Chase Nelson in Inference:

Originally proposed by Motoo Kimura, Jack King, and Thomas Jukes, the neutral theory of molecular evolution is inherently non-Darwinian. Darwinism asserts that natural selection is the driving force of evolutionary change. It is the claim of the neutral theory, on the other hand, that the majority of evolutionary change is due to chance.

Each individual in a typical mammal population has two copies of its genome in almost every cell. The exact DNA sequences they contain may differ as the result of mutations, random copying errors in which one nucleotide letter is replaced by another. Other changes can also occur, such as the deletion or duplication of larger DNA segments. The result is genetic variation, and it is estimated that, for human beings, each child acquires 100 new mutations—50 in each genome copy—that were not present in its parents’ DNA. Genetic variation means no more than spelling differences in the DNA sequences carried by different individuals in a population. When a new DNA spelling is generated, an allele is born. This alternative form of the original gene may or may not lead to changes in the organism’s physical characteristics.

Evolution involves the substitution of one allele for another in a population. Having come about by chance, a new allele becomes increasingly common, and finally replaces the old allele. An evolutionary substitution has occurred.

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