An organism's genotype is the entire set of genes that it carries in its genome, and its phenotype is the observable effect of those genes on the body and behavior of the organism. Biologists draw a distinction between genotypes and phenotypes to clarify the difference between an organism's genetic heritage and the effects that its combination of genes has.
Genotypes and phenotypes are intimately linked, and changes to the genotype of an organism can easily affect major changes to its phenotype. When these differences are visible to selective processes, the resulting success or failure of the organism tends to sculpt the gene pool of the population.
The study of evolution is, in large part, the study of how genotypes shift in response to the interaction of phenotypes with the environment. A subtle variation to the genotype of an organism can result in potentially major alterations to the phenotype. A single defective, mutated or duplicated gene, for example, can result in a different anatomical arrangement or pattern of behavior for the organism that possesses the mutation. If the new phenotype does well in the organism's environment, the genotype that caused the shift in anatomy or behavior tends to become more common in the population over time.