Extensive genetic variation is usually found within any species. This is clearly visible in the domestic dog species.
All genetic variants arise from mutations and most have no effect on the phenotype.
A mutation is a random change in a gene or chromosome. Mutations arise spontaneously and happen continually, however exposure to ionizing radiation increases the rate of mutation. A mutation rarely creates a new phenotype, but if the phenotype is suited to a particular environment, it can lead to rapid change in a 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 may be more attractive and seen as a more 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.
Generally, mutations will have no effect on the phenotype, and occasionally the mutation might have some influence on an organism's phenotype. It is only in rare cases that a mutation will fully determine an organism's phenotype.