Natural selection
Natural selection is an evolutionary process that was described by Charles Darwin in his book The Origin of Species and inspired by the ideas of the Essay on the population principle of Thomas Malthus that establishes the survival of the fittest or the preponderance of the law of the fittest in a natural environment without external intervention, so that the less fit or weaker individuals perish and their traits do not They are transmitted to subsequent generations by not reproducing, in contrast to the concept of artificial selection where there is direct intervention, by humans, with the purpose of improving the traits of individuals by manipulating them at will. Strictly speaking, it is defined as the differential survival and reproduction of the phenotypes of a biological population. The classic formulation of natural selection establishes that the conditions of an environment favor or hinder, that is, they select the reproduction of living organisms according to their peculiarities. Natural selection was proposed by Darwin as a means to explain biological evolution. This explanation is based on three premises; The first of these is that the trait subject to selection must be heritable. The second holds that there must be variability of the trait among individuals in a population. The third premise states that the variability of the trait must give rise to differences in survival or reproductive success, causing some newly appearing traits to spread in the population. The accumulation of these changes over the generations would produce all evolutionary phenomena.
In its initial formulation, the theory of evolution by natural selection constitutes the great contribution of Charles Darwin (and, independently, by Alfred Russel Wallace), and is a fundamental pillar of Darwinism, later reformulated in the current theory of evolution known as neodarwinism or modern evolutionary synthesis. In evolutionary biology, the process of natural selection is considered the main cause of the origin of species and their adaptation to the environment.
Natural selection can be expressed with the following general law, taken from the conclusion of The Origin of Species:
There are organisms that reproduce and the progenie inherits characteristics of their parents, there are variations of characteristics if the environment does not support all members of a growing population. Then those members of the population with less adapted characteristics (as determined by their environment) will die more likely. Then those members with better adapted features will survive more likely.Darwin, The Origin of Species
The result of the repetition of this scheme over time originates the evolution of the species.
In modern theory
In the synthetic theory, natural selection is not the only cause of evolution, although it is the one that plays a more prominent role. The concept of natural selection is now defined in a more precise way: as the differential reproduction of phenotypes in a population. Since there are differences in the reproductive success of the different variants (with or without a genetic basis), there is natural selection. For example: if the greenest individuals in a population of leaf-insects contribute about three offspring to the next generation, and the brown individuals contribute an average of 1.5 offspring, there is selection in favor of the green ones. Differences in reproductive success can occur for various reasons (different fertility, risk of death by predators, sexual attractiveness, ability to exploit food resources, etc.).
Important figures in the synthesis, and the three founders of population genetics, were Ronald Fisher, who wrote The Genetical Theory of Natural Selection in 1930, J.B.S. Haldane, who introduced the concept of the "cost" of natural selection, and Sewall Wright, who elucidated selection and adaptation.
Generally, there is a correlation between the reproductive efficiency of the carriers of a genotype and the adaptation to the environment that this gives them. Thus, traits that confer adaptive advantages are commonly selected for and propagated in populations (in some cases, a genotype might confer reproductive success without providing greater adaptation to the environment, and would still be selected for). The theory of natural selection provided for the first time a satisfactory scientific explanation for many scientific puzzles in the biological world, especially that of the "look of design" that exists in living beings. It allowed, therefore, that Biology could dispense with divine and supernatural elements and thus become a true science.
Today, evolution by natural selection is studied in various types of organisms, through laboratory and field experiments, and methods are developed to find out which genes have recently been subjected to natural selection and with what. intensity.
Aptitude
The concept of fitness is key in natural selection. Roughly speaking, individuals who are fitter have greater survival potential, similar to the popular phrase "survival of the fittest." However, as with the term natural selection, the precise meaning is more subtle. Richard Dawkins totally avoids it in his later books, although he devotes a chapter of his book The Extended Phenotype to discussing the various senses in which the term is used. Modern evolutionary theory defines fitness not on the basis of how long the organism lives, but on the basis of how long it reproduces. If an organism lives half as long as others of its species, but compared to the rest, twice as many of its descendants reach adulthood; then their genes will survive and spread to the next generation.
Although natural selection operates on individuals, the effects of chance mean that fitness can only be defined on average for individuals in a population. The fitness of a given genotype corresponds to the average effect on all individuals with that genotype. Very low fitness genotypes cause their carriers to have very few—or no—offspring on average. Many human genetic diseases, such as cystic fibrosis, can be cited as examples.
Since fitness is an averaged quantity, it is also possible that a favorable mutation occurring in an individual may not spread to the group if the individual dies before adulthood for other reasons. Fitness totally depends on the environment. Conditions like sickle cell anemia are very rare in the general human population. However, sickle cell disease confers immunity to malaria on the carrier, so their fitness in settings with high rates of malaria infection is very high.
Types of Natural Selection
Natural selection can act on any heritable phenotypic trait and any aspect of the environment can produce selective pressure, this includes sexual selection and competition with members of both the same and other species. However, this does not imply that natural selection always follows one direction and that adaptive evolution results. Natural selection often results in the maintenance of the status quo by eliminating less fit variants.
The unit of selection can be the individual or another level within the hierarchy of biological organization such as genes, cells, and family groups. The question of whether current natural selection at the group (or species) level to produce adaptations that benefit a larger group, without family ties, is still tenuously debated. Likewise, there is some debate about whether selection at the molecular level prior to genetic mutations and fertilization of the zygote should be considered conventional natural selection, since natural selection has traditionally been called an external and environmental force that acts on a phenotype after birth.. Some scientific journals distinguish between natural selection and genetic selection by informally calling the selection of mutations preselection.
Selection at other levels, such as the gene, can result in an improvement for the gene and at the same time in a detriment to the individual carrying the gene. This process is called intragenomic conflict. Taken together, the combined effect of all the pressures at the different levels (gene, individual, group) is what determines the fitness of an individual and therefore the result of natural selection.
Natural selection occurs at every stage of an individual's life. An organism must survive to adulthood in order to reproduce. The selection of those that reach the adult stage is called viability selection. In many species adults have to compete with each other to get sexual partners. This mechanism is called sexual selection and success in it determines who will be the parents of the next generation. When individuals can reproduce on more than one occasion, survival to adulthood increases offspring. This process is called survival selection.
Fertility, both male and female, can be constrained by "fertility selection". Thus, the viability of the gametes produced will vary. Intragenomic conflicts lead to genetic selection. Finally, the union of some combinations of eggs and sperm will be statistically more compatible than others. This is called compatibility selection.
There are four types —sometimes considered three— of natural selection, classified according to the individuals that survive in each type of selection, that is, according to how many survive:
- Stabilizing selection
- Directional selection
- Disruptive or balanced selection
- Sexual selection
Sexual selection
It is useful to differentiate between ecological selection and sexual selection. Ecological selection refers to any selection mechanism as a result of the environment. On the other hand, sexual selection refers specifically to competition for a sexual partner.
Sexual selection can be intrasexual, which is the case of competition between individuals of the same sex in a population, or intersex, which is when one sex controls access to reproduction through mate choice within the population. Normally, intrasexual selection occurs in the form of competition between males and intersex as the choice by females of the best males, due to the higher cost that breeding generally entails for females. However, some species have reversed sexual roles and it is the male that is most selective when it comes to choosing a partner. The best known example is that of certain fish of the family Syngnathidae. Similar examples have also been found in amphibians and birds.
Some characteristics present only in one of the two sexes in specific species can be explained through the pressure exerted by the other sex in their choice of partner. For example, the extravagant plumage of some male birds like the peacock. Likewise, aggression between members of the same sex is sometimes associated with very distinctive characteristics such as the deer's antlers, which are used to fight with other deer. In general, intrasexual selection is associated with sexual dimorphism, which includes differences in the body size of males and females.
Examples of Natural Selection
A well-known example of natural selection is the development of antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928 by Alexander Fleming, antibiotics have been used to combat diseases of bacterial origin. Natural populations of bacteria contain great variation in their gene pool, mainly as a result of mutations. When faced with an antibiotic, most die quickly. However, some have mutations that make them less weak to that particular antibiotic. If the antibiotic challenge is short, some of these individuals will survive the treatment. This eliminative selection of unfit individuals from a population is natural selection.
The surviving bacteria will reproduce to form the next generation. Due to the removal of maladapted individuals in the past generation, the population will contain more bacteria that have some degree of antibiotic resistance. At the same time, new mutations emerge, some of which may add more resistance to the bacterium carrying the mutant gene. Spontaneous mutations are rare and advantageous ones are even rarer. However, populations of bacteria are numerous enough that some individuals contain beneficial mutations. If a new mutation reduces susceptibility to the antibiotic, individuals carrying it are more likely to survive the antibiotic and reproduce.
With enough time and exposure to the antibiotic, a population of antibiotic-resistant bacteria eventually emerges. This new population of resistant bacteria is optimally adapted to the environment in which it evolved. However, it is no longer optimally adapted to the old antibiotic-free environment. The result of natural selection in this case is two populations that are optimally adapted to their specific environment, but are somewhat maladapted to the other environment.
The widespread use and abuse of antibiotics has brought with it increased resistance in microbes, to the point that MRSA aureus is considered a health threat due to its relative invulnerability to existing medicines. Treatment strategies include the use of more powerful antibiotics. However, new branches of MRSA have appeared that are resistant even to these drugs.
This is an example of an evolutionary arms race, in which bacteria evolve into more resistant forms and medical researchers develop new antibiotics. A similar situation occurs with pesticide-resistant plants and insects. Arms races also occur without human intervention. A well-documented case is the diffusion of a certain gene in the butterfly Hypolimnas bolina that protects the males against death caused by the Wolbachia bacterium. This gene is known to have developed only since 2002.
Evolution by natural selection
A requirement for natural selection to drive adaptive evolution, new traits, and speciation is the presence of heritable genetic variation that leads to fitness differences (i.e., that genetic variation results in individuals more and less fit for their circumstances).). Genetic variation is the result of mutations, recombinations, and alterations in the karyotype (number, shape, size, and internal organization of chromosomes). Any one of these changes can have an effect that is very advantageous or disadvantageous, but in general large effects are rare. Changes in genetic material used to be considered neutral or near-neutral because they occurred in non-coding DNA or resulted in synonymous substitutions (the protein made by the mutated gene was for all practical purposes the same as that of the unmutated gene). However, recent studies have shown that many mutations in non-coding DNA do have deleterious effects. The mutation rate and effect on the individual depend on the particular organism, however, from data based on human analyses, considers most mutations to be slightly deleterious.
By definition, the fittest individuals are more likely to contribute offspring to the next generation, while the less fit will have fewer offspring or die before reaching adulthood. As a result, alleles that on average lead to better adaptation (fitness) are more abundant in the next generation, while alleles that tend to harm carriers also tend to disappear. If selective pressures—temperature, water abundance, and any other environmental conditions—remain relatively constant, beneficial alleles spread through the population, becoming dominant (in the sense of more abundant), and harmful alleles disappear. In each generation new mutations and recombinations appear that produce a new spectrum of phenotypes. Thus, each new generation is enriched with the abundance of alleles that contribute to traits that were previously favored by natural selection, thus gradually improving these traits over successive generations.
Some mutations occur in regulatory genes. These changes can have a large effect on the phenotype of the individual because these genes are responsible for regulating the function of many other genes. Most—but not all—mutations in regulatory genes produce nonviable zygotes. Examples of non-lethal mutations in regulatory genes in man occur in the HOX genes, which can cause cervical rib or multi-finger rib formation. When these mutations result in improved fitness, natural selection will favor them and they will spread in the population.
Established traits are not immutable. Traits that are very effective in a given environment can become ineffective if conditions change. If the selective pressure on a trait disappears, it tends to acquire more variations and to deteriorate, even becoming a vestige. On many occasions, the vestigial structure can maintain certain limited functionality or be the basis of other advantageous features (a phenomenon known as preadaptation). For example, the eye is a vestigial organ of the mole, but it still provides some functionality to perceive the duration of the day and night.
Evolutionary processes and natural selection
Horizontal gene transfer and natural selection
If an organism obtains new genetic material not coming from its parents (ancestors), once the genetic material has been inserted into the organism; either by horizontal gene transfer, by processes such as endogenous viral elements, symbiogenesis, etc.; Through successive generations, these new sequences can also undergo random mutations in the same way as the rest of the genome, so natural selection can act on them in the same way. An example of this is that, among the genes involved in the development of the human placenta, a gene is involved, that of syncytin-1 and syncytin-2, whose origin is an endogenous viral element.
Epigenetics, and natural selection
Although the epigenetic process does not imply a change in the DNA nucleotide sequence but consists of a change in the expression of the genes, in any case natural selection, based on the biological result of said expression of genes, will act about the epigenetic process and about the organism that undergoes it.
Speciation
Speciation is the process by which one species splits into two different species. Speciation requires selective mating, which leads to reduced gene flow. Selective mating can be the result of:
- Geographical isolation
- Ethological isolation (behavioral)
- Temporary isolation
For example, a change in the physical environment (geographic isolation by an external barrier, such as a river or a mountain) would correspond to case 1. A change in camouflage would be an example of case 2. Finally a change in the season heat would correspond to case 3.
Over time, these isolated subgroups diverge radically, becoming different species, either because of differences in selective pressure, because different mutations appear in each group, or because of the so-called founder effect. Based on this effect, one of the subgroups might have already started with some beneficial allele by chance. A lesser known mechanism of speciation is that of hybridization. It is well documented in plants and is occasionally seen in species-rich groups such as cichlids. This type of mechanism could reflect a type of evolutionary change known as punctuated equilibrium, which suggests that evolutionary change, and in particular speciation, occurs normally. quickly after long static periods (with hardly any changes).
Genetic changes within each group lead to genetic incompatibility between the genomes of the two subgroups. With what the gene flow is further reduced. Gene flow ceases completely when the distinctive mutations of each group become fixed. With only two mutations (one in each subgroup) speciation can occur. It is enough that these mutations have a neutral or positive effect when they occur in isolation and a negative one when they occur together. From there, the fixation of these genes in each subgroup leads to two isolated reproductive populations, which according to the biological species concept, are in effect two different species.
Possible exceptions
According to Ernst Mayr, natural selection could have a different or limited action in the following cases:
- First, natural selection mechanisms have been described almost exclusively in complex animals and plants (and other groups with sexual reproduction). However, there are indications that selection can be quite different in cases where the limits of individuality are much more blurry. It is the case of colonies of invertebrates and of uniparent organisms, especially plants, protists and procariots (See article Selection Unit).
- Secondly, the first stage of the selection is based on random phenotypic variation, but a number of genetic mechanisms have been discovered that produce a non- random variation. It is the case of meiotic drift, intragenic conflict and certain "selfish genes". A variation that was drastically non- random could exceed the selection action.