Species facing widespread and rapid environmental changes can sometimes evolve quickly enough to dodge the extinction bullet.

Studies have shown that the more gradual the change, the better the chances for 'evolutionary rescue', that process of mutations occurring fast enough to allow a population to avoid extinction in changing environments. One obvious reason is that more individuals remain alive when change is gradual or moderate, meaning there are more opportunities for a winning mutation to emerge. 

Biologists using populations of microorganisms have shed light on a second reason. They found that the mutation that wins the race in the harshest environment is often dependent on a "relay team" of other mutations that came before, mutations that emerge only as conditions worsen at gradual and moderate rates. Populations of disease-causing bacteria evolve as doctors flood their environment, the human body, with antibiotics. Insects, animals and plants can make evolutionary adaptations in response to pesticides, heavy metals and overfishing, but without the winners from those first "legs" of the survival race, it's unlikely there will even be a runner in the anchor position when conditions become extreme.

Unless a species can relocate or its members already have a bit of flexibility to alter their behavior or physiology, the only option is to evolve or die in the face of challenging environmental conditions, said lead author Haley Lindsey. 

Tiny wells, each about the size of an eraser on the end of a pencil, hold individual populations of E. coli either evolving or succumbing to different levels of an antibiotic which has a red-orange hue. Credit: S. Hammarlund/U of Washington

The species studied was Escherichia coli, or E. coli, a bacterium commonly found in the lower intestine and harmless except for certain strains that cause food-poisoning sickness and death in humans. The researchers evolved hundreds of populations of E.coli under environments made ever more stressful by the addition of an antibiotic that cripples and kills the bacterium. The antibiotic was ramped up at gradual, moderate and rapid rates.

Mutations at known genes confer protection to the drug. Researchers examined these genes in surviving populations from gradual- and moderate-rate environments, and found multiple mutations.

Using genetic engineering, the scientists pulled out each mutation to see what protectiveness it provided on its own. They found some were only advantageous at the lower concentration of the drug and unable to save the population at the highest concentrations. But those mutations "predispose the lineage to gain other mutations that allow it to escape extinction at high stress," the 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," said Benjamin Kerr, University of Washington assistant professor of biology. For populations in that situation, there were only single mutations that gave protection against the antibiotic. 

"The rate of environmental deterioration can qualitatively affect evolutionary trajectories," the authors wrote. "In our system, we find that rapid environmental change closes off paths that are accessible under gradual change."

Citation: 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