Allopatric speciation: what is it, types, characteristics and examples?

Allopatric speciation is a way in which species arise. Let's take a look at its characteristics.

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 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).

So far, humans have categorized approximately 1.3 million species of living things, 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 living beings are still unknown to science.

These exorbitant figures make us ask ourselves the following question: how did 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, because the process of allopatric speciation is fascinating to say the least and, moreover, explains part of the biological variability present on Earth. In the following lines we unravel part of the miracle of life, which is soon said.

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 up. Therefore, we will begin by saying that speciation is a process by which a given 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 ramifications, therefore, is the product of differentiation and isolation among animals and other taxa.

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

• Phyletic evolution: when an E1 species transforms into an E2 species over time as a consequence of the accumulation of changes in its genes.
• Evolution by cladogenesis: a species E1 generates one or more species e2, e3, e4 or eX through a process of population divergence.

It is the second point, evolution by cladogenesis, that interests us today. In turn, different types of speciation can be distinguished. These are briefly described below:

• By hybridization: the reproductive crossing between two species ends up giving rise to a new one reproductively isolated from the parents. Common in plants.
• By instantaneous divergence: polyploidy and chromosomal divergence. We will not dwell on their peculiarities due to terminological complexity.
• By gradual divergence: allopatric, peripatric, sympatric, quantum and parapatric speciation.

It may be dizzying to know so many terms in a moment, but don't worry, because everything will be clearer in the following lines. everything will become clearer in the following lines. The general idea is that the process of speciation is not only produced by geographic 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 gene flow between two populations.. It is a phenomenon extrinsic to the organism, since it is neither its initial adaptations, nor its behavior or other internal factors that promote speciation. There is simply an insurmountable barrier that separates a population nucleus into two or more that living beings cannot circumvent.

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

A practical example: beetles and rivers

Imagine that a population of beetles with a large home range sees its territory of action split in half by a river. One part of the population will remain on one side of the water upwelling, while the other will be completely isolated from the primordial one because these invertebrates have no 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 have different demands.. Thus, over 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 east side has developed wings to escape a number of dangerous predators and, naturally, this has caused a modification in the body pattern of the individuals.

On the other hand, the western population has undergone major morphological changes in the forelimbs over the thousands of years of separation, as 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 "textbook" example, because in reality the geographic barrier may be porous and some individuals in the populations may reproduce with each other 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

Having made the term 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 as follows.

1. Allopatric vicariant or dichopatric speciation.

This model applies when a species splits into two large populations, for example by a plate tectonic movement separating two continents.. This is a "large-scale" mechanism that allows the action of natural selection in different ways between the two separated populations.

2. Peripatric allopatric speciation

This model involves the separation of a small population from a much larger one.. Some practitioners argue that there is no difference between the peripatric and the dichopatric models, but those who do consider them to be two separate entities argue that the selection forces are different between the small and the large population, which is why the process is different.

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

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

Meanwhile, if this clueless animal inadvertently kills 3 white beetles with a footfall in the large population, there are still 97 other carriers of the "white" gene. Thus, the selection forces would be quite different in the two nuclei and the smaller one would always suffer more from the effects of environmental randomness.

Summary

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

However, this does not mean that all the split populations will result in new species. Surely, out of paper, if 10 beetles were separated from a group of 300, the new population would eventually disappear due to predation or lack of resources without giving rise to the process of cladogenesis.

Moreover, allopatric speciation was long considered the most important of all, for what is more powerful than an insurmountable physical barrier? Advances in the study and use of genetics have shown this postulation to be false: sympatric speciation, which occurs without geographic isolation, is much more common. You will have to wait for future opportunities for us to explain this concept, no doubt just as fascinating as the one narrated here.

Bibliographic references:

• Speciation, bioinformatica.uab.cat.
• Speciation, Universidad Complutense de Madrid.
• Allopatric speciation, Undestanding evolution. University of Berkeley.