One Petri dish. Two bacterial strains competing to take it over. Given 500 to 1500 generations to evolve, which will prevail?
By staging survival battles that pitted clones of Escherichia coli against one another, researchers have now shown that the ultimate victors are seldom the early pacesetters, who owe their success to specific gene mutations.
Instead, it鈥檚 the 鈥減lodders鈥� that eventually prevail, mainly because, unlike the early front runners, they remain able to acquire the modest but valuable mutations that are ultimately vital for survival and domination.
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These less dramatic mutations give them the edge in the end because they play to the strengths of the underlying genome as a whole. The 鈥渇lashy鈥� mutations that confer dramatic early success don鈥檛 mesh as well with the entire genome as the slower-emerging ones.
To the death
The battles were staged by a team led by of Michigan State University in East Lansing and of the University of Texas at Austin.
Famously, Lenski studies evolution through experiments on 贰.听肠辞濒颈 bacteria that he has grown in his lab for more than 50,000 generations. In 2008, his team published new proof for evolution: by around generation 31,500, 贰.听肠辞濒颈 previously reliant on glucose alone evolved the capacity to consume citrate as well.
This time, he and Barrick selected four distinct pairs of clones and pitted each pair against one another.
The front-runner strains, which began to take over the Petri dish early on, developed beneficial mutations improving efficiency of topA, a gene that transcribes many other genes. They also developed mutations in rbs, a gene that improves production of DNA and RNA.
But in the end, as in the fable of the tortoise and the hare, these mutations proved to be their downfall because they ended up having an evolutionary 鈥渟leep鈥�: their plodding adversaries carried on evolving much less dramatic but ultimately pivotal mutations that gave them the edge.
The one and the whole
鈥淭hese are very impressive, systematic experiments showing that the course of evolution depends crucially on later mutations,鈥� says , who studies the mechanics of evolution at Stanford University in California.
Fisher says that remarkably little is known about the dynamics of evolution, and the study helps to answer questions about whether selection happens mainly through dramatic mutations in single genes or steady evolution of whole genomes.
Fisher says that the evolutionary pre-eminence of the whole genome over the effect of individual genes in bacteria has implications for human genetic studies. It may help explain why gene researchers often struggle in vain to try to identify individual genes linked with strongly heritable traits.
of Emory University in Atlanta, Georgia, says the experiments demonstrate that 鈥渆volvability鈥� is the crucial factor for long-term survival.
Journal reference: , DOI: 10.1126/science.1198914
