Sympatric speciation: what it is, definition and fundamentals.

Summary of the Biological process known as sympatric speciation, which gives rise to new species.

It is estimated that, as of 2014, a total of 1,426,337 animals have been discovered on our planet. This value is fluctuating almost on a daily basis, as it is also estimated that there are almost a total of 8 million species of living beings, of which more than ¾ are waiting to be discovered.

On the other side of the coin, the UN reports that about 150-200 species become extinct every 24 hours, a fact that counterbalances the 20,000 discovered on average annually. All these figures indicate an undeniable truth: the biological reality of our planet is fluctuating and, since we have been on it, the number and characteristics of the living beings that accompany us have changed drastically.

All this genetic and behavioral variability on the planet cannot be explained without concepts such as natural selection and genetic drift, facts that promote the presence or disappearance of species over time, as well as changes in their adaptive mechanisms. Today we are going to explain what is sympatric speciation, the phenomenon that is presumably the most important driver of the emergence of new species..

How do new species appear?

From a biological point of view, a species is defined as a group of individuals that are completely fertile among themselves, but isolated from interbreeding with other similar groups by their physiological properties. If we turn to a slightly more evolutionary description, we can say that a species is a single line of ancestor-descendant populations that maintains its identity with respect to other lines and preserves its own evolutionary tendencies and historical destiny.

In short: a species is made up of one or several populations of living beings that can reproduce among themselves, give rise to fertile offspring and, in addition, have a clear phylogenetic lineage, sharing a common ancestor. It seems a rather watertight definition, doesn't it? How can new species appear then?

The mechanisms of speciation

Speciation is the process by which a population of a given species gives rise to one or more other populations, reproductively isolated from the original, which, after a certain period of time, accumulate sufficient differences to produce new species, after a certain period of time, accumulate sufficient genetic differences to be unable to conceive fertile offspring with the original population..

Ernst Mayr, a renowned 20th century evolutionary biologist, postulated that there are two major mechanisms of speciation:

• Phyletic evolution: when an E1 species, over an extended period of time, transforms into an E2 species due to genetic changes.
• Evolution by cladogenesis: also known as bifurcation, in this case a primordial species originates two or more derivatives through a process of divergence.

So that we understand each other, in phyletic evolution the primordial species disappears to give rise to a new one, while in the cladogenesis variant the original does not have to disappear, but "bifurcates" into new taxa by differentiation through differentiation. into new taxa by differentiation through different mechanisms.

What is sympatric speciation?

It is evolution by cladogenesis that is of interest to us, since for this bifurcation to occur between two populations of a species, a barrier must first appear that prevents them from coming into contact with each other. Allopatric speciation is the clearest representation of this process because, in it, a geographical barrier literally appears (a river, a mountain or a separation of tectonic plates, for example) that makes contact between the two populations impossible.

Sympatric speciation is a little more difficult to understand, since in this case there is no tangible and observable barrier that makes contact between individuals of the same species and population impossible. Different mechanisms are postulated by which these "non-physical" isolations can appear, and among them are the following.

1. Sympatric speciation by specialization: a clear example

We don't want to go into genetic conglomerates but, in a very general way, we can say that this postulation is based on the fact that there may be alleles for a gene that encode more or less successful behaviors in the presence of certain events.. For example, a population of insects may have an A1 allele that specializes them to consume certain plants, while the mutation of the A2 allele is more efficient when it comes to preying on other animals.

Since this genetic information is inheritable from parent to offspring, and under certain conditions, it can be expected that A2 individuals will eventually exhibit sufficient behavioral differentiation from A1 individuals to give rise to different species over a long period of time. Both populations will eventually exploit different niches and accumulate very different adaptations, which is why there is no need for a physical space that produces geographic isolation to give rise to two distinct species.

2. Polyploidy and hybridization

These events are very common in the plant world, but also occur in animals. In the case of polyploidy, we are talking about an increase in the number of chromosomes in a population at the cellular level.. For example, meiosis causes the formation of haploid cells (n), which are the eggs and sperm, whose fusion will give rise to a diploid zygote (2n), as we humans are in all cells except sex cells.

If normal disjunction does not occur during meiosis, the sex cells will be diploid (2n) and, therefore, the zygote or individual born will be tetraploid (4n). As you can imagine, these offspring will be reproductively isolated from their parents and the original population, but they will be able to reproduce with each other.

As far as hybridization is concerned, in this case a new individual can be produced from parents of two different species.. Most hybrids in the animal kingdom are sterile but, especially in the case of plants, they can sometimes be reproductively viable with each other but unable to reproduce with either of the two parental species. Thus, from a theoretical framework, a new species would also emerge.

3. Speciation by change of reproductive type

The emergence of asexual lines from sexual lines in the same population automatically leads to evolutionary independence.For this reason, this mechanism can be considered a type of instantaneous sympatric speciation.

There are cases of lizards and salamanders in which this type of speciation has been documented, because once the asexual route is chosen, in some cases the exchange of genetic information involved in reproduction with the original population is no longer necessary. Again, all this is much more observable and common in plants than in other phyla.

4. Sympatric speciation by disruptive selection

In this case, we are talking about something very similar to sympatric speciation by specialization, but there are a few meanings that can be made with respect to this term. Disruptive selection promotes that, within the same population, some individuals adapt to exploit a niche, while others take a completely different path.while others take a completely different path.

For example, let's say that in a population of birds their prey begin to disappear in the environment for X or Y reason, since ecosystems are not watertight. Given this need, and at least on paper, one would expect that one group of this population would move away from the other at a behavioral level to promote the permanence of the species and that the individuals of the population would not "step on" each other's needs. Thus, some birds might adapt to hunt at night and others during the day.

You can imagine all that this entails: basically, individuals of the same population would not come into contact with each other at almost any time: some would live during the day and others at night. In the end, the number of diverse adaptations and reproductive isolation is such in both populations that, in the same space, two species end up emerging without any physical barrier.

Summary

At the foundation of evolutionary biology lies the conception that allopatric speciation (remember: differentiation of two populations by a geographic barrier) is the most important speciation mechanism, since it is basically the one that can be observed tangibly through human eyes. With the advance of science and the development of genetic tests, it has been discovered that many 20th century biologists were quite wrong.

Today, sympatric speciation is considered to be a much better explanation of biological variation than allopatric speciation.The reason for this is that there are many mechanisms of reproductive isolation that do not pass through a tangible physical barrier. This is not to say that allopatric speciation has not done its job over the centuries, but that its importance has probably been overestimated.

We hope that sympatric speciation has been made clear to you along these lines, because we are dealing with a phenomenon that is a little difficult to understand, since it happens by unobservable mechanisms. If we want you to have an idea of all this hypothetical and terminological conglomerate, this is the following: sometimes a physical barrier is not necessary for two populations to differentiate into two different species. It's as simple as that.

Bibliographic references:

• García, E. C. (2012). Mechanisms of ecological speciation in plants and animals. Biológicas Revista de la DES Ciencias Biológico Agropecuarias, 14(2), 7-13.
• Gutiérrez, L. M. H. Biological speciation.
• LAssERRE, D. F. Sympatric speciation and its genetic and morphological implications in fruit flies.
• Perfectti, F. (2002). Speciation: modes and mechanisms. Soler M., Evolution: The basis of biology. Proyecto Sur. Spain.