we know that natural selection will not result in evolution if a trait is not heritable.

we know that natural selection will not result in evolution if a trait is not heritable. However, it is also possible that a trait with heritable variation will not show an evolutionary response under natural selection. Name and describe 2 types of natural selection that do not necessarily change allele frequencies despite the presence of heritable variation, and give an example (real or hypothetical) of each. please explain!