What roles do variation and retention play in natural selection? Define your terms

What will be an ideal response?

The answer should include definitions for the three major terms in the question. Next,
the answer should focus on variation; an exceptional answer will describe the two levels at which
it may exist (at the levels of the genotype and of the phenotype) and discuss how diversity at each
level is affected by natural selection. Finally, the answer should discuss how retention is
necessary for evolutionary success. An outstanding answer to this question will describe how
variation and retention interact in natural selection.