Postzygotic Isolation

Postzygotic isolation takes effect after hybrid zygotes form. Zygotes may die early, be weak, or be sterile. Postzygotic isolation mechanisms take effect after fertilization, while the embryo is developing. Postzygotic mechanisms include zygotic mortality, hybrid inviability, and hybrid sterility.

Sometimes fertilization does occur between different species, but the hybrid embryo is weak and dies.

In some instances the hybrids are vigorous but sterile. An example would be a mule that is produced by a male donkey and a female horse.

This animation (Audio - Important) reviews prezygotic and postzygotic isolation mechanisms.

REVIEW: The primary reason for hybrid sterility is