Here's why:

* Prezygotic isolation refers to mechanisms that prevent the formation of a hybrid zygote (fertilized egg). This can happen in various ways, including:

* Gametic isolation: This is the specific scenario you mentioned, where sperm and egg from different species are unable to fuse. It's a common way to prevent hybridization.

* Habitat isolation: Species live in different habitats and thus rarely encounter each other.

* Temporal isolation: Species breed at different times of the year or day.

* Behavioral isolation: Species have different mating rituals or signals that are not recognized by other species.

* Mechanical isolation: Differences in reproductive structures prevent successful mating.

Postzygotic isolation refers to mechanisms that act after a hybrid zygote is formed. Examples include:

* Reduced hybrid viability: The hybrid offspring is unable to survive.

* Reduced hybrid fertility: The hybrid offspring is unable to reproduce.

* Hybrid breakdown: The first generation of hybrids may be fertile, but later generations are infertile or have reduced fertility.

In summary, prezygotic isolation, particularly gametic isolation, is a crucial mechanism for maintaining the genetic integrity of a population by preventing the mixing of genes from different species.