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http://www.sciencedaily.com/releases/2013/02/
130219172155.htm
How Predictable Is Evolution?
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Under a magnification of 6836x, this colorized scanning electron micrograph
(SEM) depicted a number of Gram-negative Escherichia coli bacteria of the
strain O157:H7, which is one of hundreds of strains of this bacterium. (Credit:
CDC/Janice Haney Carr)
Feb. 19, 2013 Understanding how and why diversification occurs is important
for understanding why there are so many species on Earth. In a new study
published on 19 February 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 1000 generations. Evolution, therefore,
can be surprisingly predictable.
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 3 different populations of bacteria. At the start of
the experiment, each population consisted of generalists competing for two
different sources of dietary carbon (glucose and acetate), but after 1200
generations they had evolved into two coexisting types each with a specialized
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 change."
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."
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
specializing on the alternative resource will be at an advantage.
"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.
Story Source:
The above story is reprinted from materials provided by Public Library of
Science.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal Reference:
Herron MD, Doebeli M. Parallel Evolutionary Dynamics of Adaptive
Diversification in Escherichia coli. PLOS Biol, 11(2): e1001490 DOI:
10.1371/journal.pbio.1001490
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Public Library of Science (2013, February 19). How predictable is evolution?.
ScienceDaily. Retrieved February 20, 2013, from http://www.sciencedaily.com
/releases/2013/02/130219172155.htm
Note: If no author is given, the source is cited instead.
Disclaimer: Views expressed in this article do not necessarily reflect those of
ScienceDaily or its staff.
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