Neutral Selection
Neutral selection describes changes in the gene pool of a species that are a result of accumulated random neutral changes that do not convey any particular advantage to a species. Accordingly, neutral selection does not depend upon adaptation, fitness, and natural selection.
Evolution describes biological change over time. Scientists who study evolution argue whether natural selection or random changes have more profoundly influenced the course of evolution. According to evolutionary theory based upon natural selection, changes in the gene pool are a result of differing fitness among individuals with the population. These differences in fitness manifest result in a selective advantage or disadvantage; neutral mutations are rare. According to the neutral theory, however, most changes at the molecular level do not carry a selective advantage or disadvantage and are thus, selectively neutral. To account for wide differences in genetic make-up this would mean that the rate of neutral mutations is very high. Although neither theory necessarily excludes the other, the importance of each process on evolution is remains an important question for biologists.
The concept of neutral selection led to the advancement of neutral theory by Motoo Kimura in the 1960s as a reaction to classic "cost of selection" arguments that for significant genetic changes to occur there must be an expresssed difference within individuals that causes genetic deaths. A genetic death occurs whenever individuals within a population fail to reproduce, or fail to produce fertile offspring.
The higher the selective pressures (measured by the selective coefficient) the greater percentage of a population suffers genetic death. If, for example, the pressures of selection are too high, a given population might face extinction because not enough individuals will survive to successfully reproduce. Kimura and other evolutionary theorists argued that the types of selection pressures needed to produce the genetic differences between known species were so great that these pressures would have led to extinction rather than the manifest diversity of life found on Earth.
Kimura and other proponents of the neutral theory explained genetic variation between different species (or difference between individuals with a species) as a cumulative result of neutral mutations and genetic drift. Kimura based his neutral hypothesis on the fact that most genetic changes (mutations) are harmful (deleterious) and as thus quickly acted upon by natural selection. Because such deleterious mutations are quickly removed with a population, these mutations rarely get the chance to have any profound effect on the gene pool. Kimura argued that such selection was conservative because it actually opposed change by quickly removing mutations.
Kimura further argued that, if selection pressures were the dominant force in evolution, significant genetic changes could only occur if mutations were favorable, and such favorable mutations are rare.
Based on the established mechanisms of mutation and genetic drift, Kimura based the neutral selection theory on four premises. First, Kimura argued that the rate of evolution of proteins is too high to be a function of selection. Second, the rate of molecular evolution is more constant than the rate of morphological (shape or form) evolution and is constant in different lineages. This also allowed for the development of the concept of molecular clocks that relate mutations to time.
Kimura's third argument was that proteins, and parts of the same protein, may evolve at different rates. Those parts of proteins that are not important to the function of the protein may change faster than the more functionally important parts of the protein. According to neutral theory this is explained by fewer neutral mutations in important areas. Correspondingly, selection theory states that mutations in the active sites of proteins are more likely to have an advantageous or disadvantageous effect upon which selection can act.
Kimura's final argument asserted that directional selection would never have allowed the amount of heterozygosity and polymorphism observed in populations.
According to neutral theory, molecules such as DNA, RNA and proteins with fewer functional constraints (e.g., active sites) evolve faster because the number of effectively neutral mutants is correspondingly higher. Not all DNA variation, however, translates into protein variation (e.g., silent mutations, synonymous codons, pseudogenes and non-coding DNA). Further, not all protein variation translates into phenotypic variation, and not all phenotypic variation is subsequently translated into differences in fitness.
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