Hybrid Breakdown
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State
Hybrid breakdown is a postzygotic barrier where first-generation hybrids are viable and fertile, but their own offspring in later generations show reduced fitness.
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A first-generation hybrid that reproduces successfully, but whose second-generation offspring are weak or sterile, shows hybrid breakdown a generation later.
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Hybrid breakdown's failure surfaces only in the hybrid's descendants — the first-generation hybrid can appear entirely healthy and fertile.