NATURAL SELECTION: MECHANISM, EVIDENCE, TYPES AND EXAMPLES - BIOLOGY - 2024

The natural selection is an evolutionary mechanism proposed by the British naturalist Charles Darwin, where there is a differential reproductive success among individuals in a population.

Natural selection acts in terms of the reproduction of individuals who carry certain alleles, leaving more offspring than other individuals with different alleles. These individuals reproduce more and therefore increase their frequency. The Darwinian natural selection process gives rise to adaptations.

Source: see Source, via Wikimedia Commons In the light of population genetics, evolution is defined as the variation of allele frequencies in the population. There are two evolutionary processes or mechanisms that lead to this change: natural selection and gene drift.

Charles Darwin

Natural selection has been misunderstood ever since Darwin first made his groundbreaking ideas known. Given the political and social context of the time, the naturalist theories were erroneously extrapolated to human societies, emerging phrases that today are viralized by the media and documentaries such as "survival of the fittest."

What is natural selection?

Natural selection is the mechanism proposed by the British naturalist Charles Darwin in the year 1859. The subject is treated in great detail in his masterpiece The Origin of Species.

It is one of the most important ideas in the area of ​​biology, since it explains how all the forms of life that we are able to appreciate today originated. It is comparable to the ideas of great scientists in other disciplines, such as Isaac Newton, for example.

Darwin explains through numerous examples observed during his travels how species are not immutable entities in time and proposes that they all come from a common ancestor.

Although there are dozens of definitions of natural selection, the simplest and most concrete is that of Stearns & Hoekstra (2000): “natural selection is the variation in reproductive success associated with a heritable trait”.

It should be mentioned that evolution, and natural selection, do not pursue a specific goal or objective. It only produces organisms adapted to their environment, without any kind of specification of the potential configuration that these organisms will have.

Mechanism

Some authors express that natural selection is a mathematical inevitability, since it occurs whenever three postulates are fulfilled, which we will see below:

Variation

The individuals that belong to the population show variations. In fact, variation is a sine qua non for evolutionary processes to take place.

Variation in organisms occurs at different levels, from variations in the nucleotides that make up DNA to morphologies and variations in behavior. As we lower the level, we find more variation.

Heritability

The characteristic must be heritable. These variations present in the population must pass from parents to children. To verify if a trait is heritable, a parameter called “heritability” is used, defined as the proportion of phenotypic variance due to genetic variation.

Mathematically, it is expressed as h ² = V _G / (V _G + V _E). Where V _G is the genetic variance and V _E is the variance product of the environment.

There is a very simple and intuitive way to quantify heritability: the measure of character of parents vs. character in children. For example, if we want to confirm the heritability of beak size in birds, we measure y size in parents and plot them versus size in offspring.

In case we observe that the graph tends to a line (the r ² is close to 1) we can conclude that the characteristics are heritable.

The varying character is related to the

The last condition for natural selection to act in the population is the relationship of the trait with fitness - this parameter quantifies the ability of individuals to reproduce and survive, and varies from 0 to 1.

In other words, this characteristic must increase the reproductive success of its carrier.

Hypothetical example: the tail of squirrels

Kaibaba squirrel

Let's take a hypothetical squirrel population and think about whether or not natural selection might act on it.

The first thing we must do is check if there is variation in the population. We can do this by measuring the characters of interest. Suppose we find variation in the tail: there are variants with a long tail and a short tail.

Subsequently, we must confirm if the "queue size" characteristic is inheritable. To do this, we measure the tail length of the parents and plot it against the tail length of the children. If we find a linear relationship between the two variables, it means that, indeed, the heritability is high.

Finally, we must confirm that the size of the tail increases the reproductive success of the carrier.

The shorter tail may allow individuals to move more easily (this is not necessarily true, it is for purely educational purposes), and allows them to escape predators more successfully than long-tailed carriers.

Thus, throughout the generations, the “short strain” characteristic will be more frequent in the population. This is evolution by natural selection. And the result of this simple - but very powerful process - is adaptations.

Evidence

Natural selection, and evolution in general, is supported by extraordinarily robust evidence from various disciplines, including paleontology, molecular biology, and geography.

Fossil record

The fossil record is the clearest proof that species are not immutable entities, as was thought before the time of Darwin.

Homology

The descendants with modifications raised in the origin of the species, find support in homologous structures - structures with a common origin, but that may present certain variations.

For example, the human arm, the bat's wing, and the fins of whales are homologous structures to each other, since the common ancestor of all these lineages had the same bone pattern in their upper while. In each group, the structure has been modified depending on the lifestyle of the organism.

Molecular biology

In the same way, advances in molecular biology allow us to know the sequences in different organisms and there is no doubt that there is a common origin.

Direct observation

Finally, we may observe the mechanism of natural selection in action. Certain groups with very short generation times, such as bacteria and viruses, make it possible to observe the evolution of the group in a short period of time. The typical example is the evolution of antibiotics.

What is not natural selection?

Although evolution is the science that makes sense of biology - to quote the famous biologist Dobzhansky "nothing makes sense in biology except in the light of evolution" - there are many misconceptions in evolutionary biology and the mechanisms related to it. is.

Natural selection seems to be a popular concept, not only for academics, but also for the general population. However, over the years, the idea has been distorted and misrepresented both in academia and in the media.

It's not survival of the fittest

When mentioning "natural selection," it is almost impossible not to conjure up phrases like "survival of the fittest or the fittest." Although these phrases are very popular and have been widely used in documentaries and the like, they do not accurately express the meaning of natural selection.

Natural selection is directly related to the reproduction of individuals and indirectly to survival. Logically, the longer an individual lives, the more likely it is to reproduce. However, the direct connection of the mechanism is with reproduction.

In the same way, the "stronger" or "more athletic" organism does not always reproduce in greater quantity. For these reasons, the well-known phrase must be abandoned.

It is not synonymous with evolution

Evolution is a two-step process: one that causes variation (mutation and recombination), which is random, and a second step that determines the change in allele frequencies in the population.

This last stage can occur by natural selection or by genetic or genetic drift. Therefore, natural selection is only the second part of this larger phenomenon called evolution.

Types and examples

There are various classifications of the selection. The first classifies the selection events according to their effect on the mean and the variance in the frequency distribution of the character studied. These are: stabilizing, directional and disruptive selection

We also have another classification that depends on the variation of fitness according to the frequency of the various genotypes in the population. These are the positive and negative frequency dependent selection.

Lastly, there is the hard and soft selection. This classification depends on the existence of competition between individuals in the population and the magnitude of the selection pressure. We will describe the three most important types of selection below:

Stabilizing selection

There is stabilizing selection when the individuals who have the "average" or more frequent character (those who are at the highest point in the frequency distribution) are those with the highest fitness.

In contrast, individuals found in the bell's tails, far from the average, are eliminated over the generations.

In this selection model, the mean remains constant throughout the generations, while the variance decreases.

A classic example of stabilizing selection is the child's weight at birth. Although medical advances have relaxed this selective pressure with procedures such as caesarean section, size is often a deciding factor.

Small babies lose heat quickly, while babies who are significantly heavier than average have problems with delivery.

If a researcher seeks to study the type of selection that occurs in a given population and only quantifies the average of the characteristic, he may reach the wrong conclusions, believing that evolution is not occurring in the population. For this reason, it is important to measure the variance of character.

Directional selection

The directional selection model suggests that individuals who are in any of the tails of the frequency distribution survive throughout the generations, be it the left or right sector.

In directional selection models, the mean shifts over the generations, while the variance remains constant.

The phenomenon of artificial selection carried out by humans on their domestic animals and plants is a typical directional selection. Generally, it is intended that the animals (for example, cattle) are larger, produce more milk, are stronger, etc. The same occurs in plants.

Over the generations, the mean of the selected character of the population varies according to pressure. In case larger cows are sought, the average would increase.

In a natural biological system, we can take the example of the fur of a certain small mammal. If the temperature constantly decreases in its habitat, those variants that have a thicker coat will be selected by a random mutation.

Disruptive selection

Disruptive selection works by favoring individuals who are farthest from the average. As generations go by, the queues increase in frequency, while individuals who were previously close to the average begin to decrease.

In this model, the average can be kept constant, while the variance increases - the curve gets wider and wider until it ends up dividing in two.

It is suggested that this type of selection could lead to speciation events, provided that adequate isolation occurs between the two morphologies located at the ends of the tail.

For example, a certain species of bird may have marked variations in its beak. Suppose that there are optimal seeds for very small beaks and optimal seeds for very large beaks, but the intermediate beaks do not get suitable food.

Thus, the two extremes would increase in frequency and, if the appropriate conditions are given that propitiate speciation events, it may be that over time the individuals with different variations of the peak will become two new species.

Source: Ealbert17, from Wikimedia Commons

References

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