1. Department of Biology, Faculty of Sciences, Kyushu UniversityFukuoka 812-8581, Japan
2. Program for Evolutionary Dynamics, Harvard UniversityCambridge, MA 02138, USA
3. Department of Organismic and Evolutionary Biology, Harvard UniversityCambridge, MA 02138, USA
4. Department of Mathematics, Harvard UniversityCambridge, MA 02138, USA