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2.1.5. Molecular Evolution and the Neutral Theory
Molecular evolution emerged as a scientific field in the 1960s as researchers from molecular biology, evolutionary biology and population genetics sought to understand recent discoveries on the structure and function of nucleic acids and protein. It is the process of evolution at the scale of DNA, RNA, and proteins. Depending on the relative importance assigned to the various forces, three perspectives provide evolutionary explanations for molecular evolution, . While recognizing the importance of random drift for silent mutations, selectionists hypotheses argue that balancing and positive selection are the driving forces of molecular evolution. Theses hypotheses are often based on the broader view called panselectionism, the idea that selection is the only force strong enough to explain evolution, relaying random drift and mutations to minor roles. Mutationists hypotheses emphasize random drift and biases in mutation patterns, . Neutralists hypotheses emphasize the importance of mutation, purifying selection and random genetic drift, . Kimura suggested that speciation is not due to selection of advantageous genotypes but elimination of deleterious alleles and random selection of neutral alleles. It emphasized that mutations are of neutral or nearly neutral value and genetic drift is responsible for divergence. The main assumption of Kimura's theory is that the mutations at the molecular level (amino- and nuclear-acid substitutions) are mostly neutral or slightly disadvantageous. The introduction of the neutral theory leads to a fierce debate about the relevance of neo-darwinism at the molecular level, [10-11].If molecular substitutions are neutral, then why is progressive evolution possible? To answer this question, Kimura uses the concept of gene duplication and says that gene duplications create unnecessary surplus DNA sequences, which in turn drift further because of random mutations, providing the raw material for a creation of new, biologically significant genes.