Final answer:
Monogamy evolves when ecological or social conditions favor this mating system such as mate scarcity, the necessity for male assistance in rearing offspring, or female enforcement to reduce offspring competition.
Step-by-step explanation:
Monogamy evolves in species when certain ecological or social conditions make it the most advantageous mating system. This can occur when mates are scarce and hard to find, which makes mate-guarding beneficial. The male-assistance hypothesis suggests that monogamy evolves when males that help with offspring care produce more and healthier young. This system is prevalent in birds, with a vast majority of species exhibiting monogamous pairings where both parents provide significant care to their young. Alternatively, the female-enforcement hypothesis posits that monogamy is advantageous when females prevent males from mating with others to reduce offspring competition. In scenarios where females are few, this ensures that no potential opportunity for reproduction is lost.