A theory proposed in 1983 by the Japanese geneticist Motoo Kimura, which asserts that many genetic mutations are adaptively equivalent (effectively neutral), and do not affect significantly the fitness of the carrier. Thus they can become fixed in the genome at a random rate. Changes in their frequencies are more often the result of chance than of natural selection. The theory applies only to protein evolution and does not deny the role of natural selection in shaping morphological and behavioural attributes, but it is critical of the role of selection in maintaining polymorphism, which it regards as a transient phase of molecular evolution resulting from a balance between mutational input and random extinction by drift. Kimura made important contributions to the understanding of evolutionary change by constructing mathematical models based on it.
http://www.nature.com/scitable/topicpage/Neutral-Theory-The-Null-Hypothesis-of-Molecular-839. The theory explained in more detail.
Subjects: Ecology and Conservation.