In 2003, 15 years and 33,000 generations after Richard Lenski had set up the
long term evolution experiment, one lineage of
E.coli began to show very high
turbidity. Work done by postdoctoral researcher Christina Borland showed that this elevated turbidity was due not to
contamination, and confirmed that aerobic growth on citrate had spontaneously evolved in the population. Blount went on to conduct a series of "evolutionary replay experiments" in which he replayed the evolution of the population in which aerobic citrate usage evolved from different clonal
genotypes isolated from different time points to examine how the propensity to mutate to Cit+ changed. This analysis involved examination of several trillion
bacterial
cells, and showed that clones from later generations had significantly increased rates of mutation to Cit+, indicating that evolution of the trait in the population was contingent upon a history in which "potentiating" mutations accumulated that made it mutationally accessible. Later genomic analysis of clones from the population established the immediate
genetic and
biochemical basis of the Cit+ trait to be a tandem duplication mutation that placed a silent citrate transporter gene under the control of a new promoter that caused its expression when oxygen is present, an example of promoter capture and gene
exaptation. Based on these findings, Blount proposed a model for the evolution of
complex traits: 1. potentiation, in which mutations that make the trait evolutionarily accessible accumulate, 2. actualization, in which a mutation produces a phenotypic switch in which the new trait is manifested, and 3. refinement, in which natural selection accumulates subsequent mutations that make the new trait more effective. Currently, Blount is further investigating how complex traits evolve, how novel traits impact ecology, and how
speciation occurs in
asexual populations based on models proposed by Fredrick Cohan of
Wesleyan University. == References ==