Abstract: Evolutionary theory suggests the hypothesis that among genetically isolated populations, phenotypic variation should be smaller in populations with smaller ratios of nonsynonymous to synonymous substitutions (dN/dS) because smaller dN/dS ratios occur under greater purification selections. Two distinct lineages (JPN₁ and JPN₂) of panarctic Daphnia cf. pulex sensu Hebert (1995), invaded independently into Japan from North America, provide an excellent opportunity to test this hypothesis because the earlier invader JPN₁ has a lower dN/dS ratio than JPN₂. Therefore, we examined phenotypic variations in fitness-related traits, including digestive, life-history, and morphological traits, among several genotypes within these two lineages. We found that phenotypic variations were smaller within the JPN₁ lineage than within the JPN₂ lineage. Furthermore, within-lineage variation of the phenotypic plasticity to changing food levels was smaller in the JPN₁ lineage than in the JPN₂ lineage. These results support the hypothesis that the JPN₁ lineage has been more efficiently subjected to negative selection. However, the magnitude of the phenotypic plasticity of these traits was, on average, at the same level between the JPN₁ and JPN₂ lineages and its direction differed among genotypes of these lineages, suggesting that the JPN₂ genotypes might have exploited niches that were different from those of the JPN₁ genotypes.

Key words: asexual animal,  genetic variation, invasion organism, phenotypic plasticity, phenotypic trait, zooplankton.