Allopatric Speciation: What it Is, Types, Characteristics and Examples

From a biological point of view, a species is defined as a group of natural populations whose members can reproduce with each other and generate fertile offspring, but who cannot do so under normal circumstances with populations of other different species.

The central axis of the species is the reproductive unit: if two individuals give rise to a fertile offspring, we can affirm that both parents are included in the same species (with few exceptions).

Until now, humans have categorized approximately 1.3 million species of living beings, with a total of 229 new ones discovered in 2018. We still have a long way to go taxonomically speaking, since it is estimated that 86% of terrestrial living beings and 91% of marine ones still remain unknown to science.

These exorbitant figures make us ask ourselves the following question: how do so many species arise throughout evolutionary history? Today we bring you a partial answer to this question, as we talk about speciation, focusing on its allopatric variant. Stay with us then the process of allopatric speciation It is fascinating to say the least and, furthermore, explains part of the biological variability present on Earth. In the following lines we explain part of the miracle of life, which is said quickly.

What is speciation?

Defining the concept of allopatric speciation without first encompassing it under the umbrella of generality is like starting to build a house from the roof. Therefore, we will start by saying that Speciation is a process by which a certain population of a species gives rise to a new one that cannot exchange its genes with the original population

It is a mechanism that promotes the emergence of new evolutionary lines and that, over 3.8 billion years in action, has given us thousands and thousands of species in all kingdoms of living beings. The tree of life and its branches, therefore, is a product of the differentiation and isolation between animals and other taxa.

According to Ernst Mayr (renowned contemporary biologist and taxonomist) species originate in two different ways:

It is the second point, evolution by cladogenesis, that interests us today. In turn, different types of speciation can be distinguished. We tell you about them below in summary:

It may be dizzying to know so many terms at once, but don’t worry, because everything will be clearer in the following lines The general idea is that the speciation process not only occurs due to geographical isolation between animal populations over very long periods of time, since genetic, behavioral and hybridization barriers also play essential roles, among many other things.

What is allopatric speciation?

Without further ado, we define allopatric speciation as that produced by a geographical barrier that prevents genetic flow between two populations It is a phenomenon extrinsic to the organism, since it is neither its initial adaptations, nor its behavior nor other internal factors that promote speciation. An insurmountable barrier simply arises that separates a population center into two or more that living beings cannot overcome.

For this reason, allopatry and geographic isolation are interchangeable terms. We show you the mechanism of action of this evolutionary differentiation process with a simple example.

A practical example: beetles and rivers

Imagine that in a population of beetles with an extensive range they see their territory of action divided in half by a river. One part of the population will remain on one side of the water source, while the other will be completely isolated from the original one as these invertebrates do not have adaptive mechanisms to swim through the water and cross the river.

In the most typical model of allopatric speciation, each of the “sides” of the river will present different demands Thus, throughout the generations, natural selection will act differently in each of the population groups, selecting different mutations that maximize the probability of survival of the species in the new niche created in each case. In the end, the adaptive changes between individuals will be so different between the two populations that reproduction becomes impossible.

Now, imagine that the river disappears. The population on the eastern side has evolved wings to escape a number of dangerous predators and this has naturally caused a modification in the body pattern of the individuals.

On the other hand, the western population has suffered important morphological changes in the forelimbs throughout the thousands of years of separation, since their only source of food was underground. The genetic divergence is such that, even if the two populations meet again, they can no longer reproduce with each other. Bingo: we have two different species where before there was only one

It should be noted that this is a “typical” example, since in reality the geographical barrier can present a certain porosity and some individuals in the populations can reproduce among themselves over the years. The important thing is that gene flow is greatly reduced, but it does not have to disappear completely.

Types of allopatric speciation

Once the term has been made clear with a more than visible example (or so we hope), we can close this space by explaining that there are two main types of allopatric speciation. These are the following.

1. Allopatric vicariant or dichopatric speciation

This model applies when a species splits into two large populations, for example due to a plate tectonic movement that separates two continents We are faced with a “large-scale” mechanism that allows the action of natural selection in different ways between the two separate populations.

2. Peripatric allopatric speciation

This model involves the separation of a small population from a much larger one There are certain professionals who defend that there is no difference between the peripatric and dichopatric models, but those who do consider them two separate entities argue that the selection forces are different between the small and large populations, which is why the process does. It is different.

For example, the small population is not only subject to selective pressures and natural selection. A small number of individuals favors the action of genetic drift, that is, the fluctuation of genes in the population due to chance.

Recovering the train of thought from the previous case, let’s say that 10 beetles are separated on one side of the river and 300 on the other. It turns out that ⅓ of them, in general, are white and the rest are green. If a large mammal steps on 3 white beetles in the small population by chance, the white genotype may be lost forever.

Meanwhile, if this clueless animal accidentally kills 3 white beetles with one stomp in the large population, there are still 97 other carriers of the “white” gene. Thus, the selection forces would be quite different in both nuclei and the smaller one would always suffer more from the effects of the randomness of the environment.

Summary

As you have seen, allopatric speciation is a fascinating process that It is characterized by the differentiation of two or more populations of a species due to a geographic barrier that makes correct gene flow impossible

Even so, this does not mean that all split populations will result in new species. Surely and off paper, if 10 beetles are separated from a group of 300, the new population would end up disappearing due to predation or lack of resources without giving rise to the cladogenesis process.

Furthermore, for a long time allopatric speciation was considered the most important of all, since what is more powerful than an insurmountable physical barrier? Advances in the study and use of genetics have shown that this postulation is false: sympatric speciation, that which occurs without geographical isolation, is much more common. You will have to wait for future opportunities for us to explain this concept, undoubtedly just as fascinating as the one narrated here.