Genetics, Vol. 156, 1419-1425, November 2000, Copyright © 2000 by the Genetics Society of America
Mutation-Selection Balance, Dominance and the Maintenance of Sex
J. R. Chasnov
Department of Mathematics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Communicating editor: M. J. SIMMONS
A leading hypothesis for the evolutionary function of sex postulates that sex is an adaptation which purges
deleterious mutations from the genome thereby increasing the equilibrium mean fitness of a sexual population
relative to its asexual competitor. This hypothesis requires two necessary conditions: first, the mutation
rate per genome must be of order one, and; second, multiple mutations within a genome must act with positive
epistasis, that is, two or more mutations of different genes must be more harmful together than if they acted
independently. Here, by reconsidering the theory of mutation-selection balance at a single diploid gene
locus, we demonstrate a significant advantage to sex due to nearly-recessive mutations provided the mutation
rate per genome is of order one. The assumption of positive epistasis is unnecessary, and multiple mutations
may be assumed to act independently.
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