The biological definition of a species is a group of similar organisms that can interbreed to produce fertile, viable offspring. Some extend this to say that this reproduction must occur under natural, not artificial (e.g., in captivity) situations.

When an ancestral species gives rise to two new species, what determines whether the two new species can reproduce?
Consider the mechanisms that restrict gene flow: reproductive isolating mechanisms.

PREZYGOTIC ISOLATING MECHANISMS prevent the formation of viable zygotes.

  • Ecological Isolation
    The geographic ranges of two species overlap, but their ecological needs or breeding requirements differ enough to cause reproductive isolation.


  • Temporal Isolation
    two species whose ranges overlap have different periods of sexual activity or breeding seasons


  • Behavioral Isolation
    Species with complex courtship rituals (breeding calls, mating dances, etc.) usually exhibit a stereotyped "give-and-take" between male and female before actual mating takes place.

  • Superb Bird of Paradise
  • Teminck Tragopan
  • Superb Lyrebird
  • Malaysian Fireflies

    These rituals prevent the wasted effort of mating with a partner who will not produce fertile, viable offspring with you.

  • Mechanical Isolation
    Morphological differences prevent mating/pollination.

  • The amazing partnership of the Bucket Orchid and Orchid Bee is so precise that if either one went extinct, the other would follow,. No other orchid can possibly cross-pollinate the Bucket Orchid.

  • In some snail species, the direction of shell coiling is controlled by a single (maternal effect) gene. Snails with left-coiling shells cannot mate with snails having right-coiling shells. This could eventually lead to further differentiation and speciation. example

  • Gametic Isolation
    In this case, sperm and ova of the two species are chemically (genetically) incompatible, and will not fuse to form a zygote.
    Sea urchins do not mate. They broadcast their gametes into the ocean where sperm and eggs fuse to form zygotes and then develop into larvae. The Giant Red Urchin (Strongylocentrotus franciscanus and Purple Urchin (Strongylocentrotus purpuratus) cohabit the rocky intertidal along the western U.S., but they do not interbreed. Their gametes are genetically/chemically incompatible, maintaining species integrity.

    POSTZYGOTIC ISOLATING MECHANISMS prevent hybrids from passing on their genes.

  • Hybrid Sterility
    Viable hybrid is produced (often physically more vigorous than either parent), but is unable to reproduce due to meiotic problems.

  • Hybrid Breakdown
    successive generations of hybrids suffer greatly lowered fertility --> sterility. Eventually, they are selected out of the population.

    Hybridization Between Species

    If there is interbreeding between two closely related species, there are several possible outcomes.