Variation is the differences in the characteristics of organisms. It can be caused by the genes an individual inherits or the environment it lives in.
Extensive genetic variation is contained within a population of any species and is clearly visible in the domestic dog species, as shown in the photograph below.
Genetic and environmental variation combine together to produce these different phenotypes. All genetic variants arise from mutations.
A mutation is a random change in DNA which therefore affects a gene and/or chromosome. Most mutations have no effect on the phenotype. Some influence phenotype to some extent. Very few create a new phenotype. Mutations can be inherited and therefore passed on from one individual to another. If a mutation causes a new phenotype that makes an organisms better suited to a particular environment, it can lead to rapid change in the characteristics of the individuals in that species.
For example, if a mutation led to brighter feather colouring in birds, the brighter feathers may allow those individuals with the mutation to reproduce more frequently, because they are seen as being more attractive and a desirable mate. This would result in the bright feather phenotype being more likely to be passed on, meaning that over time there will be more bright feathered birds compared to the birds of the same species without the new phenotype.