If biological species are defined in terms of reproductive compatibility, the formation of a new species hinges on reproductive isolation. This means that mechanisms must exist that prevent gene flow between populations, leading to reproductive barriers. These barriers can be prezygotic (before fertilization, such as habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, and gametic isolation) or postzygotic (after fertilization, such as hybrid inviability or hybrid sterility). The lack of gene flow due to reproductive isolation allows populations to diverge genetically, eventually forming new species that cannot interbreed successfully.