In evolution, what is the name for a reproductive barrier that acts before fertilization?

Prezygotic isolation is a reproductive barrier that acts before fertilization. It prevents two populations from successfully producing a zygote, the first cell formed when sperm and egg unite.

Examples include different breeding seasons, incompatible courtship behaviors, separate habitats, mechanical incompatibility, and gametes that cannot fuse. These barriers reduce gene flow between populations and can help drive speciation.

Postzygotic isolation acts later, after fertilization. A hybrid may fail to develop, survive poorly, or be sterile, as in the classic mule example. The distinction is useful because it identifies the stage at which reproductive isolation occurs.