Natural selection is the process by which certain traits become more or less common in a population based on their impact on the survival and reproduction of individuals. When environmental conditions change, it is the phenotype, or the observable characteristics of an organism, that is most directly affected. This is because phenotypes are the expressions of genotypes in a given environment. For example, if a change in climate favors individuals with a certain coloration or size, those phenotypes will be more likely to survive and reproduce, leading to a shift in the population's characteristics over time. While genotypes provide the genetic basis for these traits, it is the phenotypes that interact with the environment and determine the success of individuals in that environment.