NOTE:  The published version of this letter omitted several citations added in proofs.  
The corrected version of the manuscript is available as <a href="BrunoHalpern99.pdf">pdf</a> or <a href="BrunoHalpern99.ps"> in postscript</a>.  
The paper in its published form can
be obtained from <a HREF="http://www.molbiolevol.org/cgi/reprint/16/4/564.pdf"> MBE</a>.