Relative effects of selection and drift. Genetic drift and selection are two evolutionary processes that have received much attention over the years and have generated much controversy. A long-standing argument in evolutionary biology focuses on the relative strength of drift vs. selection. My students and I work on this controversy in several different ways. First, we have examined patterns of genetic variation at allozyme and molecular DNA loci in relation to environmental and physiological variation in both natural populations of salamanders and laboratory populations of mice to test whether such patterns fit expected patterns of evolution under drift (Carter 1997; Carter et al., 1999; Carter et al., 2000; Morgan et al., 2003). Second, we have used the model system of selected mice, described above, to test for the effects of genetic drift on multiple phenotypes, even as selection is occurring (Morgan et al., 2005). Several broad answers have emerged. First, some allozyme variation clearly shows patterns of variation inconsistent with drift, at least under some conditions; DNA marker variation, however, is consistent with drift. Second, drift is a very potent influence on trait values, even in populations under direct selection. These results paint a much more complex picture than is generally acknowledged in traditional arguments; both selection and drift are demonstrably important, occur concurrently, and have significant effects simultaneously.