Mechanisms of molecular evolution

Both drift and selection are important for nucleotide substitutions in evol ution. The nearly neutral theory was developed to clarify the effects of th ese processes. In this article, the nearly neutral theory is presented with special reference to the nature of weak selection. The mean selection coef ficient is negative, and the variance is dependent on the environmental div ersity Some facts relating to the theory are reviewed. As well as nucleotid e substitutions, illegitimate recombination events such as duplications, de letions and gene conversions leave indelible marks on molecular evolution. Gene duplication and conversion are sources of the evolution of new gene fu nctions. Positive selection is necessary for the evolution of novel functio ns. However, many examples of current gene families suggest that both drift and selection are at work on their evolution.