DYNAMICS OF ADAPTATION AND DIVERSIFICATION - A 10,000-GENERATION EXPERIMENT WITH BACTERIAL-POPULATIONS

We followed evolutionary change in 12 populations of Escherichia coli propagated for 10,000 generations in identical environments. Both morp hology (cell size) and fitness (measured in competition with the ances tor) evolved rapidly for the first 2000 generations or so after the po pulations were introduced into the experimental environment, but both were nearly static for the last 5000 generations. Although evolving in identical environments, the replicate populations diverged significan tly from one another in both morphology and mean fitness. The divergen ce in mean fitness was sustained and implies that the populations have approached different fitness peaks of unequal height in the adaptive landscape. Although the experimental time scale and environment were m icroevolutionary in scope, our experiments were designed to address qu estions concerning the origin as well as the fate of genetic and pheno typic novelties, the repeatability of adaptation, the diversification of lineages, and thus the causes and consequences of the uniqueness of evolutionary history. In fact, we observed several hallmarks of macro evolutionary dynamics, including periods of rapid evolution and stasis , altered functional relationships between traits, and concordance of anagenetic and cladogenetic trends. Our results support a Wrightian in terpretation, in which chance events (mutation and drift) play an impo rtant role in adaptive evolution, as do the complex genetic interactio ns that underlie the structure of organisms.