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Clock or what???

David H. Griffin griffin at mailbox.syr.edu
Wed Jul 19 15:08:39 EST 1995

The word "theory" is tossed about considerably, with considerable
confusion. For scientific usage in relation to discussions about the
molecular clock, theory is best used as a form of hypothesis. A theory
is an hypothesis that has been tested many times with a variety of
different experimental approaches and has stood the test of time, as in
the theory of chromosomal inheritance of genetic traits. At one time in
the history of science, this was clearly an hypothesis. We think of it
as a theory now, elevated to that status by repeated testing, and the
passage of time.

The molecular clock hypothesis has not yet achieved this status, but
this is certainly arguable. It is clear that different DNA sequences
change at different rates, and that these rates may vary in different
groups of organisms. Does this invalidate the concept of a molecular
clock? Well, I don't think so. These are specific variations among many
that probably average out to a reasonably consistent progression of a
clock. There will clearly be considerable problems in trying to apply a
specific clock rate to a limited set of DNA sequences over a diverse
assemblage of organisms. This does not invalidate the hypothesis.

David H. Griffin
Department of Environmental & Forest Biology
College of Environmental Science and Forestry
350 Illick Hall
One Forestry Drive
Syracuse NY 13210-2788
e-mail: griffin at mailbox.syr.edu

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