The natural selection theory is based on the hypothesis that hott girls with be chosen by dudes more often, and will not work as hard to maintain their looks, becoming ugly. Reversely, an ugly girl will work hard to get laid and become hott.
High Bitchisity + Hott = Bad; Low Bitchisity + Hott = Good
High Bitchisity + Ugly = Good; Low Bitchisity + Ugly = Bad
"My wife is hott! and has low bitchisity, so I'm assuming our sex life will stay pretty active."
"Eh, that ugly chick has got potential for a high bitchisity, ill just get wasted first..."
"Uhh, I'm so ugly, and my bitchisity is so low.... i wonder if they make solar powered vibrators."