The Fitness of the Copper Tolerance Gene of Mimulus Guttatus in Uncontaminated Soil

S ummary Two experiments looking for differences in fitness between plants of Mimulus guttatus heterozygous for the copper tolerance gene and homozygous for the non‐tolerance gene are described. Experiment 1 used self thinning to look for intraspecific competition; no differences in fitness between the genotypes could be detected. Experiment 2 looked at various fitness‐related characters in spaced plants, and again no difference in fitness was apparent. In both experiments, the trends were for increased fitness of the tolerant genotype. These results may help to explain why metal tolerant plants are often found in non‐tolerant populations at low frequency.