Final answer:
Disruptive selection is most likely responsible for the evolution of anisogamy, favoring the extremes within a population and leading to sexual dimorphism in gamete size.
Step-by-step explanation:
The model of selection most likely responsible for the evolution of anisogamy, which is the condition where male and female gametes are of different sizes, is disruptive selection. Disruptive selection favors the extremes of a trait within a population, leading to an increase in genetic variance. In the case of gamete size, small male gametes (sperm) and large female gametes (eggs) are favored because they enhance different aspects of reproductive success, such as mobility for sperm and nutrient provision for eggs, respectively. This can lead to the distinct difference in gamete size observed in many sexually reproducing organisms, a phenomenon known as sexual dimorphism, where males and females of a population look or act differently.