In , scientists showed that some species of insects have gained, lost and regained wings over millions of years. 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. So what you really want to know is: What fraction of the time is evolution reversible?
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. Evolving resistance 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.
There are 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.
In the Physical Review Letters study, they assumed an immediate switch between two environments, but they believe that more gradual changes might alter the rate of reversal. It was completely non-functional.
Here's what they suggest is behind the phenomenon: As the ancient protein evolved , five other mutations made subtle changes in the protein's structure that were incompatible with the primordial form.
He added, "The restrictive mutations in the GR glucocorticoid receptor prevented evolutionary reversal in the same way. This same restrictive process might not occur over shorter time scales, as Rose has found in his research. Too many generations have elapsed since the ancestors of the Cetaceans had functional gills as adults. Thornton hopes to study the reversibility of evolution in other proteins. Jeanna is the editor-in-chief of Live Science. The work is part of the ongoing effort to understand the paradox of altruistic behaviour in the wild, explains Fisher, a research fellow in McMaster's Department of Psychology, Neuroscience and Behaviour.
Fisher goes on to show that another way evolution can go backwards is through the evolution of an individual's negative effects on neighbours and group members. For example, a fast-growing tree may take all the sunlight, water and nutrients out of the environment, causing its neighbours to grow slowly. In the next generation, more trees are fast-growing but are also nasty neighbours. As a result, negative social effects are much more prevalent, and so everyone's growth is reduced.
Even though growing quickly is beneficial, because of these negative social effects, the population, on average, grows more slowly," he says. Fisher plans to travel to Ecuador this summer to study co-operative spiders, and whether changes in individual and group benefits can explain why co-operation diminishes at higher elevations. Materials provided by McMaster University. Note: Content may be edited for style and length. Science News.
0コメント