Understanding Evolution, a project of the University of California Museum of Paleontology, defines "phenotype" as the physical appearance of an organism. The phenotype of an organism is influenced to a large degree by the organism's genotype, the set of genes encoded in its DNA. For example, some birds have genes that produce birds with blue feathers, while other birds have genes that produce green feathers.
Evolutionary forces work to change both the genotype and the phenotype of an organism. As explained by Understanding Evolution, natural selection changes which genes are present in a given population. However, as genes determine the physical appearance of the organism, evolution indirectly affects phenotypes as well.
In some cases, an animal's phenotype is not determined genetically. Understanding Evolution describes flamingos as an example of this phenomenon. Flamingos derive their pink color from their diet; if deprived of their typical food, they turn white. In this case, portions of the animal’s phenotype are determined environmentally.
Wikipedia explains that some organisms have the ability to change their phenotype in response to environmental conditions. This ability, known as phenotypic plasticity, is more common among sessile organisms, such as plants, rather than mobile organisms, such as animals. For example, plants may develop thicker leaf cuticles during periods of dry weather.