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

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. The habitats are not going through their best moment either, as it is also estimated that a total of 13.7 million hectares of forest 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 up with the changes introduced by humans? Do living beings have enough evolutionary strategies to cope with the dizzying rate of environmental variation? This question and many others try to be answered by evolutionary salvage theory. We explain it to you below.

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

What is the theory of evolutionary rescue?

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

According to the International Union for Conservation of Nature (IUCN) more than 32,000 taxa of living beings are in danger, that is to say: one in eight species of birds, one in four mammals, almost half of the amphibians and 70% of the plants. In summary, 27% of all species assessed 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 the theory of evolutionary rescue

In the face of climate change, populations of living beings have three tools to survive over time:

• Phenotypic plasticity: refers to the genetic properties of the individual to adapt to environmental change. The genotype codes for more than one phenotype.
• Dispersal: 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, physical space is finite and the new territories explored are usually already occupied by other living beings. For this reason, the persistence of species in a changing environment largely depends on their ability to adaptively evolve, that is, specialize in new environmental variants before disappear.

The theory of evolutionary rescue is based on this last point. In other words, proposes that living things can recover from environmental stresses through advantageous genetic modification, instead of placing all their “hopes” on gene flow, migration of individuals, 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 more quickly in a short time to survive in the environment despite the rapid changes that occur in it.

• You may be interested in: "Speciation: what it is and how it develops in biological evolution"

Factors to take into account

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

1. demographic factors

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

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

2. Genetic factors

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

Naturally, the greater the genetic variability of a population, the more likely the rescue will be, since natural selection can act on a greater number of traits. This will favor the most suitable for that moment and, ideally, the least prepared will disappear and the population will fluctuate to the most effective change: adaptive evolution will take place.

The mutation rate should also promote evolutionary rescues, as non-deleterous 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 rate of change in the environment is faster than the rate of generational turnover in the population, things get enormously complicated. In the same way, interactions with other living beings play an essential role: both the intra and interspecific competitions can increase or decrease the chances of rescue evolutionary.

A practical approach

So far we've told you some of the theory, but ideally any claim should be based, at least in part, on practical observations. Unfortunately, proving the theory of evolutionary rescue 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 for 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 immunoresistant ones are the ones that have offspring and last over time.

unresolved issues

Although the previously exposed data seems promising, we must emphasize that, for each case striking, there are many others in which species have disappeared due to viruses and pandemics without power Do nothing. This is the example of the chytrid fungus in amphibians, which has caused the decline of 500 amphibian species 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 issue to be resolved is the actual distinction between evolutionary rescue and normal rates of adaptation. Differentiating both terms is at least complex, since a lot of empirical evidence is required and factors to be taken 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 finish, this is the following: evolutionary rescue is not an act performed by the human being nor a measure of the conservation, but a hypothetical situation in which living things can cope with environmental pressures thanks to rapid adaptive evolution.

Putting this concept to the test empirically presents a titanic logistical complexity, since 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, S. M., Cunningham, C. J., & Westley, P. TO. (2014). Evolutionary rescue in a changing world. Trends in Ecology & Evolution, 29(9), 521-530.
• Bell, G., & González, 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.