Difference Between Geographic and Reproductive Isolation

Main Difference – Geographic vs Reproductive Isolation

Geographic isolation and reproductive isolation are two mechanisms that lead to speciation. Speciation refers to the formation of a new, distinct species from the main population of a particular organism mainly due to genetic incompatibility. The geographical isolation is the physical separation of two populations by the geographical barriers. This occurs through adaptive radiation and allopatric speciation. The reproductive isolation is the separation of two populations of the same species, preventing interbreeding and production of a fertile offspring. The two populations can be reproductively isolated due to behavioral barriers, temporal barriers, and geographical barriers. The main difference between geographical isolation and reproductive isolation is that geographical isolation is a form of a reproductive isolation whereas reproductive isolation is the major mechanism that leads to speciation.

Key Areas Covered

1. What is Geographic Isolation
      – Definition, Function, Role, Examples
2. What is Reproductive Isolation
      – Definition, Function, Role, Examples
3. What are the Similarities Between Geographical Isolation and Reproductive Isolation
      – Outline of Common Features
4. What is the Difference Between Geographical and Reproductive isolation
      – Comparison of Key Differences

Key Terms: Adaptive Radiation, Allopatric Speciation, Behavioral Isolation, Ecological Isolation, Geographical Isolation, Mechanical Isolation, Reproductive Isolation, Speciation, Temporal Isolation       

What is Geographical Isolation

Geographical isolation is the separation of two populations by a physical barrier. The physical barriers can be a mountain, river, plateau or glacier. The geographical isolation leads to adaptive radiation followed by allopatric speciation. A physical barrier could be a geographical barrier that prevents the migration of individuals or an unfavorable habitat between the two populations. When a single population is separated into two or more populations by a physical barrier, the individuals in the each population rapidly diversify from the ancestral species into new forms. This process is called adaptive radiation. In this process, individuals change according to the environment and the types and the amounts of resources available in their habitat.

Figure 1: Allopatric Speciation

Adaptive radiation leads to different sets of morphological and physiological characteristics in different populations of the same species. Each set of characteristics builds the population as a new species. This type of speciation is called allopatric speciation. In allopatric speciation, the gene flow between two species is greatly reduced. The Dwain’s finches with diversified beak shapes is a prime example of adaptive radiation and allopatric speciation. The topography-derived geographical isolation that leads to the allopatric speciation is shown in figure 1. 

What is Reproductive Isolation

Reproductive isolation is the separation of two populations in such a way that they cannot interbreed and produce fertile offspring. This type of isolation can occur due to any environmental, mechanical, physiological or behavioral barriers. The isolation of the populations from the main population restricts the gene flow among the populations and causes speciation. The reproductive isolating mechanisms prevent the formation of a viable zygote. Therefore, they are a type of pre-zygotic isolating mechanisms. The process of reproductive isolation is shown in figure 2.

Figure 2: Reproductive Isolation

The four types of pre-zygotic isolating mechanisms are mechanical isolation, ecological isolation, temporal isolation, and behavioral isolation. In mechanical isolation, the mating is restricted by the differences of the morphological characteristics. For example, the snails with right-coiling shells cannot mate with the snails with left-coiling shells. The coiling of the snail’s shell is controlled by a single gene. The changes occurred in that gene may cause speciation. The ecological isolation occurs due to geographical barriers. It is also called the geographical isolation and is described above in this article.

The temporal isolation occurs when the populations breed in different time periods or breeding seasons. For example, though the red-legged and yellow-legged frogs are two closely-related frogs, the red-legged frog’s breeding season is from January to March while the breeding season of the yellow-legged frog is from April to May. Therefore, no interbreeding may occur between the two types of frogs. The behavioral isolation is the presence of different courtship rituals such as breeding calls and mating dances that prevent the interbreeding. 

Similarities Between Geographical and Reproductive Isolation

• Both geographical and reproductive isolation lead to the speciation in which an ancestral species to divide into new, distinct species.
• Both geographical and reproductive isolation can occur due to the separation of two populations by geographical barriers.
• Both geographical and reproductive isolation bring morphological differences to different populations.
• Both geographical and reproductive isolation lead to genetic incompatibility between populations.    
• Both geographical and reproductive isolation contribute to evolution.

Difference Between Geographical and Reproductive Isolation

Definition

Geographical Isolation: Geographical isolation is the separation of two populations by a physical barrier.

Reproductive Isolation: Reproductive isolation is the separation of two populations in such a way they cannot interbreed and produce a fertile offspring due to environmental, mechanical, physiological or behavioral barriers.

Cause

Geographical Isolation: The geographical isolation is caused by the geographical barriers.

Reproductive Isolation: The reproductive isolation is caused by the behavioral barriers, temporal barriers, and the geographical barriers.      

Significance

Geographical Isolation: The geographical isolation is a form of reproductive isolation.

Reproductive Isolation: The reproductive isolation is the major cause of the speciation.         

Examples

Geographical Isolation: The Darwin’s finches are examples of geographical isolation. 

Reproductive Isolation: The temporal isolation of red-legged and yellow-legged frogs, mechanical isolation of snails with right-coiling shells and snails with left-coiling shells are examples of reproductive isolation.

Conclusion

Geographical and reproductive isolation are two types of mechanisms that lead to speciation. Geographical isolation leads to allopatric speciation through adaptive radiation. The reproductive isolation occurs due to mechanical, ecological, temporal or behavioral isolation of populations. The geographical isolation is a form of reproductive isolation. The main difference between geographical and reproductive isolation is their extent of the contribution to speciation.

Reference:

1.“Allopatric speciation: the great divide.” Understanding Evolution, Available here. Accessed 29 Sept. 2017.
2.“Reproductive Isolating Mechanisms.” Reproductive Isolation, Available here. Accessed 29 Sept. 2017.

Image Courtesy:

1. “Topography-driven Isolation PEAK&VALLEY (allopatric speciation)” By Andrew Z. Colvin – Own work (CC BY-SA 4.0) via Commons Wikimedia

2. “Reproductive Isolation (Process diagram)” By Andrew Z. Colvin – Own work (CC BY-SA 4.0) via Commons Wikimedia