Differences in the characteristics of individuals in a population are called variation. Variation can be caused by the genes an organism has inherited (genetic causes), by the conditions in which it has developed (environmental causes), or by a combination of genes and the environment. An organism's genome interacts with its environment to produce its phenotype, which is the set of characteristics it actually shows. For example, a plant with genes for tall growth will still stay short if it grows in very poor soil.
There is usually extensive genetic variation within a population of a species. All of this variation arises from mutations, which are changes in the genetic material. Mutations occur continuously. Most mutations have no effect on the phenotype, some influence the phenotype, and very few determine the phenotype.
Very rarely a mutation leads to a new phenotype. If the new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species.