Allopatric Speciation

How Does Allopatric Speciation Occur?

Let's delve deeper into allopatric speciation.

Allopatric Speciation Requires Geographical Isolation

When a population is geographically connected, its gene pool is relatively homogenous, meaning the allele frequencies among its members are similar. Its gene flow is unrestricted: individuals can move into new areas and mate with individuals there.

When populations of a species are geographically isolated, gene flow across the groups is restricted, and the allele frequencies between the groups become more different over time. Suppose the groups are separated for a long enough period of time. In that case, the differences between their alleles become greater due to the differences in their environment (climate, predation, food sources, and other factors). Eventually, new species may form.

Allopatric Speciation can be Summarized in the Following Steps:

1. Geographic changes cause a single species population to separate into two or more groups.

2. The physical separation prevents the two groups from interbreeding with each other.

3. The groups undergo genetic divergence: each group develops new, distinct traits based on the demands of their specific habitat or the genetic makeup of the parent population.

4. Over time, the groups evolve into reproductive isolation: they become so different that the members of each group no longer interbreed, even if they are in the same habitat at the same time.

Figure 1 shows an experiment demonstrating allopatric speciation in the fruit fly (Drosophila pseudoobscura). A single population was split into two: one group was fed starch-based food, while the other group was fed maltose-based food. Over time, the populations diverged. When the two groups were mixed, they no longer mated with the other group members.

This diagram shows how allopatric speciation occurred in a fruit fly experiment.

We have mentioned that geographical changes, such as the formation of valleys and rivers, can cause allopatric speciation. But human activities such as the construction of roads and highways and environmental pollution can displace species from their habitat and cause allopatric speciation.

What Are the Types of Allopatric Speciation?

There are two types of allopatric speciation: peripatric speciation and vicariant speciation. In this section, we will briefly discuss each type.

Peripatric Speciation

Peripatric speciation—also known as founder effect speciation—occurs when members of a population become isolated in a location peripheral to the main population and establish a new population. The new population will have alleles selected at random (not because they produced more favorable traits). At the same time, gene flow is restricted between the original and new populations.

Thus, alleles in the original population can be overrepresented or underrepresented in the next generation of the new population. Eventually, the new population will have a gene pool that is different from the original population, leading to the formation of a new species. This process is illustrated in Figure 2 below.

This diagram shows how peripatric speciation occurs. Restricted gene flow between the two groups causes a new species to evolve

Vicariant Speciation

Vicariant speciation—also known as dumbbell speciation—occurs when a physical barrier, like a river or a valley, splits a population of a single species into two or more reproductively isolated groups. This process is illustrated in Figure 3 below.

This diagram shows the process of vicariant speciation. A physical barrier splits a population into two groups, where reproductive isolation evolves and leads to the formation of two distinct species.

What are Examples of Allopatric Speciation?

This section will discuss two examples of allopatric speciation: the Galapagos finches and the squirrels on the Grand Canyon.

Darwin’s Finches on the Galapagos Islands

There are over a dozen species of finches on the Galapagos Islands. While the finches were similar in size and color, their beak shapes varied. Some finches had broad, blunt beaks, which helped them crack nuts and seeds. Other finches had long, pointed beaks, which helped them snatch insects (Fig. 4).

The different finch species on the Galapagos Islands came from one parent species that first inhabited the islands millions of years ago. The populations of the parent species spread from one uninhabited island to the next, where they were geographically isolated from other populations. The different populations adapted to different ecological niches and eventually evolved into many descendant species.

An ancestral population of finch occupied different uninhabited islands in the Galapagos, forming over a dozen descendant species that are adapted to different ecological niches

Squirrels on the Grand Canyon

When Arizona's Grand Canyon formed, members of a single population of squirrels became geographically separated. The Grand Canyon was a physical barrier that prevented the isolated groups from interbreeding. Eventually, the squirrel population diverged into two subspecies: the Kaibab squirrel (Sciurus aberti kaibabensis) inhabiting the north rim and the Abert squirrel (S. a. aberti) inhabiting the south rim (Figures 5-6). In addition to their habitat, the two squirrel subspecies can be differentiated based on features like the coloration of their belly and forelimbs.

While they have not fully diverged into new species, they are well on their way; subspecies are considered to be in the first stage of speciation because the individuals rarely interbreed, and when they do, they often produce sterile male offspring.

Figures 5-6. The Kaibab squirrel (left) inhabits the north rim while the Abert squirrel (right) inhabits the south rim of the Grand Canyon. Left: Grand Canyon NPS, CC BY 2.0, via Wikimedia Commons. Right: Sally King, Public domain, via Wikimedia Commons.

Can Allopatric Speciation Occur in Plants?

Allopatric speciation occurs more often in animals than in plants; in fact, most plants undergo a different type of speciation: sympatric speciation, where two groups of the same species live in the same geographical area but no longer interbreed due to the formation of reproductive barriers. We will discuss this further in the next section.

While not common, there are instances where allopatric speciation has occurred in plants. An example of this is the formation of the various species that comprise the Hawaiian "silversword alliance".

Example of Allopatric Speciation in Plants: the Hawaiian "Silversword Alliance"

The Hawaiian silverswords are various species of plants (ranging from low bushes to tall shrubs) that are genetically similar, despite looking very different from each other. Evidence shows that all the silverswords of Hawaii descended from the tarweed (Carlquistia muirii) from North America. Like the finches on the Galapagos, the tarweed occupied the islands of uninhabited and geologically diverse islands of Hawaii. The tarweed spread into different ecological niches and formed different adaptations, leading to the formation of the different species of Hawaiian silverswords.

Due to the diverse geological conditions in Hawaii--including variations in elevation, rainfall, and soil conditions--many life forms are unique to Hawaii. The silverswords are only some of the 1,100 plant species unique to the Hawaiian islands (Figures 7-8).

Figure 7-8. Dubautia linearis (left) and Argyroxiphium sandwicense (right) are two species of Hawaiian silversword plants that look morphologically different but are genetically similar. Source (left): Forest & Kim Starr, CC BY 3.0 US, via Wikimedia Commons. Source (right): The original uploader was KarlM at English Wikipedia., CC BY-SA 2.5, via Wikimedia Commons.

Allopatric vs. Sympatric Speciation: What is the Difference?

An important difference between allopatric speciation and sympatric speciation is that allopatric speciation requires geographic separation while sympatric speciation does not. Instead of a geographic separation due to isolation or the formation of a physical barrier, sympatric speciation requires the formation of a reproductive barrier that would restrict gene flow even when members of the population are still in contact with each other. Examples of reproductive barriers include polyploidy, sexual selection, and habitat differentiation.

Additionally, as mentioned earlier, allopatric speciation occurs more frequently in animals than in plants, while sympatric speciation occurs more frequently in plants than in animals. This is because plants can form polyploidy and reproduce asexually, while not many animals can tolerate the anatomical changes accompanying these processes.