Some of Darwin's very basic studies point out that every species survived to the point we're at now by being able to stronger, faster, and more clever than other's of our species. The term "survival of the fittest" was coined through this observation. We can see today that the human species is the same. Those of use who look both ways before crossing the street, usually don't get hit by cars. The ones that check the expiration dates on food don't end up with food poisoning. Those who eat healthy and avoid an over abundance of fatty food usually maintain a healthy weight and heart. Most of the time, those are the people who survive longer, and are able to reproduce in a more effective manner passing on healthier and more fruitful genes.
That's not to say that those with lesser intelligence don't survive. In fact, they tend to procreate more, but unlike in lesser species, they have the same margin for survival that the smarter do. In all of human society, back all the way to the beginning of civilizations, it's never been about who's got the bigger IQ, it's about who is "more." When I say more, I mean more of everything. Better looking, more money, more muscle, more friends, just more of everything. This isn't an exclusive method of choosing, I mean someone's got to love the ugly people, but this seems to be the norm.
In Appleman's Darwin, Darwin himself writes, "females are most excited by... the more ornate males." In the text, this is made in observation to birds, but the same stands for humans. In this video, you can see that like all species, there are certain preconceptions about attraction.
The Chief, the green guy, shows us how there are certain ideas that we have about attracting the opposite sex. Though this is an extreme example, seeing as how the Chief is extremely socially inadequate, it makes my point. In the end, he was able to be more attractive by having more of something. In his case, it was more social skills, but it was still more than what he started with. He was able to flaunt himself, and as a result gained respect and some sort of attraction from the girl he encountered, regardless of the fact that he was about as bright as a fleck of dust.
Appleman, Philip. Darwin. 3rd. New York: Norton, 2001.