Ok I will start by defining a mutation. It is a change in gene or chromosomal structure. This means now new alleles are going to be introduced which means we now have more genetic variation in the gene pool. Where we usually used to have bb gene being expressed in the phenotype we now have Bb being expressed assuming the mutation caused a replacement of the b with B. So mutations alter the normal genetic code and the altered code is expressed in the phenotype. It increases variation in the genetic pool and these variations are expressed in the phenotype hence there is more variation in the phenotype too