Before answering this question, it is important to know, what is evolution?

The answer that I like the most, because it is simple and clear, is that evolution is a change in the genetic set over time. Evolution can be in different forms, adaptive (when it has been driven and shaped by natural selection), neutral (when it has been dominated by drift) or maladaptive (when it goes wrong), since evolution does not only produce things that work Well, sometimes evolution just wanders.

Adaptive evolution occurs when selection favors the establishment of genes that control phenotypic characters capable of optimizing the ecological and reproductive performance of a population in a certain spatial and temporal context. It is a design for reproductive success and this is where the natural selection, but what is natural selection?

Before answering, I would like to emphasize that previously I saw natural selection as a very simple process, where this evolutionary force only determined or selected the best adapted individuals, but, after reading more information and rereading Darwin, I see that more that this, natural selection is a complex process where it is always relative; It always depends on what the picture of reproductive success is in that population at the time it is happening, plus there are different types of selection. Therefore, natural selection is the process by which the selective pressure of the environment generates the differential reproduction of some individuals within a population, that is, individuals that have favorable characteristics for a certain environment are more likely to survive and leave fertile offspring who will inherit these characteristics.

Why and how does Natural Selection cause evolution and adaptation?

Because the environment changes, which causes the population to change over time and adapt. Natural selection produces evolution when the environment changes and also when in a constant environment a new way of surviving “better” than the current way of the population emerges, so, over time, natural selection generates adaptation.

What are the four conditions that Natural Selection requires?

  • The reproduction. To form new generations, individuals must reproduce.
    • The Heritage. The offspring must tend to resemble their parents.
      • Variation in individual characters among members of the population. The different individuals of the population must have differences, which make them different from the other members, genetic variability.
        • Variation in the aptitude of organisms depending on the state they have for a heritable character. This means that individuals in the population with some traits should be more likely to reproduce than others.

          Fast and slow natural selection

          We can see natural selection in two ways: fast or slow. Evolution can be very rapid when populations are large and selection is strong. And the reason for this is that large populations have a lot of genetic variation. Therefore, there is the possibility of a large response to selection, on the other hand, small populations do not have as much genetic variation. So while the selection may be strong, they can't respond as well.

          An example of rapid selection is antibiotic resistance, a problem caused by strong directional natural selection, which causes a rapid evolutionary response. Since the population of bacteria or viruses, when administered that drug that changes the environment, causes the millions of individuals to mutate and after being selected they reproduce and inherit said resistance.

          In slow selection, the individuals in the population manage to generate stabilizing selection for hundreds of millions of years, they find a fairly stable way of living and surviving, they always try to find the same type of environment, so they are never exposed On the other hand, an example is living fossils, such as coelacanths, horsetails, lycopodia, etc., that have survived millions of years without molecular or morphological changes, or at least with very subtle changes.

          Types of Natural Selection

          There are different types of selection, directional, stabilizing and disruptive selection; natural and sexual selection; frequency-dependent selection; and selection that acts on individuals and groups.

          In directional natural selection, this favors smaller individuals and, if the trait is inherited, will produce a decrease in average body size, it can also produce an evolutionary increase in body size if larger individuals had greater fitness physical.

          In stabilizing selection, intermediate phenotypes predominate over extreme phenotypes; in this type of selection, organisms with characteristics of the "average" range are the ones that survive the most. And disruptive natural selection occurs when both extremes are favored relative to the intermediates.

          Frequency-dependent selection is a type of selection where the environment selects based on the frequency of a certain characteristic that individuals have. The greater the number of individuals with that trait, the environment tends to select them positively or negatively, depending on the context.

          Another type of selection is sexual selection, which involves a direct trade-off between mating success and survival. Sexual selection is a component of natural selection. Sexual selection can be intrasexual, within the same sex, when males compete among themselves for a certain female, or intersexual selection, where both sexes participate, for example, when the female selects certain males that have the same traits.

          Finally, other types of selection are the selection that acts on individuals and groups, the one that acts on individuals is the one that we have already explained, where particular individuals with a favorable character are selected that can reproduce and leave offspring, in order to evolve to through time. On the other hand, there is group selection, which proposes the idea that selection can operate at levels higher than the individual, however, it would be very unstable because some individuals would be more at risk than others, some would take advantage and others would be subjected or suffer.

          So, as we can see, natural selection is found in different ways, we can see it quickly or slowly, at different levels, we classify it in different ways, but always with the same objective and forming part of the evolutionary changes, explaining a part of the process of biological evolution. After all, it is one of the causes of our evolutionary history.

          Thanks for reading me, what did you think? I'll read you in the comments.

          More information: Ridley, M. 2004. Natural Selection and Variation (Pp.71-92). In: Evolution. USA:Blackwell Publishing company.Ridley, M. 2004. Natural Selection and Variation (Pp.71-92). In: Evolution. USA:Blackwell Publishing company.