A mutation is a change in a gene or chromosome. Mutations arise spontaneously and happen continually. A mutation rarely creates a new phenotype, but if the phenotype is changed as a result of a mutation and the new phenotype is suited to a particular environment, it can lead to a change in a species over time. For example, if a mutation leads to a change in phenotype, such as feather colouring in birds, this change may allow those individuals to reproduce more frequently, due to them being more attractive and seen as a more desirable mate. This would result in the mutated gene being passed on more frequently than the original gene and would result in an increase in the proportion of birds with the new feather colour compared to the original feather colour. This is the basis of natural selection.