What is the result of hybrid sterility in postzygotic isolation?

Hybrid sterility is a form of postzygotic isolation that occurs after fertilization, leading to the production of hybrid offspring that are unable to reproduce. This biological phenomenon happens when two different species interbreed, producing hybrids that might develop normally in other respects but lack the necessary reproductive structures or mechanisms to produce gametes, such as sperm or eggs.

For example, mules, which are hybrids of horses and donkeys, are typically sterile. Although they are viable and can survive and grow to maturity, they cannot reproduce. This inability to reproduce effectively prevents gene flow between the parent species, thereby enforcing the boundaries of species and contributing to speciation over time.

The other options do not accurately reflect the concept of hybrid sterility. While hybrids may show physical differences or have survival issues, the definitive characteristic of hybrid sterility specifically focuses on the inability to reproduce.