Describe how a new phenotype may arise in the population and increase in frequency using the theory of natural selection.

A mutation arises in an individual that confers a selective advantage.  The phenotype the mutation produces is selected for by natural selection and so the individual survives to reproductive age.  This mutation is passed on to the next generation and then subsequent generations for the same reason.  The frequency of the genotype increases in subsequent generations and therefore the phenotype increases in frequency.