This Is A Law Of Life: The More Complex The System, The More Risky It Is To Introduce Random Mutational Diversity

"Furthermore, if the mutation rate is high, the size of the genome in question must be small to stay within the bounds of the speed limit -- thus organisms that tend to mutate quickly are those with concise genomes, such as viruses and bacteria.

The Shakhnovich speed limit also offers an explanation for observed differences in genome sizes between organisms with genome error correction -- such as bacteria, mammals, birds, and reptiles -- and those without..." From Harvard's Science Daily at http://www.sciencedaily.com/releases/2007/10/071001172753.htm