<P> In humans, visible secondary sex characteristics include pubic hair, enlarged breasts and widened hips of females, and facial hair and Adam's apple on males . </P> <P> Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species . Biologists today distinguish between "male - to - male combat" and "mate choice", usually female choice of male mates . Sexual characteristics due to combat are such things as antlers, horns, and greater size . Characteristics due to mate choice, often referred to as ornaments, include brighter plumage, coloration, and other features that have no immediate purpose for survival or combat . </P> <P> Ronald Fisher, the English biologist developed a number of ideas concerning secondary characteristics in his 1930 book The Genetical Theory of Natural Selection, including the concept of Fisherian runaway which postulates that the desire for a characteristic in females combined with that characteristic in males can create a positive feedback loop or runaway where the feature becomes hugely amplified . The 1975 handicap principle extends this idea, pointing out that a peacock's tail, for instance, displays fitness by being a useless impediment that is very hard to fake . Another of Fisher's ideas is the sexy son hypothesis, whereby females will desire to have sons that possess the characteristic that they find sexy in order to maximize the number of grandchildren they produce . An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism . This idea is known as the good genes hypothesis . </P> <P> Male jumping spiders have visual patches of UV reflectance, which are ornamentations used to attract females . </P>

The difference in size shape or color between the sexes within a species is referred to as