Sexual dimorphism is the systematic difference in form between individuals of different sex in the same species. In most cases, it is the male that shows extravagant or exaggerated secondary sexual characteristics. Sexual selection was Darwin's solution to the problem of why conspicuous traits such as the bright colors, elaborate horns, and fancy displays are seen in the males of many species. He proposed two forms of sexual selection: one caused by direct competition among males for access to females or other resources,and one caused by preferential mating of females to males with "attractive" traits (female choice). Differential selection pressure between the sexes has been postulated to explain the substantial between-sex differences observed in morphology, physiology, and behavior, indicating the existence of different optimal sex-dependent phenotypes. Especially traits that are involved in male reproduction tend to evolve fast.
One question which arises is whether sexual dimorphism affects the rate of evolution at the molecular level. Studies of gene expression during the life cycle of Drosophila melanogaster have found that, for sexually mature males and females, a substantial fraction of the transcriptome displays sex-dependent regulation. Increasing evidence suggests that molecular mechanisms associated with sex and reproduction change substantially faster than those more narrowly restricted to survival. In order to obtain gene expression levels in males and females, high-throughput and large-scale technologies are required. In recent years, this has been possible due to the development of microarray technologies.