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. among themselves and produce fertile offspring, but cannot do so under normal circumstances with populations of other species different.

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 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 living beings on land and 91% of marine beings remain 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, since we are talking about speciation, focusing on its allopatric variant. So stay with us

the process of allopatric speciation It is fascinating, to say the least, and also explains part of the biological variability present on Earth. In the following lines we describe part of the miracle of life, which is said soon.

• Related article: "The theory of biological evolution: what it is and what it explains"

What is speciation?

Defining the concept of allopatric speciation without first including it in 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, throughout 3.8 billion years in action, has given us thousands and thousands of species in all kingdoms of beings alive. The tree of life and its ramifications, therefore, is the 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:

• 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: an E1 species generates one or more e2, e3, e4 or eX species through a process of population divergence.

It is the second point, evolution by cladogenesis, which interests us today. In turn, different types of speciation can be distinguished. We will tell you briefly 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. We are not going to dwell on its 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 one moment, but don't worry, because everything will be clearer in the following lines. The general idea is that the speciation process is not only produced by geographic isolation between animal populations in very long times, since genetic and behavioral barriers and hybridizations also play essential roles, among many others things.

• You may be interested in: "The differences between clade, taxon and monophyletic group"

What is allopatric speciation?

Without further ado we define allopatric speciation as that produced by a geographic barrier that prevents gene flow between two populations. It is a phenomenon extrinsic to the organism, since it is not its initial adaptations, nor its behavior, nor other internal factors that promote speciation. An insurmountable barrier simply arises that separates a population nucleus 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 process of evolutionary differentiation with a simple example.

A practical example: beetles and rivers

Imagine that in a population of beetles with a large home range they see their home range cut in half by a river. A part of the population will remain on one side of the water surge, while the other will be isolated completely from the primal by not having these invertebrates adaptive mechanisms to swim through the water and to 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 east side has developed wings to escape a number of dangerous predators and this has naturally caused a change in the body pattern of the individuals.

On the other hand, the western population has undergone important morphological changes in the extremities previous 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 book" example, since in reality the geographical barrier can present certain porosity and some individuals of the populations can reproduce with each other throughout the years. 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 become 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. Vicariant or dicopatric allopatric speciation

This model applies when a species splits into two large populations, for example by a tectonic movement of plates that separates two continents. We are dealing with a “large-scale” mechanism that allows natural selection to act 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 argue that there is no difference between the peripatric and dicopatric models, but those who do consider them to be two entities Separate authors argue that selection forces are different between small and large populations, which is why the process is indeed distinct.

For example, the small population is not only subject to selection pressures and natural selection. A reduced 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 separate 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 inadvertently kills 3 white beetles with one footstep in the large population, there are still another 97 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 environmental randomness.

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 the correct gene flow impossible..

Still, this is not to say that all splintered 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 process of cladogenesis.

Furthermore, for a long time allopatric speciation was 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 that this postulation is false: sympatric speciation, that which occurs without geographic isolation, is much more common. You will have to wait for future opportunities for us to explain this concept to you, no doubt just as fascinating as the one narrated here.

Bibliographic references:

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