Reproductive Isolation

What prevents different species from interbreeding? When two different species mate, what prevents the development of hybrid populations? Are there exceptions where different species can successfully breed to produce viable offspring? This article will discuss the definition of reproductive isolation, elaborate on reproductive isolation mechanisms, discuss what mechanisms prevent interbreeding and the exceptions, and then delve into how reproductive isolation leads to speciation.

What Is the Definition of Reproductive Isolation?

There are notable exceptions to this rule. Being able to successfully interbreed and produce viable offspring does not always indicate that two animals are the same species.

What Are the Types and Some Examples of Reproductive Isolation Mechanisms?

Reproductive isolation mechanisms prevent gene flow between different species and limit the development of hybrids, the offspring of two different species.

One reproductive isolation mechanism may not completely block gene flow, but a combination can effectively separate the gene pool of a species. There are various types of reproductive isolation mechanisms. These can be grouped into two broad categories: prezygotic and postzygotic barriers.

What Are Prezygotic Barriers?

A zygote is a fertilized egg cell that is formed through the union of two reproductive cells. ‘Prezygotic’ means before the formation of the zygote. Prezygotic barriers are reproductive isolation mechanisms that prevent the formation of the zygote.

The following section will discuss four prezygotic barriers: temporal isolation, geographic isolation, behavioral isolation, and gametic barrier.

Temporal Isolation

Many organisms mate only during specific times of the year. This means that, for many organisms, their breeding schedules do not match. Differences in breeding schedules are called temporal isolation. Temporal isolation prevents reproduction between organisms that have different breeding schedules.

For example, the western spotted skunk (Spilogale gracilis) and the eastern spotted skunk (S. putorius) cannot interbreed because the western spotted skunk breeds in fall while the eastern spotted skunk breeds in late winter.

Geographic Isolation

A population could become physically separated from other populations of the same species. That means its members no longer interbreed with the original population. This is called geographic isolation. Figure 1 shows how a population could be geographically isolated as a result of climatic changes in a mountainous region.

This diagram shows how geographic isolation can develop in a population inhabiting a valley

When a population is geographically isolated, its succeeding generations develop new, distinct traits based on the demands of their specific habitat, or the genetic makeup of the parent population. Over time, they become so different that they no longer interbreed with the other populations even if they are in the same habitat at the same time. At this point, they have developed reproductive isolation and have evolved into separate, distinct species.

Figure 2. This collage shows four examples of ratites (clockwise from top left): Greater Rhea, Ostrich, Emu, and Cassowary. Source: Bernard DUPONTUser:KoreUser:KadellarDonald Hobern, CC BY-SA 2.0, via Wikimedia Common

Behavioral Isolation

Some organisms use particular behaviors to attract mates. Consequently, when these behaviors are different or absent, reproduction does not take place. This mechanism is called behavioral isolation.

For example, male fireflies display lights in specific patterns to attract mates. These patterns are unique for each species. If a male firefly of one species tried to attract a female firefly of another species, the light pattern used by the male firefly would be unfamiliar to the female firefly, and they would not mate.

Gametic Barrier

Gametes are reproductive cells. Some organisms may attempt to mate but they have gametes that are not compatible with each other. Differences in gametes prevent fertilization. This mechanism is called a gametic barrier.

For example, plants have structures that vary in length and diameter. The aim is to attract certain pollinators while preventing a different pollinator from accessing the pollen. These unique structures prevent cross-pollination with other species.

What Are Postzygotic Barriers?

'Postzygotic' means after the formation of the zygote. Postzygotic barriers are reproductive isolation mechanisms that prevent reproduction after the zygote is formed. Postzygotic barriers result in hybrid offspring that are either inviable or sterile. In the following section, we will discuss hybrid inviability and hybrid sterility.

Hybrid Inviability

Viable refers to the ability to sustain life. Hybrid inviability is the inability of a hybrid individual to sustain life. In many cases, hybrid individuals cannot survive past the embryonic stages. If they do survive, they may not mature into healthy adults.

Hybrid Sterility

If the hybrid individual is able to survive, it is likely to be sterile. Hybrid sterility is the inability of a hybrid individual to produce offspring. An example of this is the mule, the hybrid offspring of a horse and a donkey (Fig. 3). Mules are sterile; they are incapable of producing their own offspring.

Figure 3. Mules are the sterile hybrid offspring of horses and donkeys. Mules are incapable of producing their own offspring. Source: w:User: Dario u / User: Dario urruty, Public domain, via Wikimedia Commons.

Gene flow occurs between populations of the same species but not between populations of different species. Members of a species can interbreed, so the species as a whole shares a common gene pool. On the other hand, members of different species usually do not interbreed. If they do, they will produce inviable or sterile offspring and will be unable to pass on their genes. This means that the presence or absence of gene flow can distinguish one species from another.

Reproductive isolation mechanisms limit or prevent gene flow between different species. This causes the allele frequencies of the groups to become more and more different from each other. If the groups do not interbreed for a long period of time, the differences between their alleles become greater because of the differences in their environment (various factors including climate, food sources, and predation). Eventually, the groups will become genetically divergent, forming new species.

This process is illustrated in Figure 4 below. A single species population (teal) splits into two populations (green and blue). The black lines represent gene flow. As we can see in the diagram, the two populations become reproductively isolated, and over time, they become two distinct species.

This diagram illustrates how reproductive isolation takes place in a population

Reproductive isolation helps define the boundaries of the species as a reproductive community and as a gene pool, as well as maintain the cohesion of the species as a genetic system. Reproductive barriers are why members of a species share more similarities with each other than they do with members of other species.