Evolutionary rescue : what it is and how it affects the preservation of species

 

Evolutionary rescue : Climate change and anthropization take their toll on ecosystems and, therefore, experts estimate that 150 to 200 species of living beings become extinct every 24 hours. Habitats are not going through their best moment either, as it is also estimated that a total of 13.7 million hectares of forest are cut down per year worldwide, the equivalent of the area occupied by Greece.

All these data show us a reality that is hard to recognize: the Earth is approaching a point of no return. Will nature be able to keep pace with the changes introduced by humans? Do living things have enough evolutionary strategies to cope with the dizzying pace of environmental variation? This question and many others try to be answered by evolutionary rescue theory. We explain it to you below.

Evolutionary rescue : What is evolutionary rescue theory?

The human being is in the sixth mass extinction (Holocene extinction), since the extinction rate of species today is 100 to 1,000 times the natural average in evolution. Unfortunately, these data have been supported by scientific evidence on multiple occasions.

According to the International Union for Conservation of Nature (IUCN) more than 32,000 taxa of living things are in danger, that is to say: one in eight species of birds, one in four mammals, almost half of amphibians and 70% of plants. In summary, 27% of all species evaluated by humans are in some category of threat.

This raises the following question for conservation professionals: Do living beings have tools to face the growing threat that is human action? How have some species survived other extinction events? Evolutionary rescue theory tries to partially cover these answers, at least on paper.

Theoretical foundation of evolutionary rescue theory

Faced with a climatic variation, populations of living beings have three tools to endure over time:

  • Phenotypic plasticity: refers to the genetic properties of the individual to adapt to an environmental change. The genotype encodes more than one phenotype.
  • Dispersion: any population movement that has the potential to lead to gene flow between individuals of a species.
  • Adaptive evolution: rapid speciation of one or more species to fill many new ecological niches.

Although in the short term dispersion phenomena may be the solution, the physical space is finite and the new territories explored are already usually occupied by other living beings. For this reason, the persistence of species in a changing environment largely depends on their ability to evolve adaptively, that is, to specialize in new environmental variants before disappearing.

Evolutionary rescue theory is based on this last point. In other words, proposes that living things can recover from environmental pressures through advantageous genetic modification, instead of placing all their “hopes” on gene flow, individual migration, or dispersal.

The “typical evolution” proposes that living beings evolve slowly, but we are no longer in a typical situation. Thus, a new concept of “contemporary evolution” is explored, or what is the same, that living beings can evolve faster in a short time to survive in the environment despite the rapid changes that occur in it.

Evolutionary rescue : Factors to take into account

Various factors play a key role in evolutionary rescue theory. We present them briefly in the following lines.

Evolutionary rescue : 1. Demographic factors

Theoretical postulations stipulate that the size of the population evaluated is an essential factor to know if evolutionary rescue can occur or not. In populations there is a value called “minimum viable population” (MVP), the lower limit that allows a species to survive in the wild. When taxa are below this value, extinction is made much more plausible by stochastic or random processes, such as genetic drift.

Thus, the longer a population is below the MVP, the less likely it is that evolutionary rescue will occur. Furthermore, the faster the population decreases, the more the viability of this theory is reduced: the species must be given “time” to generate a viable adaptation before it is evoked to extinction.

Evolutionary rescue : 2. Genetic factors

The genetic variability of a species, the rate of mutations that it presents and its rate of dispersion are also key for an evolutionary rescue phenomenon to take place in it.

Naturally, the greater the genetic variability of a population, the more likely rescue will be, since natural selection can act on a greater number of traits. In this way, the most suitable for that moment will be favored and, ideally, the less prepared will disappear and the population will fluctuate at the most effective change: adaptive evolution will occur.

The mutation rate should also promote evolutionary rescues, as non-deleterious or beneficial mutations are another way of acquiring genetic variability in species. Unfortunately, in animals this phenomenon is usually quite slow.

3. Extrinsic factors

Clearly, the probability of a successful evolutionary rescue also depends on the environment. If the speed of change in the environment is faster than the generational turnover rate of the population, things get enormously complicated. In the same way, interactions with other living beings play an essential role: both intra and interspecific competitions can increase or decrease the probabilities of evolutionary rescue.

A practical approach

So far we have told you part of the theory, but ideally any application should be based, at least in part, on practical observations. Unfortunately, proving the evolutionary rescue theory is tremendously complex, even more so when we take into account that genetic tests and population follow-ups are required that must be maintained for decades.

A very clear example (although not entirely valid due to its anthropic nature) is resistance to antibiotics by various groups of bacteria. Bacteria mutate at a much faster rate than evolutionarily expected, as drugs unintentionally select the most resistant and viable individuals on an ongoing basis. The same happens with some species of insects and the application of insecticides on crops.

Another ideal case could be that of rabbits, since viral myxomatosis reduced their populations in some areas of Europe and Australia by up to 99% during the 20th century. This led to the selection, in the long term, of those individuals with mutations resistant to infection (up to 3 effective genetic variations have been identified). This fact has prevented, at least in part, the complete disappearance of the species, since the immune-resistant are those that have offspring and last over time.

Unresolved issues

Although the data previously presented seem promising, we must emphasize that, for each striking case, there are many others in which the species have disappeared due to viruses and pandemics without being able to do anything. This is the example of the chytrid fungus in amphibians, which has caused the decline of 500 species of amphibians and the complete extinction of almost 100 of them in just 50 years. Of course, in no case are we dealing with a miraculous adaptive mechanism.

Another question to be solved is to perform the real distinction between evolutionary rescue and normal adaptation rates. Differentiating both terms is at least complex, since many empirical evidences and factors are required to take into account for each species analyzed.

Summary

Perhaps these terms may sound a bit confusing to the reader, but if we want you to have an idea before finishing, this is the following: evolutionary rescue is not an act carried out by humans or a conservation measure, but a hypothetical situation in which living things can cope with environmental pressures thanks to rapid adaptive evolution.

Testing this concept empirically presents a titanic logistical complexity, since it requires very long-term population monitoring, genetic analysis and many other parameters. In any case, we cannot trust that nature itself will fix the disaster that we have created: if anyone can reverse this situation, at least in part, it is man.

Bibliographic references:

  • Data on extinctions: International Union for Conservation of Nature (IUCN).
  • Carlson, SM, Cunningham, CJ, & Westley, PA (2014). Evolutionary rescue in a changing world. Trends in Ecology & Evolution, 29 (9), 521-530.
  • Bell, G., & Gonzalez, A. (2009). Evolutionary rescue can prevent extinction following environmental change. Ecology letters, 12 (9), 942-948.
  • Bell, G. (2017). Evolutionary rescue. Annual Review of Ecology, Evolution, and Systematics, 48, 605-627.
  • Bell, G. (2013). Evolutionary rescue and the limits of adaptation. Philosophical Transactions of the Royal Society B: Biological Sciences, 368 (1610), 20120080.

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