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

When we think about 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 today: The Origin of the species. Despite the fact that it has been reformulated on several occasions and new knowledge has been obtained regarding the subject, this evolutionary phenomenon is indisputable.

Natural selection deals with a series of very simple premises: the genome of living beings mutates, recombines (in the case of sexual reproduction) and chromosomes can change shape and / or number. As genes are not watertight throughout generations, new traits sometimes appear that favor the individuals who carry them. At other times, the mutations are silent or deleterious, so they don't look at the species.

Let's 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 rest of the individuals of its species. If this trait is heritable, more and more specimens with long tails will appear, as they would have more offspring on average. In the end, this beneficial character would end up being fixed on the species.

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

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What is genetic drift?

Evolutionary mechanisms are not perfect, as much as they seem when studying certain animal adaptations in biology classes. Natural selection acts as an involuntary and unconscious force, but living beings "do what they can with what they have". Certainly some traits would be ideal for an animal in a specific environment, but the mutation may It is impossible in the species or that, simply, the body of the animal is not designed to exploit a niche dice.

In addition to this, it should be noted that natural selection is not the only evolutionary mechanism in living beings. There is also genetic drift, a stochastic (non-deterministic) effect that causes the variation of genes through random generations, 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 greens blend better with the environment and, therefore, reduce the chances of being predated and could become more easily reproduced than reds. There is no doubt that green invertebrates, in this case, are "more fit at an evolutionary level."

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

So that, Genetic drift tends to reduce genetic diversity: if 3 red beetles had been stepped on (the most common trait), there would still be another 4 that could reproduce. As much as the green color would be beneficial for the species, it has been the 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, the chances of being stepped on are assumed to be the same for green and red beetles. If not, the sampling would not be random.

The evolutionary bottleneck in genetic drift

For a moment, imagine that in the example above the population is 10,000 beetles, 7,000 red and 3,000 green: in this In this case, no matter how much a cow crushes 3 specimens of a specific color, the green genes will continue to remain long term. With this premise, it is understood that genetic drift affects small populations much more.

The evolutionary bottleneck, meanwhile, is an event in which a sudden drastic population decline is experienced by 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 easier in the battered population depleted.

In order to understand the implications of an evolutionary bottleneck, we must dissect a series of terms that are as concrete as they are exciting. Go for it.

The minimum viable population

In conservation biology, the minimum viable population (MVP) is the minimum number of individuals in a population that can survive without it collapsing over time. At the theoretical level, the population with a number of individuals greater than the MVP may exist despite the normal natural disasters, the lack of expected 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 is not the same. annually than an elephant (Loxodonta africana), a species whose females only give birth to one calf at birth and have a gestation period of 22 months. Depending on the development 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 population permanence of 95-99% in 1,000 years, understanding that disasters and harmful events can occur during this interval temporary. As you can imagine, if a bottleneck results in a population with a number below the MVP, it will be doomed.

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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 for a specific quantity of interest to be the same in the idealized population as in the actual 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 again. In the initial population of 10,000 specimens we have many living beings, but this does not imply that all of them go to reproduce every year, perhaps because they compete with each other or because the space for laying eggs 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 many 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, minute effective sizes are very common in wild amphibian populations. Males compete intensely with other contenders for access to females and, for Unfortunately, many years there are droughts and they do not find enough water sources to deposit the eggs. Thus, even if 1,000 adults are surveyed in a given population, only 100 may have reproduced that year (being very optimistic).

Resume

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 gives rise to an evolutionary bottleneck that, on top of it, leaves a population of a species below the MVP that is characterized by having a low Ne, you can imagine the outcome.

The effects of this event may not be noticed in the first instance, but with each generation of the affected population, the gene pool will erode and, therefore, those involved will end up suffering from inbreeding and disappearing due to diseases, mutations, lack of adaptations and biological viability depleted.

Bibliographic references

• Barbadilla, A. (2012). Population's genetics. Autonomous University of Barcelona. On: http://biologia. uab. es / divulgacio / genpob. html # factors, consulted, 27 (10), 2012.
• López, S. F. (2001). Evolution of class I histocompatibility genes in radiation from South American goldfinches (lúganos) (Doctoral dissertation, Universidad Complutense de Madrid).
• Roffé, A. (2014, August). Genetic drift as an evolutionary force. In IX Meeting AFHIC / XXV Epistemology and History of Sciences Conference.
• SEOANE, C. AND. S., KAGEYAMA, P. Y., RIBEIRO, A., MATIAS, R., Reis, M. S., BAWA, K., & SEBBENN, A. M. (2005). Effects of forest fragmentation on seed migration and temporary genetic structure of Euterpe edulis Mart populations. Revista do Instituto Florestal, 17 (1), 23-43.