11 May, 2011
Can evolutionary adaptations be reversed? It’s a question that’s intrigued scientists since the publication of The Origin of Species. In the late 19th century, paleontologist Louis Dollo argued that evolution could not retrace its steps to reverse complex adaptations — a hypothesis known as Dollo's law of irreversibility.
Teams investigating the hypothesis have found various parts of the jigsaw, but putting them together has proved difficult, especially as some seemed to conflict.
“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,” summed up Anne Trafton of MIT.
Will altering the question get us somewhere closer to an answer? 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.
Hi and his students combined a computational model with experiments on the mutations in bacteria which confer resistance to certain key antibiotics. 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”.
Gore says his team's results offer support for Dollo's law, but with some qualifications.
"It's not that complex adaptations can never be reversed," he says. "It's that complex adaptations are harder to reverse, but in a sense that you can quantify."
The study may also go some way to explaining why humans still have an appendix, despite it being no longer needed. "You can only ever really think about evolution reversing itself if there is a cost associated with the adaptation," Gore says. "For example, with the appendix, it may just be that the cost is very small, in which case there's no selective pressure to get rid of it."
