Small wells contain individual populations E. coli prospering or perishing under different levels of a red-orange colored antibiotic (Credit: S Hammarlund/U of Washington)
Rapid environmental change is a worldwide phenomenon, and in order to survive, many species must biologically respond to changing environmental conditions, such as increased heavy metals or temperatures. Logically, previous studies have demonstrated that species respond best to gradual change in environmental conditions, as potentially-beneficial mutations are given more time to arise in a greater number of individuals still alive in the population. Now, biologists at the University of Washington have demonstrated another reason: many beneficial mutations are dependent on prior mutations that may only emerge as conditions slowly worsen.
Using populations of a species of bacteria, E. coli, researchers evolved hundreds of populations under environments with a lethal antibiotic which was increased at gradual, moderate and rapid rates. Researchers then examined the genes in surviving populations and found multiple mutations. Then, using genetic engineering, the scientists isolated each mutation and found that some mutations were only able to ‘protect’ the population at low antibiotic concentrations. However, those mutants “predispose the lineage to gain other mutations that allow it to escape extinction at high stress,” the study’s authors wrote.
“That two-step path leading to the double mutant is not available if a population is immersed abruptly into the high-concentration environment,” Ben Kerr, the study’s corresponding author, said. Such populations under rapid environmental change must then rely on single mutations for protection against the antibiotic, thus environmental change can significantly impact evolutionary trajectories. In this system in particular, the authors have found that “rapid environmental change closes off paths that are accessible under gradual change.”
Read more here: Haley A. Lindsey, Jenna Gallie, Susan Taylor, Benjamin Kerr. Evolutionary rescue from extinction is contingent on a lower rate of environmental change. Nature, 2013; DOI: 10.1038/nature11879
[This is a reposted excerpt from my work in the BEACON Buzz]