1. Department of Genetics, University of Georgia, Athens, GA 30602, USA
2. Department of Zoology, Oregon State University, Corvallis, OR 97331, USA
3. Department of Zoology, Norwegian University of Science and Technology, N-7491 Trondheim, Norway
4. Department of Animal Ecology, Uppsala University, Norbyvagen 18d, S–752 36 Uppsala, Sweden