In general, no two individuals of a sexually reproducing species are genetically alike (except for identical twins). Thus there is considerable variation within each population of a species. A look around the room will confirm this for humans. In addition, populations of the same species often differ from one another depending upon where they are found. This variation is frequently the result of natural selection operating in different directions in different environments, but can also result from variation due to chance (genetic drift) or from different environments influencing the plasticity of the genotype.

Examples of variation NOT related to gender:

1. At the individual level
   • albisim in alfalfa butterflies
   • individual color variation in ladybird beetles
2. At the population level
   • size variation in different populations of house sparrows
   • subspecific variation in checkerspot butterflies