Final answer:
Reproductive success would be reduced if both sexes had small gametes because the division of reproductive roles, which optimizes successful fertilization and offspring survival, would not exist.
Step-by-step explanation:
If both sexes had small gametes, reproductive success would be reduced for several reasons. In a biological context, gametes (sperm and egg) differ in size to optimize reproductive success.
Typically, small gametes (sperm) are mobile and numerous, while larger gametes (eggs) are nutrient-rich but fewer in number. This difference exists because it maximizes the likelihood of successful reproduction. For instance, studies such as those on sperm-depleted male fruit flies show that these males prefer to mate with larger, more fecund females, as this increases the chances of their limited sperm resulting in offspring. If both sexes produced small gametes, this division of reproductive roles would not exist, leading to a less efficient system with potentially wasted gametes and lower overall fecundity.