In species with sexual reproduction, one sex, more often female, initially contributes greater parental investment, since she produces gametes that are fewer in number but bigger in size. This leads to intrasexual competition among individuals of the opposite sex to achieve reproduction. The rise of polygamic reproductive systems is a direct consequence of sexual selection, as Darwin has already stated. During evolution, polygamy appeared before monogamy. The latter probably evolved as an adaptation to harsh habitats, where survival was possible only for two individuals who cooperated during the reproductive period.