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Molecular clocks and the tree of life

David Steffen steffen at mbir.bcm.tmc.edu
Mon Sep 16 09:33:03 EST 1991

lamoran at gpu.utcs.utoronto.ca (L.A. Moran) writes:
>The question of rates of molecular evolution is a vexing one. Are there 
>really changes in the rate with time or in different lines? If the rate
>of evolution is not constant then it is possible to argue that some living
>organisms are more primitive than others.

I believe that there are really changes in the rate of evolution with
lineage.  There are massive changes in the rates of morphological
evolution with lineage, of course (Alan Wilson talked about this some
years ago; he had a cover of Science illustrating this fact very
graphically), but I assume you are not talking about that.  The reason
I believe that the rate of neutral evolution varies with time is when
I was writing a paper comparing rat, mouse, and human c-myc genes, I
came across a reference to the "generally accepted fact" that
molecular clocks in rodents ran faster than in other mammals.  I do
not have instant access to this reference, but if it is important, I
can try to track it down.

BTW, the extreme variance in rates of morphological evolution, which I
believe to be well established, could be used to argue that some
organisms are more primitive than others.  I personally think that the
statement that all existing species are equally evolved is a
convenient half truth useful for counteracting the tendency of
thinking of one modern organism evolving into another.  However, I do
not think that it needs to be precisely, quantitatively correct.
David Steffen
Department of Cell Biology, Baylor College of Medicine, Houston TX 77030
Telephone = (713) 798-6655, FAX = (713) 790-0545
Internet = steffen at mbir.bcm.tmc.edu

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