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The Wilhelmine E. Key 1994 invitational Lecture. Plant genetic diversity and the struggle to measure selection.

The fundamental research program of population genetics has been to seek a quantitative assessment of the role of the various forces of evolution in shaping patterns of genetic variation. This goal has been pursued on both empirical and theoretical fronts. The introduction of biochemical and molecular techniques into population genetics more than 25 years ago revealed vast stores of genetic variation within populations. This level of genetic diversity is difficult to reconcile with balancing selection, and as a consequence, recent thinking has emphasized the role of mutation and genetic random drift as the primary determinants of genetic diversity. The resulting neutral theory of molecular evolution has dominated population genetic thought for more than 20 years. Nonadaptive theories have also emphasized the role of deleterious mutations in driving evolutionary change. New insights into the relative importance of selection and genetic random drift can now be obtained from samples of DNA sequences of genes drawn from within species. The elaboration of coalescence theory, together with the accumulation of data on gene genealogies, permits an integration over relatively long periods of evolutionary time. The ability to integrate over long periods of evolutionary time permits the detection of small selection intensities and it reveals some information about the mode of selection. When the genealogy is consistent with a neutral process, the effective population size can be estimated, as can the age of the coalescent, thus providing new empirical approaches to the estimation of these important parameters. Applications of these approaches in plant population genetics are still in their infancy, but they have already provided new insights into effective population sizes and they are beginning to illustrate how selection for domestication has affected plant genomes.