Allopatric speciation: process and examples

The allopatric speciation or geographical speciation, it's a type of speciation which occurs because of the geographical isolation between biological populations of the same species. "Alopatrica"​​derives from the Greek allos which means 'separate' and patris which means 'country'.

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

Kaibab squirrel, example of allopatric speciation

With the passage of time, the individuals of the populations on each side of the barrier differ. Some of these differences can be reflected in the reproductive biology of the species, so that when the two populations rejoin by eliminating the barrier, they can no longer interbreed. Separate species are then considered.

Allopatric speciation can occur even if the barrier is a bit"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,"the 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 among themselves.

Steps of allopatric speciation

Geographic change

In the first step, a geographical 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 contamination they can have an effect on habitable 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 of the 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 they can no longer reproduce and leave fertile offspring, even if they are again 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 by an experiment with fruit flies, in which the population 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.

Squirrel Kaibab

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

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

Just a few kilometers 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 the changes in time, both in appearance and ecology, the Kaibab squirrel ( Sciurus kaibabensis ) is on the way to becoming a new species.

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

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

Biologists think that these amazing changes arose in Kaibab squirrels as a result of an evolutionary process called gene drift. Some scientists consider that the Kaibab squirrel and the Abert squirrel are 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 ).

Rabbits of Porto Santo

Allopatric speciation has the potential to occur rather quickly. In 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 prospered.

In the nineteenth century, these rabbits were markedly different from their European ancestors. They were only half as big (they weighed a little more than 500 g), with a different color pattern and a more nocturnal lifestyle.

The most significant thing is that attempts to breed rabbits from Porto Santo with rabbits from continental Europe 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 they 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 species.

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

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

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

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

References

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