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Online Document
Brian Clarke,
"Darwinian Evolution of Proteins," Science 168 (1970),
1009-1011.
[Summary] [PDF 737K]
In this article,
Bryan Clarke presents a critique of King and Jukes' theory that most
evolution at the molecular level is selectively neutral. In the first
paragraph, he explicitly states that he is doing so from the
perspective of classical (neo-Darwinian) evolutionary theory. Clarke
begins by claiming that they manipulated their statistical results by
removing "an arbitrary number" of invariant sites (i.e. sites in which
there is no variation from species to species) from their dataset. He
then goes on to argue that King and Jukes made an unfortunate choice in
concentrating on the S-regions of globulins, since scientists have long
believed that amino acid variation at these sites is controlled by an
unusual mechanism that is very different than normal allelic
substitutions. Clarke then goes on to punch several other holes in King
and Jukes' theory and evidence, including: many of the changes that
they label neutral actually have adaptive significance, that some of
the variability in protein sequence may be a result of the differential
availability of amino acids in various species, and that there may be
selection acting to maintain a certain amount of structural variability
at the molecular level. Clarke also expands on Ernst Mayr's concept of
"evolutionary interia" to explain how the effects of natural selection
of an organism over a long period of time could mimic the effects of a
constant rate of molecular evolution. Finally, he points out that we
have strong evidence in at least two species (man and Drosophila) that
selection acts on protein polymorphisms. In his conclusion, Clarke
points out that he does not discount the role of non-Darwian evolution
in all cases. Instead, he argues that we need to find out the overall
importance of neutral evolution in relation to the dominant force of
natural selection. (jda)
This page was written by Michael Dietrich and Jay
Aronson. It was last updated on May 15, 2004.
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