| Citation | Distinguished for his studies of natural selection and evolution in natural plant populations. His early work on the genetics of tolerance to heavy metals exposed the relative roles of selection and gene flow in determining the differentiation of local populations and local tolerances to extreme environments. He has made important contributions to understanding the forces that determine the evolution of plant life cycles. His studies of the demography of natural plant populations have involved the deliberate perturbation of density and frequency in the field and reciprocal transplantation of plants between communities. These experiments have given new insights into the role of plant competition in natural selection. This work has lead directly to very subtle field and laboratory experiments to test theories of the evolutionary significance of sexual reproduction, especially the roles of frequency and density dependent selection and the part played by pests and diseases. |