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POSTZYGOTIC
BEHAVIORS
Postzygotic behaviors—hybrid inviability,
hybrid sterility, and hybrid breakdown—prevent
gene flow in the unlikely event that fertilization
occurs between two closely related species.
In hybrid inviability,
the hybrid offspring of two species do not mature
normally and usually die in the embryonic stage
of development. In crosses between different species
of irises, for example, the embryos die before
seeds form.
Sometimes hybrid offspring mature
normally but are not able to reproduce successfully.
Hybrid sterility often occurs because the
gametes are abnormal in some way. Mules are sterile
hybrids formed by mating a female horse with a
male donkey.
If a mating between two F1
hybrids produces a second hybrid generation, this
F2 generation may be unable
to reproduce because of hybrid breakdown.
The second-generation hybrids are defective in
some way that prevents successful reproduction.
Hybrid breakdown has been demonstrated in sunflower
hybrids.
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