A test of Fisher's theory of dominance.

One of the first patterns noticed by geneticists was that mutations are almost always recessive to their wild-type alleles. Several explanations of this striking pattern have been offered. The two most influential are Fisher's theory--which argues that dominance results from natural selection against recurring deleterious mutations--and Wright's theory--which argues that dominance results from the physiology of gene action. The debate over which of these theories is correct represents one of the most protracted controversies in the history of evolutionary biology. Here I test Fisher's theory by assessing the dominance of mutations in an organism that is typically haploid, the alga Chlamydomonas reinhardtii. The results show that mutations are recessive just as often among haploid as among diploid species. This result falsifies Fisher's theory of dominance and provides strong support for the alternative physiological theory.