All life has midichlorians. It's just that they noticed a higher concentration of midichlorians in particularly force sensitive beings, which is why the Jedi have adopted midichlorian testing as a way to pick out Jedi hopefuls before they are old enough to demonstrate pronounced force ability.
The fact that they are based on Mitochondria (according to Lucas himself) tells me that he's not considering force sensitivity a mutation, just that some beings have a stronger innate ability with the Force.
Before the Midichlorians, there was a stronger case for force sensitivity being a mutation, because for whatever reason, the Force ran strong only in certain family lines, or at least that was strongly implied by the end of the classic trilogy.
I think the Midichlorian concept clarifies that all beings can tap into the Force to verying degrees based on both their midi-count, and their discipline. Someone like Han for example could potentially use the Force if he took it seriously and leanred the art and put a lot of effort into perfecting his skill.