Mutation = any change in the quantity or structure of an organism's DNA.
Allele = different forms of the same gene caused by a mutation that changes the sequence of bases in the gene.
If a mutation occurs in a gamete (sex cell), the allele will be passed onto offspring, which may change the phenotype (characteristics), increasing the phenotypic variation of a population.
The frequency of this allele in the population may increase if it provides a selective advantage (making the organism more likely to survive and reproduce).
An example of this is…