Why does hybrid sterility help keep species separate?

Hybrid sterility serves as a postzygotic reproductive isolating mechanism that helps maintain the boundaries between species by preventing the successful exchange of genetic material.1

While individuals from different populations may sometimes mate and produce hybrid offspring—such as the mule—these hybrids often fail to develop functional gametes. Because these hybrids are sterile, they cannot reproduce or pass their genes on to a subsequent generation. Consequently, this sterility acts as a biological barrier that prevents the gene pools of the two parent species from merging. By ensuring that genetic changes remain confined to their respective, independently evolving gene pools, hybrid sterility reinforces the reproductive isolation necessary for species to remain distinct evolutionary units.23