PERIODIC SELECTION AND HITCHHIKING IN A BACTERIAL POPULATION

The accumulation of a neutral marker in a bacterial population under b alanced growth in a chemostat follows a jagged curve as adaptive varia nts continuously appear and sweep the population. Such periodic select ion curves are simulated in the present work using deterministic equat ions that, in contrast to previous models, take full account of the st ochastic character of the process. The uncertainties due to the appear ance time, the survival probability, and the extent of early growth at small numbers are included as stochastic initial conditions for every new variant-adaptive or neutral-that appears. The model is used to ca lculate the substitution rate via hitchhiking where a neutral or weakl y selected mutation is carried along when a new adaptive one takes ove r the population. The expected ratio for the probabilities of the pres ence or absence of a weakly selected or counterselected mutation in th e population is also calculated. This can be related to the standard r esult without hitchhiking if the average time between adaptive shifts is interpreted as an effective population size.