Where fecundity, mortality, immigration, or emigration differ among species, genotypes, environments, sexes, or age and size classes, we must expect differences in the composition and dynamics of populations and communities. However, as shown in this study on all-hybrid populations of the hemiclonal waterfrog Pelophylax esculentus, the reverse conclusion is not necessarily true: Population differences in genotype composition were not paralleled by differences in genotype-specific age and size distributions. We investigated 12 ponds with different ratios of diploid (LR) and triploid (LLR and LRR) hybrids and tested whether the pond-to-pond ratio differences result from differential survival of the three genotypes under different ecological conditions (selection hypothesis). This should then be reflected in the age and size distribution of the frogs. Age (determined through skeletochronology) and size (measured as snout–vent length) were related to eight ecological variables that in previous studies had been found to affect genotype ratios: temperature, dissolved oxygen, pH, pond size, amounts of submerged and floating pond vegetation, and proportions of forest and wetland area within 20 m of the pond. Only two of these eight variables had a significant effect on growth and survival: frogs in larger ponds were, on average, older and larger than frogs in smaller ponds, and body size also increased with water temperature. Since these relationships did not differ between genotypes and sexes, we conclude that ecological conditions are unlikely to exert their influence on population structure via differential individual performance. The conclusion is supported by our finding that across the 12 study ponds there was no significant relationship between the proportion of a particular genotype × sex category and the average age and size of the individuals belonging to that category. An alternative to the selection hypothesis is offered by the recently proposed gamete pattern hypothesis. It states that pond-to-pond differences in genotype ratios arise from different gamete production patterns of LLR, LR, and LRR males and females. But, so far, support for this hypothesis is also weak.