What stops members of different species from breeding with each other?

Members of different species are prevented from breeding with each other primarily due to reproductive isolation mechanisms. These can be prezygotic, such as differences in mating behaviors, timing, or physical barriers, and postzygotic, such as hybrid inviability or sterility. Additionally, genetic differences can result in incompatible gametes or developmental processes, further ensuring that even if mating occurs, successful reproduction does not take place. These mechanisms help maintain the integrity of each species and their distinct evolutionary paths.