Question

biological evolution
postzygotic barriers can contribute to speciation by reducing the fitness or reproductive success of hybrid offspring. hybrid infertility, one type of postzygotic barrier, refers to the reduced fertility of hybrids. for instance, when two different species interbreed, the resulting hybrid offspring may be sterile or have reduced fertility.
another postzygotic barrier is hybrid breakdown, where the first - generation hybrids may be viable and fertile, but subsequent generations have reduced viability or fertility. these barriers further limit gene flow between species and promote the maintenance of distinct genetic lineages.
which postzygotic barrier refers to subsequent generations of hybrids having reduced viability or fertility?
mechanical isolation
hybrid breakdown
gametic isolation
hybrid longevity

Explanation:

The text clearly states that hybrid breakdown is the postzygotic barrier where the first - generation hybrids may be viable and fertile, but subsequent generations have reduced viability or fertility.

Answer:

hybrid breakdown