When Darwin published his "on the Origin of species" in 1859, his major theme was that natural selection works upon the group of individuals, ultimately selecting for traits which will help the progeny better adapt to the environment. Say we have a set of 10 girls and one guy who must decide which girl to mate with. Without himself even being conscious of the fact, he will mate with the best looking female, not because he will have bragging rights, but because beauty is sometimes a good indicator of health, a healthy mate means a healthy baby, a healthy baby means that your genes will have a better chance of being passed on towards subsequent generations. This isn't just about mating, for it is the general behavior, innate, instinctual and is with us whether we are in a serious relationship or just flirting. Lets face it people, none of us are perfect, that means that when looking for a relationship, we are looking for the best choice into which we could invest our genes. What this means is that we will not likely to bond with a person who deviates from the ideal beauty by more than one standard deviation. None of us are in it for the leftovers so to say, from a biological standpoint. Agree or disagree with my opinion, or present an alternative explanation.