9616190 Dykhuizen Two competing species, each a winner on a single substitutable resource, may coexist when both resources are available. Will there be a sympatric divergence of the two species, each specializing on its preferred resource, or will one evolve to replace the other? Because mutation is a stochastic process, many believe that the adaptive divergence of populations is irreproducible: if the tape of life is rerun, the evolutionary outcome will be dramatically different. Drs. Dean and Dykhuizen propose to distinguish these hypotheses experimentally using replicated populations of clonal bacteria. Because the strains are almost genetically identical, the ecological question of sympatric speciation becomes congruent with the genetic question of the evolution of specialization through trade-offs. Part 1 will establish two strains of E. coli harboring two different lactose operons and competing for two alternative galactosides. This study will test the prediction, made from a knowledge of biochemistry (a bottom up approach), that the two strains will coexist only within a certain range of ratios of the two alternative galactosides. Part 2 investigates the evolutionary dynamics of coexistence. Long-term chemostat experiments (a top-down approach) will allow adaptive mutations to accumulate in each strain. These experiments are designed to address: 1) whether trade-offs at the molecular level force specialization on particular resources; and, 2) whether increasing specialization (or increasing generalization) perturbs the initial equilibrium frequencies or leads to competitive exclusion (i.e. only one strain wins).
