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DEFINING. "E. coli cells cannot grow on citrate under oxic conditions, and that inability has long been viewed as a defining characteristic of this important, diverse, and widespread species." This is to be exploited in an experiment of unicellular evolution. http://www.pnas.org/content/105/23/7899.full View in LinkedIn

RANDOM VS DETERMINISTIC EVOLUTION. "At its core, evolution involves a profound tension between random and deterministic processes. Natural selection works systematically to adapt populations to their prevailing environments. However, selection requires heritable variation generated by random mutation, and even beneficial mutations may be lost by random drift. Moreover, random and deterministic processes become intertwined over time such that future alternatives may be contingent on the prior history of an evolving population." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

HISTORICAL CONTINGENCY. "More generally, we suggest that historical contingency is especially important when it facilitates the evolution of key innovations that are not easily evolved by gradual, cumulative selection." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

INDEPENDENT CONFIRMATION. "Other findings suggest that E. coli has the potential to evolve a Cit+ phenotype. Hall reported the only documented case of a spontaneous Cit+ mutant in E. coli." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

WHY SO SLOW? "A citrate-using (Cit+) variant finally evolved in one population by 31,500 generations, causing an increase in population size and diversity. The long-delayed and unique evolution of this function might indicate the involvement of some extremely rare mutation. Alternately, it may involve an ordinary mutation, but one whose physical occurrence or phenotypic expression is contingent on prior mutations in that population." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

SUDDEN APPEARANCE OF CITRATE EVOLUTION. "Here we report that a Cit+ variant finally evolved in one population by 31,500 generations, and its descendants later rose to numerical dominance." For 30,000 generations there were no citrate metabolizes evolving in this defined population. http://www.pnas.org/content/105/23/7899.full View in LinkedIn

NO CITRATE EVOLUTION. "Despite this potential, none of the 12 LTEE populations evolved the capacity to use the citrate that was present in their environment for over 30,000 generations. During that time, each population experienced billions of mutations, far more than the number of possible point mutations in the ≈4.6-million-bp genome. It must be difficult, therefore, to evolve the Cit+ phenotype, despite the ecological opportunity." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

CHANGE OF ENVIRONMENT. "In media containing other carbon sources, such as maltose or lactose, the variation in performance is much greater. And while the same genes often harbor substitutions, the precise location and details of the mutations almost always differ between the populations." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

DIVERGENT MUTANTS. "At the same time, there has also been some divergence between populations. Four have evolved defects in DNA repair, causing mutator phenotypes. There is subtle, but significant, between-population variation in mean fitness in the glucose-limited medium in which they evolved." http://www.pnas.org/content/105/23/7899.full View in LinkedIn

GENE SUBSTITUTIONS. "At least three genes have substitutions in all 12 populations, and several others have substitutions in many populations, even though most loci harbor no substitutions in any of them." http://www.pnas.org/content/105/23/7899.full View in LinkedIn