February 20, 2013
Evolution Of Bacteria Has Become Predictable
Brett Smith for redOrbit.com — Your Universe Online
The principles of evolution are widely accepted, but the causes and mechanisms that drive evolution are still heavily debated.
In a new study published in the open access journal PLOS Biology, two researchers found similar or identical genetic mutations can emerge in separate populations of E. coli evolving in different environments for over 1000 generations, leading the team to conclude that evolution can be fairly predictable.
Matthew Herron, a researcher at the University of Montana, and Michael Doebeli, a professor at the University of British Columbia, looked at three different populations of bacteria in their experiment. Initially, each E. coli population consisted of generalists competing for two different food sources: glucose and acetate. After 1,200 generations, each population had evolved into two specialized coexisting types with a physiology adapted to one of the food sources.
The researchers were able to sequence each population of bacteria at 16 different points during the course of its evolution. The analysis found a significant amount of similarity in the bacteria´s evolution.
"In all three populations it seems to be more or less the same core set of genes that are causing the two phenotypes that we see," Herron said. "In a few cases, it's even the exact same genetic change."
The analysis was possible because of new advances in sequencing technology and it allowed Herron and Doebeli to analyze large numbers of whole bacterial genomes. While any evolutionary process is a combination of predictable and random processes, the same genetic changes in different populations showed selection can be deterministic.
"Not only did similar genetic changes occur, but the temporal sequence in which the changes occur over evolutionary time was also similar between the different evolving populations,” Doebeli said. “This 'parallelism' implies that diversification is a deterministic process driven by natural selection."
"There are about 4.5 million nucleotides in the E. coli genome," Herron said. "Finding in four cases that the exact same change had happened independently in different populations was intriguing."
The scientists said a particular form of selection -- negative frequency dependence -- was important for driving diversification in their experiment. When the bacteria specialized for the consumption of either glucose or acetate, a higher density of one type translates into fewer available resources for that type, driving the specialization of bacteria toward the alternative resource.
"We think it's likely that some kind of negative frequency dependence–some kind of rare type advantage–is important in many cases of diversification, especially when there's no geographic isolation," Herron said.
In their report, Herron and Doebeli said the genetic analysis closely mirrored the evolutionary dynamics found using mathematical models “of adaptive diversification due to frequency-dependent ecological interactions.”
With additional advancement in sequencing technology, the scientists said they believe future experiments will soon be possible on larger organisms. While some plants and animals are known to diversify their populations without geographic isolation, it is unknown whether or not the evolutionary mechanisms driving these processes are similar to those found in the E. coli population.