Prezygotic vs. Postzygotic Isolations

Zedonk, a hybrid of zebra and donkey
A zebra/donkey hybrid is called a zedonk. (Getty/Fox Photos)

Diversity in life on Earth is due to evolution and speciation. In order for species to diverge into different lineages on the tree of life, populations of a species must be isolated from each other so they are no longer able to reproduce and create offspring together. Over time, mutations then build up and new adaptations become evident, making new species that came from a common ancestor.

There are many different isolating mechanisms, called prezygotic isolations, that prevent species from interbreeding with each other.

If they do manage to produce offspring, there are more isolating mechanisms in place, called postzygotic isolations, that ensure the hybrid offspring are not selected for by natural selection. In the end, both types of isolations are designed to drive evolution and make sure that speciation is the desired outcome.

Which types of isolations are more effective in the view of evolution? Are prezygotic or postzygotic isolations the preferred deterrent for species interbreeding and why? While both are very important, they have their strengths and weaknesses in speciation.

Prezygotic Isolations Strengths and Weaknesses

The biggest strength of prezygotic isolations is that it prevents a hybrid from even happening in the first place. Since there are so many prezygotic isolations (mechanical, habitat, gametic, behavioral, and temporal isolations), it stands to reason that nature prefers these hybrids not even form in the first place.

There are so many checks and balances in place for prezygotic isolation mechanisms, that if species manage to avoid being caught in the trap of one, then another will prevent the hybrid of the species from forming. This is especially important to prohibit mating between very different species.

However, especially in plants, hybridization does occur.

Usually, this hybridization is between very similar species that have much more recently diverged into different lineages from a common ancestor in the relatively recent past. If a population is divided by a physical barrier that leads to speciation due to the individuals not being able to get to each other physically, they are more likely to form hybrids. In fact, there is often an overlap of habitat called the hybridization zone where this type of interaction and mating occurs. So while prezygotic isolation is very effective, it cannot be the only type of isolation mechanism in nature.

Postzyogtic Isolations Strengths and Weaknesses

When prezygotic isolation mechanisms fail to keep species in reproductive isolation from each other, the postzygotic isolations will take over and ensure that speciation is the preferred route for evolution and diversity among species will continue to increase as natural selection acts. In postzygotic isolation, hybrids are produced but tend not to be viable. They may not survive long enough to be born or have major defects. If the hybrid makes it to adulthood, it is often sterile and cannot produce its own offspring. These isolation mechanisms ensure that hybrids are not the most prevalent and species remain separate.

The main weakness of postzygotic isolation mechanisms is that they must rely on natural selection to correct the convergence of species. There are times this does not work and the hybrid actually makes a species regress in their evolutionary timeline and revert to a more primitive stage. While this sometimes is a desirable adaptation, more often than not it is actually a set back on the evolution scale.

Conclusion

Both prezygotic isolations and postzygotic isolations are necessary to keep species separate and on divergent paths of evolution. These types of reproductive isolations increase biological diversity on Earth and help drive evolution. Even though they are still dependent upon natural selection to work, it ensures that the best adaptations are kept and species do not regress back to a more primitive or ancestral state through hybridization of once-related species.

These isolation mechanisms are also important to keep very different species from mating and producing weak or not viable species from taking up important resources for individuals that actually should reproduce and pass down their genes to the next generation.

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Scoville, Heather. "Prezygotic vs. Postzygotic Isolations." ThoughtCo, Jul. 7, 2017, thoughtco.com/prezygotic-vs-postzygotic-isolations-1224814. Scoville, Heather. (2017, July 7). Prezygotic vs. Postzygotic Isolations. Retrieved from https://www.thoughtco.com/prezygotic-vs-postzygotic-isolations-1224814 Scoville, Heather. "Prezygotic vs. Postzygotic Isolations." ThoughtCo. https://www.thoughtco.com/prezygotic-vs-postzygotic-isolations-1224814 (accessed January 16, 2018).