Successive adaptive radiations have played a pivotal role in the evolution of biological diversity1,2,3. The effects of adaptive radiation are often seen4,5,6, but the underlying causes are difficult to disentangle and remain unclear7,8,9. Here we examine directly therole of ecological opportunity and competition in driving genetic diversification. We use the common aerobic bacterium Pseudomonas fluorescens10, which evolves rapidly under novel environmental conditions to generate a large repertoire of mutants11,12,13. When provided with ecological opportunity (afforded by spatial structure), identical populations diversify morphologically, but when ecological opportunity is restricted there is no such divergence. In spatially structured environments, the evolution of variant morphs follows a predictable sequence and we show that competition among the newly evolved niche-specialists maintains this variation. These results demonstrate that the elementary processes of mutation and selection alone are suifficient to promote rapid proliferation of new designs and support the theory that trade-offs in competitive ability drive adaptive radiation14,15.
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We thank K. McCallum for technical assistance; J. Baker for photography; D. Ebert, S. Kahn, B. Haubold, and E. R. Moxon for discussion; and R. E. Lenski, P. Sniegowski and I. Moore for comments ont he manuscript. This work was supported in part by grants from the B.B.S.R.C. and BTP Plc.
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