1) Zygotes cannot survive due to different sets of chromosome.
2) Hybrid Sterality:
- Offsping may be born, but the adults are stearlity
- Cross a horse and a donkey, result is a mule which is usually sterile.
- complications during meiosis - cannot produble viable gametes
3) F2 fitness : hybrids reproduce, but their offsping are unable to reproduce (the F2 generation). In some cases, mules are fertile, but their offspring are not.