Evolutionary clock: The rate of evolution of rattlesnake cytochromec

Abstract

It is shown that the method used by Jukes and Holmquist [Science177, 530 (1972)] is not able to lead to any conclusion on the rate of evolution of rattlesnake cytochromec because the method does not consider the time period over which the observed differences occurred. In an attempt to overcome this problem the phylogenetic relationship between rattlesnake, turtle and birds is examined from the paleontological evidence and from phylogenetic trees constructed from cytochromec sequences by “matrix methods” and by “ancestral sequence” methods. The paleonto-logical evidence and the “ancestral sequence tree” are in agreement for the positioning of rattlesnake. This ancestral sequence tree is used to estimate the rate of amino acid substitution and minimum base changes for different lines of descent among 20 vertebrate species. The rate of amino acid substitution is faster than average on the rattlesnake line but is not the fastest among the vertebrates and it is concluded that no “species specific” effect has yet been demonstrated for rattlesnake. However there is a large amount of diversity in the rates of amino acid substitution and this is discussed from the concept that at any point in time only a few codons (the covarions) are able to accept an amino acid substitution. It is suggested that some fluctuations in the rate of amino acid substitutions should occur.