Final answer:
The model of sexual selection that best explains why males in promiscuous mammalian species have greatly enlarged testicles compared to monogamous species is sperm competition.
Step-by-step explanation:
In promiscuous mammals, where males often have greatly enlarged testicles compared to closely related monogamous species, sperm competition is the model of sexual selection that best explains this observation. In species where multiple males may mate with the same female in a short time frame, sperm competition is fierce, leading to the evolution of larger testicles that can produce more sperm. This adaptation increases a male's chances of fathering offspring by ensuring that their sperm are more likely to fertilize the female's eggs.
Sexual selection can lead to the evolution of traits that enhance an individual's chances of reproductive success. Secondary sexual characteristics such as enlarged testicles in promiscuous species are evolved to maximize reproductive success, even if they do not directly benefit the individual's survival. Other forms of sexual selection include traits like the large and ornate tail of a peacock, which, despite making the male more prone to predation, is favored by females as it indicates the male's genetic fitness (handicap principle).