The process by which certain organisms produce more offspring by mating more frequently than other organisms of the same sex and thereby ensure the survival of more of their genetic traits. Sexual selection is a form of natural selection in which organisms are competing not for food or other resources in the environment but for mates. The development of size difference between males and females in mammals and birds, in which the greater strength (and often aggressiveness) of larger males allows them to have greater success mating, is seen as a consequence of sexual selection. The development of secondary sex characteristics, such as colored feathers in male birds or large antlers in male deer, which are attractive to the opposite sex as signs of fitness but are not directly involved in reproduction is also attributed to sexual selection. These features are often disadvantageous to the organism's survival—the colored feathers make the male bird more visible to predators, for instance—but can provide the organism with a competitive advantage over rivals in mating. The theory of sexual selection was first proposed by Charles Darwin in The Origin of Species.