Postzygotic barriers prevent a hybrid zygote from developing into a fertile adult.
Gametes of red and purple sea urchins are released into the water, but are unable to fuse. Some salamander subspecies of the genus Ensatina can hybridize, but hybrids do not complete development or are frail. A mule is the robust but sterile hybrid between a male donkey and a female horse. Hybrids of two rice strains are vigorous and fertile, but the next generation (center) may be sterile.