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Neutral theory of molecular evolution

The neutral theory of molecular evolution (also, simply the neutral theory of evolution) is an influential theory that was introduced with provocative effect by Motoo Kimura[?] in the late 1960s and early 1970s. Although the theory was received by some as an argument against Darwin's theory of evolution by natural selection, Kimura and most evolutionary biologists today maintain that the two theories are compatible. The theory attributes a large role to genetic drift.

According to Kimura, when one compares the genomes of existing species, or looks between a species and its forebears, the vast majority of single-nucleotide differences are selectively "neutral." That is, these differences do not influence the fitness of either the species or the individuals who make up the species. As a result, the theory regards these genome features as neither subject to, nor explicable by, natural selection. This view is based in part on the genetic code, according to which sequences of three nucleotides (codons) may differ and yet encode the same amino acid (GCC and GCA both encode alanine, for example). Consequently, many potential single-nucleotide changes are in effect "silent" or "unexpressed" (see synonymous or silent substitution[?]). Such changes are presumed to have little or no biological effect.

A second assertion or hypothesis of the neutral theory is that most evolutionary change is the result of genetic drift acting on neutral alleles. A new allele arises typically through the spontaneous mutation of a single nucleotide within the sequence of a gene. In single-celled organisms, such an event immediately contributes a new allele to the population, and this allele is subject to drift. In sexually reproducing, multicellular organisms, the nucleotide substitution must arise within one of the many sex cells that an individual carries. Then only if that sex cell participates in the genesis of an embryo and offspring does the mutation contribute a new allele to the population. Neutral substitutions create new neutral alleles.

Through drift, these new alleles may become more common within the population. They may subsequently decline and disappear, or in rare cases they may become "fixed"--meaning that the substitution they carry becomes a universal feature of the population or species. When an allele carrying one of these new substitutions becomes fixed, the effect is to add a substitution to the sequence of the previously fixed allele. In this way, neutral substitutions tend to accumulate, and genomes tend to evolve.

According to the mathematics of drift, when looking between two species or two isolated populations, most of their single-nucleotide differences can be assumed to have accumulated at the same rate as individuals with mutations are born. This latter rate, it has been argued, is predictable from the error rate of the enzymes that carry out DNA replication--enzymes that have been well studied and are highly conserved[?] across all species. Thus, the neutral theory is the foundation of the molecular clock technique, which evolutionary molecular biologists[?] use to measure how much time has passed since species diverged from a common ancestor. While the mutation rate is no longer considered a constant, diverse and more sophisticated clock techniques have emerged.

Many molecular biologists and population geneticists, besides Kimura, contributed to the development of the neutral theory, which may be viewed as an offshoot of the modern synthesis.

Table of contents

The "neutralist-selectionist" debate

A heated debate arose on the initial publication of Kimura's theory, in which discussion largely revolved around the relative percentages of alleles that are "neutral" versus "non-neutral" in any given genome. Contrary to the perception of many onlookers, the debate was not about whether or not natural selection acts at all. The neutralist-selectionist debate has largely subsided, even though the question of the relative percentages remains.

As of the early 2000s, the neutral theory is widely used as a "null model" for so-called null hypothesis testing. In such a test, the number of differences that exist between two actual sequences is compared to the number that the neutral theory predicts, given the assumption of a certain elapsed time since the species diverged (typically measured from fossil records or historical records in the case of shorter time intervals). Such tests are used to detect the influence or the absence of influence of selection on an allele. Thus they contribute to the ongoing investigation into the extent to which molecular evolution is neutral.

See also

External references

  • Motoo Kimura Neutral theory of molecular evolution Cambridge University Press, 1983 ISBN 0-521-23109-4
  • Richard Lewontin The Genetic Basis of Evolutionary Change Columbia University Press, 1974 ISBN 0-231-03392-3

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