I saw a very good study predicting evolution of humans into multiple species based on pair mating and obesity. Essentially, people mate with the people who are of similar levels of "attractive" as themselves. Seeing as obesity is unattractive (say whatever you want, tubblr, it isn't), obese people will pair with obese people. This will eventually cause a bimodal distribution of body fat percentage, which could even lead to the development of two different subspecies of H. sapiens sapiens. This, in turn, could have the end that the fatties die out or proliferate as another species even more parasitic than our own.