The main difference between prezygotic and postzygotic isolation is that the prezygotic isolation acts before fertilization. But, the postzygotic isolation acts after fertilization. Furthermore, prezygotic isolation plays a key role in the natural selection of a population, while postzygotic isolation prevents successful inter-population crossing. Moreover, habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation are the mechanisms of prezygotic isolation. On the other hand, zygote mortality, non-viability of hybrids, and hybrid sterility are the mechanisms that lead to postzygotic isolation. 

Prezygotic and postzygotic isolation are the two types of reproductive isolation. Generally, reproductive isolation is a collection of evolutionary mechanisms, behaviors, and physiological processes, which lead to speciation. Therefore, the main purpose of reproductive isolation is to prevent nascent species from inbreeding. 

Key Areas Covered 

1. What is Prezygotic Isolation
     – Definition, Features, Mechanisms
2. What is Postzygotic Isolation
     – Definition, Features, Mechanisms
3. What are the Similarities Between Prezygotic and Postzygotic Isolation
     – Outline of Common Features
4. What is the Difference Between Prezygotic and Postzygotic Isolation
     – Comparison of Key Differences

Key Terms 

Fertilization, Hybridization, Natural Selection, Postzygotic Isolation, Prezygotic Isolation, Reproductive Isolation, Speciation

What is Prezygotic Isolation 

Prezygotic isolation is one of the two types of reproductive isolation, occurring prior to undergoing fertilization. Therefore, there is no mating and environmental factors responsible for the isolation of individuals, preventing reproduction. Nature does not allow the formation of hybrids even in the first stage. This is by preventing mating through environmental barriers.

Figure 1: In Coral Reefs, Gamete Incompatibility Prevents the Formation of Numerous Inter-Species Hybrids

Furthermore, the main mechanisms of prezygotic isolation include habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation. However, the division of a population by physical barriers permits the formation of hybrids by making a hybridization zone. In fact, the formation of hybrids is a kind of a waste of resources as it produces weak, nonviable, and sterile offsprings. Therefore, prezygotic isolation prevents the formation of hybrids by means of physiological or systemic barriers.  

What is Postzygotic Isolation 

Postzygotic isolation is the second type of reproductive isolation, occurring after undergoing fertilization. However, during reproductive isolation, the formation of a hybridization barrier allows the arising of hybrids. Therefore, postzygotic isolation mechanisms such as zygote mortality, non-viability of hybrids, and hybrid sterility make the hybrid not viable. Thus, the hybrid may not either be born, have major defects or may be sterile. Ultimately, postzygotic isolation prevents the formation of an offspring by the hybrids.

Figure 2: Mules are Hybrids with Interspecific Sterility

Moreover, the main disadvantage of postzygotic isolation is that it has to rely on natural selection to correct the convergence of species. When this does not occur, hybrids regress from the evolutionary timeline, reverting to a more primitive species.  

Similarities Between Prezygotic and Postzygotic Isolation 

• Prezygotic and postzygotic isolation are the two types of reproductive isolation. 
• Their main purpose is to prevent the arising of an offspring from the parents of the same population. 
• Both of them depend on natural selection, permitting the best adaptations to be kept while not allowing to regress back to the primitive or ancestral level. 
• They lead to speciation through evolutionary mechanisms, behaviors, and physiological processes. 
• They allow very different species to reproduce, producing weak or non-viable species to prevent the passing genes to the next generations. 

Difference Between Prezygotic and Postzygotic Isolation 

Definition 

Prezygotic isolation refers to the type of reproductive isolation that occurs before the formation of the zygote. Postzygotic isolation refers to the second type of reproductive isolation which occurs after the production of the zygote. 

Mating 

No mating occurs in the prezygotic isolation while mating occurs in the postzygotic isolation. 

Type of Mechanism 

Prezygotic isolation is an extrinsic mechanism that permits the physical isolation of parents, and thus, preventing mating. But, postzygotic isolation is an intrinsic mechanism which does not allow the formation of an offspring through the formation of hybrids.  

Evolving 

Moreover, prezygotic isolation can evolve as mating signals and preferences adapt to different environments. Postzygotic isolation, on the other hand, evolves since hybrids have intermediate phenotypes and are poorly adapted to parental habitats. 

Dependence of the Environment 

While prezygotic isolation depends on the environment, postzygotic isolation does not depend on the environment. 

Genetic Incapabilities 

Prezygotic isolation does not occur through genetic incapabilities, but postzygotic isolation occurs through genetic incapabilities, which in turn cause sterility. 

Mechanisms 

Habitat isolation, behavioral isolation, mechanical isolation, and gametic isolation are the mechanisms that result in prezygotic isolation. Meanwhile, zygote mortality, non-viability of hybrids, and hybrid sterility are the mechanisms of postzygotic isolation. 

Significance 

Prezygotic isolation prevents the formation of viable zygotes, while postzygotic isolation prevents the hybrids from passing on their genes. 

Importance 

Furthermore, prezygotic isolation plays a key role in the natural selection of a population, while postzygotic isolation prevents successful inter-population crossing.  

Conclusion 

Prezygotic isolation is the type of reproductive isolation, occurring prior to mating and fertilization. Therefore, it prevents the formation of a zygote. Significantly, it provides chances for natural selection. However, prezygotic isolation can occur through habitat, behavioral, mechanical, gametic isolation. In comparison, postzygotic isolation is the second type of reproductive isolation, occurring after fertilization. Therefore, individuals can undergo mating and form zygotes. However, the main purpose of postzygotic isolation is to prevent the passing genes of hybrids through non-viable zygotes and hybrid sterility. Therefore, the main difference between prezygotic and postzygotic isolation is the mechanism of reproductive isolation. 

References:

1. Kozak, Genevieve M., et al. “Postzygotic Isolation Evolves before Prezygotic Isolation between Fresh and Saltwater Populations of the Rainwater Killifish, Lucania Parva.” International Journal of Evolutionary Biology, vol. 2012, 2012, pp. 1–11., doi:10.1155/2012/523967.
2. Scoville, Heather. “How Do Prezygotic and Postzygotic Isolations Drive Evolution?” ThoughtCo, ThoughtCo, 3 Sept. 2018, Available Here.

Image Courtesy:

1. “Reef0484” (Public Domain) via Commons Wikimedia   
2. “Maultier grau” (CC BY-SA 3.0) via Commons Wikimedia  

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