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. |
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