RARE CONVERGENCE. “It is clear that the vast majority of sequences in the genomes have domain architectures which have arisen by evolutionary descent rather than due to functional necessity. In short, convergent evolution of domain architectures is rare…we conclude that between 0.4 and 4% of sequences are involved in convergent evolution of domain architectures, and expect the actual number to be close to the lower bound.”
http://bioinformatics.oxfordjournals.org/content/21/8/1464.full