  Recently, Lenski et al [Elena,Lenski,Travisano] have carried out several
experiments on bacterial evolution. Their findings support the theory of
punctuated equilibrium in biological evolution. They have further quantified
the relative contributions of adaptation, chance and history to bacterial
evolution. In this paper, we show that a modified $M$-trait Bak-Sneppen model
can explain many of the experimental results in a qualitative manner.
