Understanding how and why diversification occurs is
important for understanding why there are so many species on Earth. In a new
study published in the open access journal PLOS Biology, researchers show that
similar—or even identical—mutations can occur during diversification in
completely separate populations of E-coli evolving in different environments
over more than 1 000 generations. Evolution, therefore, can be surprisingly
The experiment, conducted by Matthew Herron, research
assistant professor at the University of Montana, and Professor Michael Doebeli
of the University of British Columbia, involved three different populations of
How the study was
At the start of the experiment, each population consisted of
generalists competing for two different sources of dietary carbon (glucose and
acetate), but after 1 200 generations they had evolved into two coexisting
types each with a specialised physiology adapted to one of the carbon sources.
Herron and Doebeli were able to sequence the genomes of populations of bacteria
frozen at 16 different points during their evolution, and discovered a
surprising amount of similarity in their 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
Recent advances in sequencing technology allowed Herron and
Doebeli to sequence large numbers of whole bacterial genomes and provide
evidence that there is predictability in evolutionary diversity. Any
evolutionary process is some combination of predictable and unpredictable
processes with random mutations, but seeing the same genetic changes in
different populations showed that selection can be deterministic.
"There are about 4.5 million nucleotides in the E-coli
genome," he said. "Finding in four cases that the exact same change
had happened independently in different populations was intriguing."
A rare advantage
Herron and Doebeli argue that a particular form of
selection—negative frequency dependence—plays an important role in driving
diversification. When bacteria are either glucose specialists or acetate
specialists, a higher density of one type will mean fewer resources for that
type, so bacteria specialising on the alternative resource will be at an
"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.
As technology advances, Herron believes that similar
experiments in larger organisms will soon be possible. Some examples of
diversification without geographic isolation are known in plants and animals,
but it remains to be seen whether or not the underlying evolutionary processes
are similar to those in bacteria.