  Evolution by natural selection happens when there are heritable differences among types of organisms in a population and, as a result of these differences, some types leave more offspring than others. This leads to changes in the frequencies of those different types of organisms in a population. Darwin described sexual selection in terms of “the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction (1871, vol. 1 256). In other words, this advantage need not be one of physiological or mechanical efficiency or longevity, but rather concerns increasing reproductive potential. Darwin notes two kinds of sexual selection: male-male competition and female choice. This twofold characterization of sexual selection remains standard in current biological literature ranging from academic publications to textbooks to the popular press. Darwin writes that “the male is generally eager to pair with any female” whereas females tend to choose the most attractive partner (1859, 70; 1871, 582). Darwin considered the competition among males “for the possession of the other sex” to result in the improvement of sensory and locomotory features and the development of strong passions.