Abstract

RECENT advances in molecular genetics have had a great deal of influence on evolutionary theory, and in particular, the neutral mutation-random drift hypothesis of molecular evolution^1,2 has stimulated much interest. The concept of neutral mutant substitution in the population by random genetic drift can be extended to include random fixation of very slightly deleterious mutations which have more chance of being selected against than of being selected for^3,4. If this class of mutant substitution is important, we can predict that the evolution is rapid in small populations or at the time of speciation⁵. Here I shall organize the observed facts which indicate that this class is in fact important.