Punctuated equilibrium in an evolving bacterial population

Recently, Lenski et al have carried out an experiment on bacterial evolution. Their findings support the theory of punctuated equilibrium in biological evolution. We show that the M=2 Bak-Sneppen model can explain some of the experimental results in a qualitative manner. There are two major schools of thought in evolutionary biology: gradualism which implies continuous evolutionary changes and the theory of punctuated equilibrium (PE) which states that evolutionary activity occurs in bursts. Long periods of ‘stasis’ are followed by short periods of rapid changes. Recent exhaustive studies of fossil beds lend support to the second theory [1]. In a remarkable but controversial experiment, Lenski et al [2, 3] studied an evolving bacterial population for approximately 10,000 generations. They inoculated a flask of low sugar broth with a dollop of bacteria. At the end of a day, a bit of the bacterial broth was siphoned into a fresh flask of food to keep the cells growing and dividing. Every 15 days, a sample bacterial population was frozen for later analysis. After four years, data for 10,000 generations were available. Lenski et al found evidence of PE when they measured the average cell size every 100 generations. The relative fitness, a measure of the increase in the growth rate of the descendant population over that of the ancestral population, also increases in a step-like manner. Further, the average cell size and the mean fitness appear to be correlated. Lenski et al have interpreted the experimental results as suggesting that natural selection of rare beneficial mutations may be responsible for the punctuated growths seen in their experiments. During the periods of stasis, the bacteria are in a sense waiting for the beneficial mutations to occur. The rate of evolution is limited by the availability of such mutations. The bacterial