Darwin's (1859) theory of natural selection states that advantageous characteristics replace their less advantageous counterparts, to become the dominant characteristics of the population. Mate-seeking individuals would therefore be expected to preferentially chose individuals with a minimum of unusual features, or, stated differently, individuals whose characteristics are all close to the mean of the population. These individuals are the least likely to carry harmful mutations. This form of mate choice is known as koinophilia, which explains why, what humans determine to be a beautiful face, is a face that contains no extreme features.