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. Monogamy also developed strategies for the fertilization of already mated females by unmated males. As a consequence, various anatomical and behavioural adaptations in defense of monogamy evolved, e.g. prolonged copulations, partner surveillance, sperm competition. Parental care is another adaptation which arose to defend “fitness”. Parental care has several forms, depending on the type of mating system used by each species. A particular position is held by helpers at the nest. These, as foreseen by the theory of kin selection, are generally relatives of the reproductive couple. Nevertheless, the presence of helpers not genetically linked, suggests the existence of reciprocal altruism. From our present knowledge, the hypothesis is put forward that helping also originated through individual selection.
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