Evolutionary bottleneck: what is it and how does it affect species?

Bottlenecks are phenomena that affect genetic diversity. Let's see how they work.

When we think of the evolution of living beings, the first thing that comes to mind is natural selection, that famous postulation that was made by Charles Darwin in his timeless work to this day: The Origin of Species. Although it has been reformulated on several occasions and new knowledge has been obtained on the subject, this evolutionary phenomenon is indisputable.

Natural selection is based on a series of very simple premises: the genome of living beings mutates, recombines (in the case of sexual reproduction) and chromosomes can change in shape and/or number.. As genes are not watertight over the generations, sometimes new characters appear that favor the individuals that carry them. At other times, mutations are silent or deleterious, so they are not fixed in the species.

Say, for example, that a mutation in a particular gene causes a bird to have slightly longer tail feathers. If this trait attracts females, the long-tailed male will reproduce more than the other individuals of his species. If this trait is inheritable, more and more individuals with long tails will appear, as they will have more offspring on average. In the end, this beneficial trait would become fixed in the species.

This is a clear example of sexual natural selection, since it is the choice of females that codifies the process. However, what not everyone knows is that in nature "not everything has a reason". You will know what we mean if you read on, as we tell you what genetic drift is and a particularly striking variant of it: the evolutionary bottleneck.

What is genetic drift?

Evolutionary mechanisms are not perfect, however much they may seem so when studying certain animal adaptations in biology classes. Natural selection acts as an involuntary and unconscious force, but living things "do what they can with what they have.". Surely some traits would be ideal for an animal in a particular environment, but mutation may be impossible in the species, or the animal's body may simply not be designed to exploit a given niche.

In addition to this, it should be noted that natural selection is not the only evolutionary mechanism in living things. There is also genetic drift, a stochastic (non-deterministic) effect that causes variation of genes over generations at random, due to sampling error.

A practical example

Let's take an example. In a dwarf population there are 7 red and 3 green beetles. It turns out that the green ones mimic the environment better and, therefore, reduce the probability of being predated and could reproduce more easily than the red ones. There is no doubt that green invertebrates, in this case, are "evolutionarily fitter".

Unfortunately, before these 3 specimens can copulate, a cow steps on the ground and crushes them. The mammal did not consciously choose to end the lives of the beetles, as it was not trying to prey on them, nor did it interact with them in any way. The trait of these beetles was undoubtedly positive, but by chance, the beneficial genes have disappeared.

Thus, through genetic drift, there is a tendency for the beneficial genes to disappear, genetic drift tends to reduce genetic diversity: if 3 red beetles (the most common trait) had been stepped on, there would still be 4 others left that could reproduce.. As much as the green color would be beneficial to the species, it has been a random misfortune that the gene has been erased from the population by a completely anecdotal act. This is how genetic drift works.

In this scenario, it is assumed that the odds of being stepped on are the same for green and red beetles. If this were not the case, sampling would not be random.

The evolutionary bottleneck in genetic drift.

For a moment, imagine that in the above example the population is 10,000 beetles, 7,000 red and 3,000 green: in this case, no matter how much a cow crushes 3 specimens of a particular color, the green genes will still be maintained in the long run. With this premise, it is understandable that genetic drift affects small populations much more.

The evolutionary bottleneck, on the other hand, is an event in which a sudden drastic population decline is experienced due to an environmental event, such as an earthquake, famine, disease or, unfortunately, human activities.. If in our population of 10,000 multicolored beetles there is a flood that leaves only 10 specimens alive, it is not difficult to imagine how genetic drift will be able to act much more easily on the depleted population.

In order to understand the implications of an evolutionary bottleneck, we must dissect a number of terms that are as concrete as they are exciting. Let's get down to it.

The minimum viable population

In conservation biology, the minimum viable population (MVP) is the minimum number of individuals in a population. the minimum number of individuals in a population that can survive without collapse over time.. At a theoretical level, the population with a number of individuals greater than the MVP will be able to exist despite the normal natural disasters, the expected lack of food or the effects of genetic drift previously described.

There is no specific minimum viable population number, since a species such as a common toad (Bufo spinosus) that lays thousands of eggs annually is not the same as an elephant (Loxodonta africana), a species whose females give birth to only one young per birth and have a gestation period of 22 months. Depending on developmental time, gestation, reproductive cycles and many other parameters, the MVP can be much higher or lower..

In general, what can be universally established is that an optimal MVP in any species is one that ensures the permanence of the population by 95-99% in 1,000 years, understanding that disasters and harmful events can occur during this time interval. As you can imagine, if a bottleneck results in a population with a number below the MVP, it will be doomed to disappear.

• You may be interested in, "What is the genetic code and how does it work?"

Effective population size (Ne)

Another very interesting parameter (but much more difficult to understand) is the effective population size (Ne). This is defined as the number of individuals that an idealized population should have so that a specific quantity of interest is the same in the idealized population as in the real population.. Put much more simply, Ne helps geneticists understand the actual number of individuals that reproduce in a population.

Let's go back to our beetles. In the initial population of 10,000 specimens we have many living beings, but this does not imply that all of them will reproduce every year, perhaps because they compete with each other or because the space for egg laying is limited. Therefore, even if the total number of the population is 10,000 (N:10,000), the effective population size could be, for example, 300 individuals (Ne: 300). This has a lot of implications at the evolutionary level, since it is this parameter that really matters to us when quantifying the possible effects of a bottleneck.

This example may sound far-fetched, but for example, tiny effective sizes are very common in wild amphibian populations. Males compete intensely with other contenders for access to females and, unfortunately, many years there are droughts and females do not find sufficient water sources to deposit eggs. Thus, even if 1,000 adults are counted in a given population, only 100 may have reproduced that year (being very optimistic)..

Summary

In summary, here we have taught you what genetic drift is, what the bottleneck is, and what its effects depend on. If a catastrophic event results in an evolutionary bottleneck that, on top of that, leaves a population of a species below the MVP that is characterized by a low Ne, you can imagine the outcome.

The effects of this event may not be noticeable at first, but with each generation of the affected population, the gene pool will be eroded and, consequently, those involved will end up suffering from inbreeding and disappearing due to diseases, mutations, lack of adaptations and reduced Biological viability.

Bibliographic references

• Barbadilla, A. (2012). Population Genetics. Autonomous University of Barcelona. In: http://biologia. uab. en/divulgacio/genpob. html# factors, accessed, 27(10), 2012.
• Lopez, S. F. (2001). Evolution of class I histocompatibility genes in the radiation of South American goldfinches (lúganos) (Doctoral dissertation, Universidad Complutense de Madrid).
• Roffé, A. (2014, August). Genetic drift as an evolutionary force. In IX Encuentro AFHIC/XXV Jornadas Epistemología e Historia de las Ciencias.
• SEOANE, C. E. S., KAGEYAMA, P. Y., RIBEIRO, A., MATIAS, R., Reis, M. S., BAWA, K., & SEBBENN, A. M. (2005). Efeitos da fragmentação florestal sobre a imigração de sementes e a estrutura genética temporal de populações de Euterpe edulis Mart. Revista do Instituto Florestal, 17(1), 23-43.