Originally proposed by Darwin (in his book The descent of man, 1871), it is a form of selection that is typically contrasted with natural selection. According to the theory, sexual selection results from mating preferences (most often by females) in favor of those members of the opposite sex expressing certain genetically determined characters (e.g., the best courtship display, the brightest plumage, the largest size). The consequence is that the genes for these characters tend to spread through the male population and to become exaggerated in subsequent generations. In many species, but especially polygynous ones where only a few males monopolise all the females, sexual selection has led to pronounced sexual dimorphism. In these species, males compete against other males for mates, either directly or mediated by female choice (one of the most cited examples being the feathers of the male peacock assumed to have evolved through sexual selection by peahens). It seems that Erasmus Darwin (1731-1802), Darwin’s grandfather, gave the first outline of a theory of sexual selection in his book Zoomania (1795), but he did not consider it to be an evolutionary mechanism. Wallace agreed with Darwin that male competition was a decisive factor in sexual selection, but he could not accept that female choice was also involved. All told, the theory of sexual selection continues to promote much debate and disagreement about its importance in biological evolution.
See Biological evolution, Co-evolution, Darwinism, Delivery position (or presentation), Evolutionary biology, Quantitative genetic theory, Sex, Sex ratio, Theory of natural selection