Molecular Evolution Activities



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.