Selection: The Mechanism of Evolution

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A fitness value of greater than one indicates that the frequency of that genotype in the population increases, while a value of less than one indicates that it decreases. Natural selection can act on any phenotypic trait, and any aspect of the environment, including mates and competitors, can result in a selective pressure. However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection is considered to often result in the maintenance of the situation.

Natural selection is often discussed in terms of a struggle among individual organisms for reproductive success. However, other objects of natural selection have been suggested on levels both below and above the individual.

Some have proposed the gene as the principal object of selection. Dawkins argued that "the fundamental unit of selection, and therefore of self-interest, is not the species , nor the group, nor even, strictly, the individual. It is the gene, the unit of heredity…. Selection occurs at only one lowest level—the gene. While a number of evolutionists support this view, Mayr , for one, considers gene selection as invalid, both because a gene is only one part of the genotype and natural selection acts on the phenotype , and because it fails to recognize that genes do not act independently of other genes.

Likewise, Gould insists that only individuals can reproduce or die, and hence genes could not be the unit of selection.


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Some, such as Gould , recognize other hierarchical levels of selection, including groups of individuals, species, and higher taxa. Species selection also has been tied to the theory of punctuated equilibrium, developed by Gould and Eldredge. Such levels of selection remain controversial.

Many evolutionists recognize "kin selection," that being selection for traits that favor the survival and reproduction of close relatives who share similar genotypes Mayr A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. Antibiotics have been used to fight bacterial diseases since the discovery of penicillin in by Alexander Fleming. However, the widespread use of antibiotics has led to increased microbial resistance against antibiotics, to the point that the methicillin-resistant Staphylococcus aureus MRSA has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.

Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, primarily as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some may have mutations that make them a little less susceptible.

If the exposure to antibiotics is short, these individuals will survive the treatment. This selective elimination of "maladapted" individuals from a population is natural selection in action. These surviving bacteria will then reproduce again, producing the next generation. Due to the elimination of the maladapted individuals in the past generation, this population contains more bacteria that have some resistance against the antibiotic.

At the same time, new mutations occur, contributing new genetic variation to the existing genetic variation. Spontaneous mutations are very rare, very few have any effect at all, and usually any effect is deleterious. However, populations of bacteria are enormous, and so a few individuals may have beneficial mutations. If a new mutation reduces their susceptibility to an antibiotic, these individuals are more likely to survive when next confronted with that antibiotic.

Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge.


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Recently, several new strains of MRSA have emerged that are resistant to vancomycin and teicoplanin. This exemplifies a situation where medical researchers continue to develop new antibiotics that can kill the bacteria, and this leads to resistance to the new antibiotics. A similar situation occurs with pesticide resistance in plants and insects. See also: Evolution and Darwinism. The theory of modification through natural selection , or the theory of natural selection, postulates a process by which the mechanism of natural selection can lead to biological evolution.

This theory is used to explain both evolution at or below the level of species microevolution , such as changes in gene frequencies in populations and speciation phenomena, as well as major genetic changes above the species level macroevolution , such as the development of novel traits wings, feathers, jaws, etc.

Natural selection

In the theory of natural selection, a prerequisite for natural selection to result in evolution, novel traits, and speciation is the presence of heritable genetic variation. Genetic variation is the result of mutations , recombinations , and alterations in the karyotype the number, shape, size, and internal arrangement of the chromosomes. Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are very rare.

In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in non-coding DNA. However, recent research suggests that many mutations in non-coding DNA do have slight deleterious effects. Overall, of those mutations that do affect the fitness of the individual, most are slightly deleterious, some reduce the fitness dramatically, and some increase the fitness. Individuals with greater fitness are more likely to contribute offspring to the next generation, while individuals with lesser fitness are more likely to die early or fail to reproduce.

As a result, genotypes with greater fitness become more abundant in the next generation, while genotypes with a lesser fitness become rarer. If the selection forces remain the same for many generations, beneficial genotypes become more and more abundant, until they dominate the population, while genotypes with a lesser fitness disappear.

In every generation, new mutations and recombinations arise spontaneously, producing a new spectrum of phenotypes. Therefore, each new generation will be enriched by the increasing abundance of alleles that contribute to those traits that were favored by selection, enhancing these traits over successive generations. Some mutations occur in so-called regulatory genes. Changes in these can have large effects on the phenotype of the individual because they regulate the function of many other genes.

Most, but not all, mutations in regulatory genes result in non-viable zygotes. Mutations in some HOX genes in humans result in polydactyly, an increase in the number of fingers or toes Zakany et al. According to the theory of natural selection, when such mutations result in a higher fitness, natural selection will favor these phenotypes and the novel trait will spread in the population. Established traits are not immutable: an established trait may lose its fitness if environmental conditions change.


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The power of natural selection will also inevitably depend upon prevailing environmental factors; in general, the number of offspring is far greater than the number of individuals that can survive to the next generation, and there will be intense selection of the best-adapted individuals for the next generation. The theory of natural selection is one of two major theories presented by Darwin , the other being the theory of descent with modification.

The theory of descent with modification deals with the pattern of evolution, while the theory of natural selection deals with the cause of evolutionary change.

Mechanisms: the processes of evolution

In other words, the theory of natural selection is an explanation offered for how evolution might have occurred, i. It was the most revolutionary and controversial concept advanced by Darwin.

Mechanisms of Evolution - SC.912.L.15.14

According to this theory, natural selection is the directing or creative force of evolution. A specimen of the living fossil fish, a coelacanth. Exterior of a horseshoe crab, an example of a living fossil. Directional selection tends to favor phenotypes at one extreme of the range of variation. Insecticide resistance is an example.

New mechanism for evolution that helps explain the origin of new functions

DDT was a widely used insecticide. After a few years of extensive use, DDT lost its effectiveness on insects. Directional selection. Another example is the peppered moth Biston betularia. Before the Industrial Revolution in the 18th and early 19th centuries, only light-colored moths were collected in light-colored woodlands in England. There was a rare, dark form. With the pollution caused by the buring of coal, the light-colored tree trunks became darker due to soot. The once rare dark-colored moths became more prevalent, while the once-common light-colored moths became increasingly rare.

Reason: predation by birds. The color that had the greatest contrast with the background tree trunk was at a disadvantage. Cleanup of the forest during the s caused the allele frequencies of light and dark moths to reverse to pre-Industrial Revolution levels, dark moths are now rare, light moths are now common. The resistance of many bacterial species to antibiotics ia another example of directional selection.

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Over speciews show some degree of antibiotic resistance, necessitating the development and more prudent use of a new generation of antibiotic medicines. Disruptive selection favors individuals at both extremes of variation: selection is against the middle of the curve. This causes a discontinuity of the variations, causing two or more morphs or distinct phenotypes. The African swallowtail butterfly Papilo dardanus produces two distinct morphs, both of which resemble brightly colored but distasteful butterflies of other species.

Each morph gains protection from predation although it is in fact quite edible. Disruptive selection. As populations diverge, they form similar but related species. When are two populations new species? When populations no longer interbreed they are thought to be separate species. As natural selection adapts populations occupying different environments, they will diverge into races, subspecies, and finally separate species. A species can be defined as one or more populations of interbreeding organisms that are reproductively isolated in nature from all other organisms.

Genetic divergence results when adaptation, drift and mutation act on populations. Barriers to gene flow between populations isolate those populations, ultimately leading to the formation of new and separate species. Populations begin to diverge when gene flow between them is restricted. Geographic isolation is often the first step in allopatric speciation.

Other mechanisms may develop that further restrict reproduction between populations: these are the reproductive isolating mechanisms. Sympatric speciation happens when members of a population develop some genetic difference that prevents them from reproducing with the parent type. This mechanism is best understood in plants, where failure to reduce chromosome number results in polyploid plants that reproduce successfully only with other polyploids.

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