Evolution of sexual dimorphism over four stages. Each panel shows the frequency distribution of trait values for a hypothetical population (females (red), males (blue), overlap (purple)) and fitness surfaces (solid lines). Mean phenotypic trait values given by dashed vertical lines, optimum trait values given by asterisks, where fitness is maximized. (a) The trait experiences stabilizing selection to a single optimum trait value and the trait is sexual monomorphic; (b) the trait experiences sex-specific selection (red and blue fitness surfaces and optima) but is sexually monomorphic. As a consequence the population experiences a gender load (sum of Δf and Δm), which is the difference between the maximum possible fitness (upper horizontal gray dotted line) and the fitness achieved by the population mean (lower horizontal gray dotted line); (c) the trait experiences sex-specific selection but has evolved sexual dimorphism, the population therefore experiences a reduced gender-load; (d) the trait experiences sex-specific selection but since the extent of sexual dimorphism matches the fitness optima, the gender-load has been eliminated.