Here's how it works:

* Initial hybridization: Two different species sometimes interbreed, producing hybrid offspring.

* First generation viability: These hybrids might be healthy and fertile in the first generation (F1).

* Subsequent generations: However, when these hybrids reproduce with each other or with either parent species, the offspring (F2 and subsequent generations) often experience reduced viability, fertility, or both. This means the offspring might be weak, sterile, or have other developmental problems that prevent them from surviving or reproducing successfully.

The result: Hybrid breakdown prevents the establishment of a new hybrid species, effectively isolating the original two species.

Example: Some cultivated plants, like cotton, show hybrid breakdown. While the first generation hybrids are vigorous, subsequent generations become increasingly weak and less productive.

Let me know if you have any other questions!