One of the first and central mechanisms proposed by Darwin during his life was that of "natural selection." Some may refer to this as "survival of the fittest," whereby species that produce offspring have been successful in doing so because they have produced random individual features which have lead them to live longer and pro-create offspring with the same inheritable desired features. Not all individuals will be able to reproduce, therefore only the fittest will "multiply, vary, let the strongest live and the weakest die," (Darwin, Charles (1968, 1859)) Origin of the Species.
Therefore, different desired traits are passed down through generation-to-generation, and those traits, which are not so popular, will eventually die out. However, although Darwin proposed this mechanism, he also proposed a second which tied into the process of natural selection; that of "sexual selection." Natural selection tends to focus on the degree of difference in survival of offspring, whereas sexual selection in more concerned with the mate choice and how that can differentiate. And how sexually desired traits may have in fact speeded up the process of how humans have evolved. One example may be that intelligence may have become a desirable and attractive trait in a male 500 000 years ago, hence why the human brain has become larger, because intelligence became the target for a mate for women. Intelligence may have been just one of many sexual signalling plans used by individuals to notify others of the potential we possess as future mates.
For example, scientists have found that men have a preference in regards to hip-waist ratio. This may have resulted from sexual selection in regards to both intelligence and physical appearance many years ago. Man became more intelligent and believed that low waist-hip ratio indicated a high oestrogen ratio, which may favour reproductive function, and therefore also became sexually sought after in mates.