No two individuals in a population are exactly alike. The differences between them are called variation. Some variation is down to the genes an organism has inherited from its parents, which are genetic causes. Some is down to the conditions in which it has developed, such as the soil, light or food it has had, which are environmental causes. Often both play a part together.
The genome of an organism and its interaction with the environment influence how the phenotype develops. The phenotype is the set of characteristics the organism actually shows. For example, a plant with genes for tall growth may stay short if it grows in poor soil.
There is usually extensive genetic variation within a population of a species. All of this variation comes from mutations, which are changes in the genetic material. Mutations occur continuously. Most mutations have no effect on the phenotype, some influence it, and very few determine it. Very rarely, a mutation leads to a new phenotype. If that phenotype is suited to a change in the environment, it can lead to a relatively rapid change in the species.