Evolutionary biologists study the dynamics of the adaptation of organisms to their environment and the divergence of populations and species from each other. But it can be very difficult to observe evolutionary change in populations because it often happens over long periods of time. In 1988 Dr. Richard Lenski started an evolution experiment that is still running today. On February 24, 1988 12 populations of E. coli were placed in identical environments. Each population was founded by a single cell from an asexual clone, and these populations have been evolving ever since. A remarkable feature of this experimental system is that the mean fitness of a derived population can be measured relative to a clone of its ancestor. This allows scientists to measure evolutionary fitness as the relative increase in reproductive rate of the descendant compared to its ancestor. The data provided in this exercise includes measurements of mean fitness in all 12 populations over time. Two different datasets allow students to explore the first 10,000 generations, and then make predictions about the trajectory of evolutionary change over 50,000 generations. Students can then test their hypotheses - and if desired, identify the best model that describes changes in fitness over time.