1. Division of Biological Sciences, The University of Michigan, Ann Arbor, Michigan 48109
2. Department of Statistics, The University of Michigan, Ann Arbor, Michigan 48109
3. Instituto Nacional de Pesquisas da Amazonia, P.O. Box 478, 69.000 Manaus, Amazonas, Brasil
4. Depto. de Genetica, Faculdade de Medicina de Ribeira-o Preto, Universidad de Sao Paulo, 14.000 S.P., Brasil