Reproductive Isolation

What is reproductive isolation? Ranging from prezygotic barriers to postzygotic barriers, the different mechanisms in nature which keep species differentiated.

According to the biological species concept, developed by Ernst Mayr in 1940: a species is a group of populations which interbreed and form viable, fertile offspring. More specifically, this group of populations, doesn't interbreed in nature with other species. Why doesn't a species interbreed with other species? The answer to this question is reproductive isolation mechanisms. There are two types of reproductive isolation mechanisms: prezygotic, and postzygotic. Prezygotic is before the union of the gametes, and postzygotic is after the union of the gametes.

The first prezygotic isolating mechanism is called temporal isolation. Temporal isolation prevents fertilization because the two different species reproduce at different times. The time periods could differ simply by hours, or by seasons. If one species reproduces in the spring, while the other reproduces in the fall, the two species aren't able to breed.

Behavioral isolation is another isolating mechanism. This mechanism operates through courtship behavioral patterns. If one species displays a certain courtship pattern, it won't be recognized by those of the other species. This is the mechanism which separates wolfs from dogs, their courtship patterns are different in the wild.



Mechanical isolation deals with the actual mechanics of the reproductive organs. Mechanical isolation acts a prezygotic barrier by preventing sexual intercourse between two different species. The last three isolation mechanisms: temporal, behavioral, and mechanical, can all be overcome in the laboratory. This is to say that artificial selection can overcome the above isolation mechanisms. However, gametic isolation, and postzygotic barriers can't be overcome by any known means. Gametic isolation is where the gametes of the two species are chemically incompatible, thus preventing fertilization.

Hybrid inviability, the first of three postzygotic barriers, causes an inviable offspring to be produced through hybridization. Hybridization is the practice of two different species interbreeding. So hybrid inviability is one of three safeguards against hybridization. Hybrid sterility, another postzygotic barrier, causes the F1 generation to be sterile. Hybrid breakdown causes the F2 or F3 generations to be sterile.

Prezygotic and postzygotic isolating mechanisms have preserved the integrity of the different species throughout millions of years. Had these safeguards not been put into place, the gene pool would be muddled up and evolution would be a circular process.

In order to better understand the above detailed isolating mechanisms, here are a few examples. The wolf and dog have preserved their differentiated gene pool. How? Behavioral isolating, however not in the traditional respect. The wolf is more aggressive than the dog. This provides a behavioral barrier which doesn't relate to courtship, yet is a prezygotic isolating mechanism.

Looking at mechanical isolation in a nontraditional sense could involve flowers. Imagine a flower that is not accessible to a bee, this plant self-pollinates. Thus, this plant could not hybridize with other flowers due to physical inhibitions in the reproductive organs.

The mule: is a cross between a female horse and a male donkey. It is such a hybrid because it has hybrid sterility. Thus, the creation of the mule is only possible by hybridization, due to postzygotic isolating mechanism: hybrid sterility.

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