The meaning of near-neutrality at coding and non-coding regions

The nature of weak selection differs between coding and non-coding regions. Coding regions contain genetic information, whereas most non-coding regions do not have any information. Genetic information may be regarded as interaction systems, and the NK model of Kauffman was analysed. This model assumes that each amino acid makes a fitness contribution that depends on the amino acid and on K other amino acids among the N that make the protein. Through simulations, it was found that there are numerous nearly-neutral mutations under this model. Therefore, evolution is rapid in small populations, and slow in large populations. The variance of the evolutionary rate is not quite as large as data indicate under the model, and additional factors, such as environmental change or population-size fluctuation need to be considered. Weak selection at non-coding regions may come from chromosome organization, and may be regional in character, which differs from that at coding regions. The problem of genetic load is thought to disappear in these circumstances. (C) 1997 Elsevier Science B.V.