Module 7: Molecular Evolution

 

  Evolution by natural selection occurs when certain genotypes produce more offspring than other genotypes in response to the environment. It is a non-random change in allele frequencies from one generation to the next.  Charles Darwin described four necessities for evolution by natural selection to occur which are; the trait under selection must be variable in the population so that the encoding gene has more than one variant or allele, the trait under selection must be heritable, encoded by a gene or genes, the struggle of existence that many more offspring are born than can survive in the environment, and that individuals with different alleles have differential survival and reproduction that is governed by the fitness of the organism to its environment. Evolution by natural selection occurs when the environment exerts a pressure on a population so that only some phenotypes survive and reproduce successfully. The stronger the selective pressure or the selection event the fewer individuals make it through the sieve of natural selection.

  The neutral theory of molecular evolution put ahead by Motoo Kimura contends that at the molecular level, most evolutionary changes and polymorphisms within species are not caused by natural selection, but by random genetic drift. This differs from Darwin’s claim that natural selection is the main process that brings about evolution. Kimura compared the amino acid sequences of hemoglobin α and cytochrome c in several mammalian species and found that the number of mutant substitutions was too large to be tolerable within the theory of natural selection if the substitution number was extrapolated to the total genome. The neutral theory puts more prominence on the average rate at which species’ genome accumulates mutations used to measure their evolutionary divergence, the molecular clock. Here the case for the neutral theory was the obvious disconnection between molecular and phenotypic changes. Another important observation for the neutral theory was the inverse relationship between the importance of a protein and its rate of evolution. Natural selection could account for protein polymorphism whereas, the neutral theory of molecular evolution claims that most the molecular-genetic diversity is non-selective.

  Some similarities in Kimura’s theory of neutral evolution and the theory of evolution by natural selection are, the neutral theory applies only for evolution at the molecular level and is compatible with phenotypic evolution being shaped by natural selection as postulated by Darwin. The neutral theory also allows for the possibility that most mutations are deleterious but holds that because these are rapidly removed by natural selection, they do not make significant contributions to variation within and between species at the molecular level.

Sources

Neutral Theory of Molecular Evolution - an overview | ScienceDirect Topics