The mean of the distribution is negative, and the variance is dependent upon the environmental diversity. Some facts relating to the theory are reviewed. Not only nucleotide substitutions but also duplications, deletions and gene conversions have indelible marks on gene evolution. Gene duplications and conversions are sources of the evolution of gene families of various kinds from those of uniform members to those with diverse functions. Also evolution of new functions is due to gene duplication followed by differentiation.
Positive selection is needed for the evolution of novel functions.
Examples of current gene families suggest again that both drift and selection are at work.