Brian Charlesworth1, Russell Lande2, Montgomery Slatkin3. 1. Population Biology Group,-School of Biological Sciences, University of Sussex, Falmer, Brighton, BN1 9QG, England. 2. Department of Biophysics and Theoretical Biology, University of Chicago, Chicago, Illinois, 60637. 3. Department of Zoology, University of Washington, Seattle, Washington, 98195.