Evidence of Natural Selection in Molecular Evolution

Abstract

HOMOLOGOUS proteins have similar structure and function and are encoded by genes which have descended by gradual mutation from the same ancestral gene. Studies of genetically controlled protein variation have shown that at any given locus different species often have different alleles. Several authors^1,2 have proposed that allelic differences between species in genes coding for protein variants lack adaptive significance. Many amino-acid substitutions can, allegedly, occur in a protein without modifying its functional efficiency. Different alleles would, then, become fixed in different species by random sampling through the generations. According to this theory, the widespread protein polymorphisms found in natural populations of a given species are adaptively neutral as well; they simply represent transient states in the process of random substitution of a protein for another adaptively equivalent².