s factored into molecular clock calculations.
Note that this does not support the claim that there are no
transitional forms. There are plenty of transitional forms, but
identifying a particular fossil as the shared great-great...great
grandparent of both modern lineages (and thus the starting time for
the mutation rate calculation), as opposed to a cousin, uncle, etc.
is probably impossible.
The other problem is that the mutation rate is unlikely to be
constant over long periods of time, and thus a simple division of
number of mutations by time is often inadequate. Environmental
factors such as the level of UV, radioactivity, oxygen, and
competitive pressure (if the going is easy more mutants survive)
vary. Organisms may evolve changes in their DNA copying systems that
affect the mutation frequency. All of this complicates the picture.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted
Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at
Droitgate Spa
---------- Original Message ----------------------------------
From: "J Burgeson" <hoss_radbourne@hotmail.com>
Date: Tue, 04 Jun 2002 10:30:41 -0600
>
>Wally wrote: "I did however note how barbaric I thought it was to require
>that kind of
>indirect measurement of a fundamental quantity."
>
>Interesting idea. Of course most of the "fundemental quantities" we measure
>are done indirectly.
>
>I suppose even the measurement of a board with a steel tape is indirect -- I
>do this all the time at the H4H houses I'm working on this summer. One
>always assumes the steel tape has been correctly calibrated. There seems to
>be a continuum of "indirectness" in scientific measurements. I don't see
>that "mutation rate" differs in kind from my board measurements, only in
>degree of difficulty.
>
>Burgy
>
>_________________________________________________________________
>Chat with friends online, try MSN Messenger: http://messenger.msn.com
>
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