dimorphism was taxonomically widespread and remarkably variable in the magnitude and form of its expression100,101. It is now quite obvious that sex-specific differences occur not only in conspicuous morphological traits (i.e. size, shape, and coloration) but also in a diverse suite of behavioral97-99,102, psychological102,103, biochemical69,103, and gene expression23,24,26 phenotypes.Variation in the magnitude of sexual dimorphism among closely related species, and sometimes within a species, motivated biologists to test Darwin's (1859) hypothesis that sex-specific differences were largely due to sexual selection, particularly male–male competition, in dozens of different taxa101. The results of these studies consistently reaffirmed the importance of sexual selection (via male–male competition and/or female choice) as a major driver of sexual dimorphism, but also suggested a significant role for natural selection and non-selective forces, i.e. genetic, ecological, and developmental pressures and constraints, in the evolution of sex-specific phenotypic divergence100,104. Indeed, future research into the nature and consequences of intersexual genetic correlations105 and intersexual ontogenetic conflict106 will lead to a more sophisticated understanding of the evolution and expression of sexual dimorphism.Box 2. Microchimerism and DiseaseAs a result of bi-directional cell trafficking between the mother and fetus during pregnancy, mothers may harbor cells from their children and children may harbor cells
