Darwin errou de novo: adaptações evolucionárias podem reverter

sábado, maio 14, 2011

Evolutionary Adaptations Can Be Reversed, but Rarely

ScienceDaily (May 11, 2011) — Physicists' study of evolution in bacteria shows that adaptations can be undone, but rarely. Ever since Charles Darwin proposed his theory of evolution in 1859, scientists have wondered whether evolutionary adaptations can be reversed. Answering that question has proved difficult, partly due to conflicting evidence. In 2003, scientists showed that some species of insects have gained, lost and regained wings over millions of years. But a few years later, a different team found that a protein that helps control cells' stress responses could not evolve back to its original form.

Colorized scanning electron micrograph depicted a number of Gram-negative Escherichia coli bacteria. Researchers used an experimental model system to study the evolution of a gene conferring resistance to the antibiotic cefotaxime in bacteria. (Credit: Janice Haney Carr)

Jeff Gore, assistant professor of physics at MIT, says the critical question to ask is not whether evolution is reversible, but under what circumstances it could be. "It's known that evolution can be irreversible. And we know that it's possible to reverse evolution in some cases. So what you really want to know is: What fraction of the time is evolution reversible?" he says.

By combining a computational model with experiments on the evolution of drug resistance in bacteria, Gore and his students have, for the first time, calculated the likelihood of a particular evolutionary adaptation reversing itself.

They found that a very small percentage of evolutionary adaptations in a drug-resistance gene can be reversed, but only if the adaptations involve fewer than four discrete genetic mutations. The findings will appear in the May 13 issue of the journal Physical Review Letters. Lead authors of the paper are two MIT juniors, Longzhi Tan and Stephen Serene.

Gore and his students used an experimental model system developed by researchers at Harvard University to study the evolution of a gene conferring resistance to the antibiotic cefotaxime in bacteria.

The Harvard team identified five mutations that are crucial to gaining resistance to the drug. Bacteria that have all five mutations are the most resistant, while bacteria with none are very susceptible to the drug. Susceptible bacteria can evolve toward resistance by gaining each of the five mutations, but they can't be acquired in any old order. That's because evolution can only proceed along a given path if each mutation along the way offers a survival advantage.

Scientists study these paths by creating a "fitness landscape": a diagram of possible genetic states for a particular gene, and each state's relative fitness in a given environment. There are 120 possible paths through which bacteria with zero mutations could accumulate all five, but the Harvard team found that only 18 could ever actually occur.

The MIT team built on that study by asking whether bacteria could evolve resistance to cefotaxime but then lose it if they were placed in a new environment in which resistance to the original drug hindered their ability to survive.

Genetic states that differ by only one mutation are always reversible if one state is more fit in one environment and the other is more fit in the other. The MIT researchers were able to study how the possibility of reversal decreases as the number of mutations between the two states increased.

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Phys. Rev. Lett. 106, 198102 (2011) [4 pages]
Hidden Randomness between Fitness Landscapes Limits Reverse Evolution



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Supplemental Material

Longzhi Tan*, Stephen Serene†, Hui Xiao Chao, and Jeff Gore‡
Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Received 16 August 2010; published 11 May 2011

In biological evolution, adaptations to one environment can in some cases reverse adaptations to another environment. To study this “reverse evolution” on a genotypic level, we measured the fitness of E. coli strains with each possible combination of five mutations in an antibiotic-resistance gene in two distinct antibiotic environments. While adaptations to one environment generally lower fitness in the other, we find that reverse evolution is rarely possible and falls as the complexity of adaptations increases, suggesting a probabilistic, molecular form of Dollo’s law.

© 2011 American Physical Society

DOI: 10.1103/PhysRevLett.106.198102
PACS: 87.23.Kg, 87.14.ej, 87.18.Vf, 87.23.Cc

*Corresponding author. tantan@mit.edu

†Corresponding author. sserene@mit.edu

‡Corresponding author. gore@mit.edu


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