Climatic change is expected to affect individual life-histories and population dynamics, potentially increasing vulnerability to extinction. The importance of genetic diversity has been highlighted for adaptation and population persistence. However, whether responses of life-history traits to a given environmental condition depend on the genetic characteristics of a population remains elusive. Here we tested this hypothesis in the lizard Zootoca vivipara, by simultaneously manipulating habitat humidity, a major climatic predictor of Zootoca's distribution, and adult male colour morph frequency, a trait with genome-wide linkage. Interactive effects of humidity and morph frequency had immediate effects on growth and body condition of juveniles and yearlings, and on adult survival, and delayed effects on offspring size. In yearlings, higher humidity led to larger female body size, and lower humidity led to higher male compared to female survival. In juveniles and yearlings, some treatment effects were compensated over time. The results show that individual responses to environmental conditions depend on the population's colour morph frequency, age class and sex, and that these affect intra- and inter-age class competition. Moreover, humidity affected the competitive environment, rather than imposing trait-based selection on specific colour morphs. This indicates that species' responses to changing environments, e.g. to climate change, are highly complex, and difficult to accurately reconstruct and predict without information on the genetic characteristics and demographic structure of populations.