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molecular clocks

RANBRUH at YaleVM.YCC.Yale.edu RANBRUH at YaleVM.YCC.Yale.edu
Sat Sep 7 15:04:21 EST 1991

It is important to distinguish between "mutation rate" and "rate of
evolution" when discussing a molecular clock. Eukaryotic mutations
may occur in somatic cells as well as gametes; obviously the gametic
mutations, which may be passed to subsequent generations will have
the only evolutionary significance. With regard to intrinsic rates
of mutation, these obviously do vary to some degree; if we consider
RNA-based viruses, for example, these have rates of mutation which
appear orders of magnitude greater than rates observed for other DNA-
based organisms. Evolutionary biologists observe the products of
many possible forces: selection, sampling error (small population
size), and other non-Mendelian phenomena such as transposable
elements. Recognizing a constant mutation rate, a constant substitution
rate, or a constant rate of evolution is a hierarchical progression;
the punctuated equilibrium theory is at the peak of this hierarchy
and is rather weaker, in terms of its theoretical basis, for this
reason. I found your comments interesting.

B.H. Rannala
Division of Evolutionary Biology and Ecology
Biology Department
Osborn Memorial Laboratories
Yale University
New Haven, CT 06511-7444

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