BIO 304. Ecology & Evolution: Macroevolution

Macroevolution: Reproductive Isolating Mechanisms

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Reproductive isolating mechanisms

Prezygotic mechanisms

temporal

Distribution of mating activity in five sympatric frog species. Differences in mating seasen prevents gene flow among them.

behavioral

Differences in courtship behaviour in the sympatric and morphologically similar Hawaiian fruitflies. Females respond only to behaviors characteristic of her own species. Drosophila heteroneura left, Drosophila simulans middle, and Drosophila ?right.

mechanical
Reproductive structures or genitalia of species are morphologically very different so that fertilization or efficient pollen transfer between the two is not possible.

ecological
Individuals of the two species live in the same general area (sympatric), but inhabit different habitats, such as open grassland versus woodland, swampy places versus dry rocks, different soil types, etc. Habitat separation can effectively prevent gene flow.
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gametic
Gametes can not meet or fuse together due to the lack of appropriate chemical cues.

Postzygotic mechanisms

hybrid inviability
hybrid sterility

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INVERSIONS

TRANSLOCATIONS

Hybrid sterility can be due to incompatibility between the genes of two species or to chromosomal differences that prevent normal meiosis. In some cases, species' chromosomes do not pair because of lack of homology. In other cases, pairing occurs but inviable gametes are formed when species differ by inversions and translocations. Inversions involve single pairs of homologous chromosomes, whereas translocations involve two pairs of homologous chromosomes.

Hybrids, like these zebroids (crosses between horses and zebras, shown
among horses) are often viable, but sterile.

hybrid breakdown
Hybrids sometimes can be partially fertile, but their offspring is inviable, or sterile. This prevents the perpetuation of the hybrid line.