The type of reproductive barrier that occurs when two species mate but fail to produce fertile hybrids is called postzygotic isolation.

Here's a breakdown:

* Reproductive barriers are mechanisms that prevent two species from interbreeding and producing viable, fertile offspring.

* Postzygotic isolation occurs *after* the formation of a zygote (fertilized egg). This means that the initial mating and fertilization are successful, but the hybrid offspring are either unable to survive or are infertile.

There are several types of postzygotic isolation:

* Reduced hybrid viability: The hybrid offspring is less likely to survive to adulthood.

* Reduced hybrid fertility: The hybrid offspring can survive but is infertile (e.g., mules).

* Hybrid breakdown: The first generation of hybrids may be fertile, but subsequent generations lose fertility.

Let me know if you'd like more detail on any of these types of postzygotic isolation!