postzygotic

Postzygotic refers to effects that occur after a zygote has formed, reducing the viability or reproductive capacity of offspring arising from mating between individuals of different species or populations. In reproductive isolation, postzygotic barriers prevent gene flow by producing inviable or sterile hybrids, or by reducing the fitness of hybrid offspring in subsequent generations.

Typical postzygotic barriers include hybrid inviability, where hybrid embryos fail to survive or die early in development; hybrid sterility, where hybrids are unable to produce functional gametes; and hybrid breakdown, where first-generation hybrids are viable and fertile, but their offspring suffer reduced viability or fertility. These effects often arise from genetic incompatibilities, such as Dobzhansky-M Muller incompatibilities, and from chromosomal differences that disrupt meiosis or development.

Familiar examples include the mule, a sterile hybrid of a horse and a donkey, and various plant hybrids that are vigorous but fail to produce viable offspring after several generations. Postzygotic isolation complements prezygotic barriers in shaping species boundaries and can be important drivers of speciation when mating occurs between diverging populations.