MICROORGANISMS are well suited to experimental studies of evolution. They have such short generation times that observations of evolutionary change over thousands of generations are possible not only within the lifetime of a scientist, but even within the duration of a grant. Such experiments often give rise to surprising and unexpected results, as demonstrated by the report of Bennett, Dao and Lenski on page 79 of this issue'. The authors adapted a population of Escherichia coli to growth at 37 oc in a minimal medium by serially transferring the population for 2,000 generations. During this time, the population became well adapted to its environment. From this population a single cell was isolated. The clone derived from the cell was split into six experimental and six control populations, the control populations then being grown in the same environment as before-minimal medium at 37 oc. The experimental populations were grown in the same way and in the same medium, but at