Hmmm... Really interesting question! Problem is: it cannot evolve to zero unless it is not zero. If mutation rate slows down to near zero, evolution will also slow down, and the probability of a "beneficial" mutation of "no mutation" will be astronomically low. It reminds me of that story dating back to the time when everybody's wishes came true... until... until one man's wish was: "I wish that nobody's wish comes true!"

It's really interesting you're bringing this up now. It was a heavily debated question last month in the ICiS workshop http://www.icis.anl.gov/programs/summer2011-2b . The debate was led by Gary Olsen and Rob Edwards questioning why there are so many species in each habitat, and why the dominant species do not take over. "Kill the winner" was discussed. The definition of species was debated. Many ecological models were brought up. Alcohol was heavily consumed. After all this, the most satisfactory conclusion was that *IT Happens*. Mutations happen. Horizontal transfer happens. My question is: How many's too many?