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.
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   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.