ALLOPATRIC SPECIATION: PROCESS AND EXAMPLES - BIOLOGY - 2025

The allopatric speciation or geographic speciation, is a type of speciation that occurs because of geographical isolation between biological populations of the same species. "Allopatric" derives from the Greek allos which means 'separate' and patris which means 'country'.

During this speciation, a population is divided by some geographic barrier. For terrestrial organisms, this barrier could be a mountain range or a river. In contrast, a land mass would be a geographic barrier to a population of aquatic organisms.

Kaibab squirrel, example of allopatric speciation

Over time, individuals in the populations on either side of the barrier differ. Some of these differences may be reflected in the reproductive biology of the species, so that when the two populations rejoin by removing the barrier, they can no longer interbreed. They are then considered separate species.

Allopatric speciation can occur even if the barrier is somewhat "porous," that is, even if some individuals can cross the barrier to mate with members of the other group.

For a speciation to be considered 'allopatric', gene flow between future species must be greatly reduced, but it does not have to be completely reduced to zero.

Speciation is a gradual process by which populations evolve into different species. A species in itself is defined as a population whose individuals can interbreed.

Thus, during speciation, members of a population form two or more distinct populations that can no longer reproduce with each other.

Steps of allopatric speciation

Geographic change

In the first step, a geographic change separates the members of a population into more than one group. Such changes could include the formation of a new mountain range or a new waterway, or the development of new canyons, for example.

Human activities such as civil engineering, agriculture, and pollution can have an effect on living environments and cause some members of a population to migrate.

Genetic mutations

Different genetic mutations occur and accumulate in different populations over time. Different variations in genes can lead to different characteristics between the two populations.

Differentiation between populations

The populations become so different that the members of each of the populations can no longer reproduce and leave fertile offspring, even if they are found in the same habitat at the same time. If this is the case, allopatric speciation has occurred.

Examples

Fruit fly

A typical example of speciation is observed through an experiment with fruit flies, in which the population was purposely separated into two groups and each received a different diet.

After many generations, the flies looked different and preferred to mate with the flies of their own group. If these two populations continued to diverge for a long time, they could become two different species through allopatric speciation.

Kaibab squirrel

About 10,000 years ago, when the southwestern United States was less arid, the forests in the area supported a population of arboreal squirrels with tufts of hair sprouting from their ears.

A small population of tree squirrels that lived on the Grand Canyon's Kaibab Plateau became geographically isolated when the climate changed, causing areas to the north, west, and east to become desert.

Just a few miles to the south lived the rest of the squirrels, known as Abert squirrels (Sciurus aberti), but the two groups were separated by the Grand Canyon. With changes over time, both in appearance and ecology, the Kaibab squirrel (Sciurus kaibabensis) is on its way to becoming a new species.

During its many years of geographic isolation, the small population of Kaibab squirrels has separated from the widely distributed Abert squirrels in a number of ways.

Perhaps the most obvious changes are in skin color. The Kaibab squirrel now has a white tail and a gray belly, in contrast to the gray tail and white belly of the Abert squirrel.

Biologists think that these surprising changes arose in Kaibab squirrels as a result of an evolutionary process called gene drift. Some scientists consider the Kaibab squirrel and the Abert squirrel to be different populations of the same species (S. aberti).

However, because the Kaibab and Abert squirrels are reproductively isolated from each other, some scientists have classified the Kaibab squirrel as a different species (S. kaibabensis).

Porto Santo rabbits

Allopatric speciation has the potential to occur fairly quickly. On Porto Santo, a small island off the coast of Portugal, a population of rabbits was released. Because there were no other rabbits or competitors or predators on the island, the rabbits thrived.

In the 19th century, these rabbits were markedly different from their European ancestors. They were only half as large (they weighed just over 500g), with a different color pattern and a more nocturnal lifestyle.

Most significantly, attempts to mate Porto Santo rabbits with continental European rabbits failed. Many biologists concluded that within 400 years, an extremely short period in evolutionary history, a new species of rabbit will have evolved on the island.

Not all biologists agree that the Porto Santo rabbit is a new species. The objection comes from a more recent breeding experiment and arises as a consequence of the lack of consensus on the definition of the species.

In the experiment, the adoptive mothers of the wild Mediterranean rabbit raised newborn rabbits from Porto Santo. When they reached adulthood, these Porto Santo rabbits were successfully mated with Mediterranean rabbits to produce healthy and fertile offspring.

For some biologists, this experiment clearly shows that Porto Santo rabbits are not a separate species, but rather a subspecies, which is a subdivision in the classification of a species. These biologists regard the Porto Santo rabbits as an example of speciation in progress (much like the Kaibab squirrels).

Other biologists think that the Porto Santo rabbit is a separate species, since it does not cross with other rabbits under natural conditions.

They note that the breeding experiment was successful only after the baby Porto Santo rabbits were raised under artificial conditions, which likely modified their natural behavior.

References

1. Allopatric speciation: the great divide. Recovered from: berkeley.edu/evolibrary/article/_0/speciationmodes_02
2. Brooks, D. & McLennan, D. (2012). The Nature of Diversity: An Evolutionary Voyage of Discovery (1st ed.). University of Chicago Press.
3. Givnish, T. & Sytsma, K. (2000). Molecular Evolution and Adaptive Radiation (1st ed.). Cambridge University Press.
4. Kaneko, K. (2006). Life: An Introduction to Complex Systems Biology (Il. Ed.). Springer.
5. Rittner, D. & McCabe, T. (2004). Encyclopedia of Biology. Facts On File.
6. Russell, P. (2007). Biology: The dynamic Science (1st ed.). Cengage Learning.
7. Solomon, E., Berg, L. & Martin, D. (2004). Biology (7th ed.) Cengage Learning.
8. Tilmon, K. (2008). Specialization, Speciation, and Radiation: The Evolutionary Biology of Herbivorous Insects (1st ed.). Univerisity of California Press.
9. White, T., Adams, W. & Neale, D. (2007). Forest Genetics (1st ed.). CABI.