Homogenous test tube populations of E. coli were fed easy-to-digest glucose and harder-to-stomach acetate. In each test tube, 2 groups emerged, each specializing in 1 of the 2 food sources.
Then they gained the ability to better switch between meal types. This last adaptation would have not been useful until the specialization had emerged, which helped exhaust food supplies faster. The different bacterial populations genetically came about their adaptations in the same way and in the same order.
The results provide empirical evidence of adaptive diversification as a predictable evolutionary process. ~ Matthew Herron & Michael Doebeli