1. School of Animal & Microbial Sciences, University of Reading, Whiteknights, PO Box 228, Reading, RG6 2AJ, U.K.
2. Department of Applied Statistics, University of Reading, Reading RG6 2FN, U.K.
3. Department of Genetics, University of Georgia, Athens, GA 30602‐7223, U.S.A.
4. Computer Services Centre, University of Reading, Reading, RG6 2AF, U.K.
5. Department of Zoology, University of Oxford, Oxford, 0X1 3PS, U.K.