Also known as the neutral theory of protein evolution or the neutral mutation-genetic drift theory. According to Motoo Kimura, one of the originators of the theory in the late 1960s, most of evolution at the molecular level is adaptively neutral, based on the finding of widespread protein polymorphisms in natural populations. Much of this polymorphism appears to be without phenotypical significance. Since it is neither useful nor harmful, it is considered to be the product of neutral mutations, a kind of ‚aanoise‚aa in the genetic system generated by spontaneous errors in DNA replication and giving rise to ‚aaDNA junk‚aa. Consequently, a lineage might evolve molecularly, while the phenotype of descendants remains unchanged. In effect, the theory is a null hypothesis implying an absence of systematic effects for natural selection in creating characters of adaptive significance. Having reduced the significance of natural selection (necessity) and accentuated the role of accidental changes (chance), it is not surprising that the neutralist theory has been declared non-Darwinian evolution. If natural selection is largely unimportant in evolution, what is significant according to the theory? One process that is emphasised is random genetic drift. The theory claims that neutral alleles underlie most of the genetic diversity in natural populations, and that they are fixated or eliminated by genetic drift. Emphasizing as it does random non-adaptive processes such as genetic drift as well as gene flow and neutral mutations at the expense of natural selection coupled with adaptation, the neutralist theory has led to a division of opinion between selectionists and neutralists. The selectionist riposte to the theory is that the ‚aright‚aa genes are not being taken into account. If they were, the theory would be redundant. It is somewhat surprising that the selectionists object to the neutralist theory given that both Darwin and Alfred Russel Wallace (1823-1913) admitted a place for non-adaptive processes in their respective theories of natural selection. And for this reason, it is also surprising that the neutralists should depict their theory as non-Darwinian evolution. What we have, in effect, is a divide between two different levels of evolution, below and at the Darwinian (i.e., individual) level, and it may be the case that they are in some way decoupled from each other. In fact, the neutralists readily admit that at the Darwinian level and above (i.e., the group level), the evolution of characters and traits is largely governed by natural selection, but they treat it as being negligible at the molecular level. A step in the direction of trying to achieve a meaningful synthesis of the selectionist and neutralist standpoints was taken with the advent of a second-generation neutralist theory of protein evolution proposed by Tomoko Ohta, and termed the mutation-equilibrium theory. This theory is based on the idea of a continuum of mutations from those that have neither advantageous nor detrimental effects to those on which selection can act. While initially criticised for not really being a selectionist compromise but more an extension of the neutralist‚was original position, the subsequent development of this theory has led to a focus on alleles that are ‚aanearly neutral‚aa rather than being strictly neutral. It is has now become the main theory for explaining evolutionary change at the molecular level. If anything, what these neutralist theories suggest is that when mutations arise they may be ‚aastored away‚aa in neutral form (or kept in abeyance by canalisation), and then ‚aareleased‚aa under the auspices of natural selection at a later stage in evolution to meet new adaptive demands. Put another way, neutral mutations may create a healthy stockpile of genetic redundancy that provides a very flexible insurance policy against the vagaries of the ‘evolutionary game’.
See Adaptation, Allele, Biological evolution, Canalization, DNA, Genetic drift, Gene flow, Lineage, Mutation (biology), Polymorphism, Theory of balanced polymorphisms