Mechanical isolation acts a prezygotic barrier by preventing sexual intercourse between two different species. This is to say that artificial selection can overcome the above isolation mechanisms. However, gametic isolation, and postzygotic barriers can t be overcome by any known means. Gametic isolation is where the gametes of the two species are chemically incompatible, thus preventing fertilization. Hybrid inviability, the first of three postzygotic barriers, causes an inviable offspring to be produced through hybridization.
Hybridization is the practice of two different species interbreeding.
So hybrid inviability is one of three safeguards against hybridization. Hybrid sterility, another postzygotic barrier, causes the F1 generation to be sterile. Hybrid breakdown causes the F2 or F3 generations to be sterile. Prezygotic and postzygotic isolating mechanisms have preserved the integrity of the different species throughout millions of years. Had these safeguards not been put into place, the gene pool would be muddled up and evolution would be a circular process.