Evolution photos
Evolution photos
Genetic variation in a population of Sabatia campestris
Genetic variation in a population of Sabatia campestris 
 
In order for natural selection to occur, there must be genetic variation in populations. A visible example is color forms in Sabatia campestris (Gentianaceae), as seen in rural Oklahoma. Photograph by Stanley A. Rice. Appears in Encyclopedia of Biodiversity (Facts on File).