Final answer:
Reproductive isolation refers to the inability of sperm to fertilize eggs of a different species, serving as a prezygotic barrier to maintain species' genetic uniqueness.
Step-by-step explanation:
The inability of sperm to fertilize eggs of a different species is known as reproductive isolation. This type of prezygotic barrier is essential for maintaining the genetic uniqueness of a species. When reproductive structures do not match or if the gamete cells are incompatible, successful mating and fertilization is prevented. This process is an important aspect of gradual speciation where species diverge over time.