Jeff Gore, study leader and assistant professor of physics at MIT, and a team of researchers, have worked to figure out what the likelihood of reversing an evolutionary adaptation would be in bacteria in certain environments.

Charles Darwin, an English naturalist, proposed his theory of evolution in 1859 in the book, "Origin of Species," which theorizes that all species of life have descended over time from common ancestry, resulting from natural selection. 

Darwin's ideas raised a lot of questions, such as whether evolutionary adaptations can be undone. In 2003, a study showed that some insects have gained the ability to fly through the use of their wings, then lost it, then regained it again over millions of years. But then a study conducted a few years later contradicted this finding through the discovery of a certain protein, which aids in the control of stress responses and cannot evolve to its original form. 

In response to these contradictory findings, Gore set out to find whether evolutionary reversal was possible, and if so, under what circumstances and what fraction of the time.  

Gore built his research upon a previous study conducted by Harvard University scientists, which determined that five mutations that are vital for bacteria to gain resistance to an antibiotic called cefotaxime. With all five mutations, bacteria are most resistant. With less than five or none, bacteria become more susceptible. But for those with no mutations, bacteria can gain resistance by obtaining each of the five mutations.  

To obtain these mutations, evolution must "proceed along a given path if each mutation along the way offers a survival advantage." These paths are studied by scientists through fitness landscapes, which are diagrams of possible genetic states for genes as well as their relative fitness in a certain environment. For bacteria with no mutations at all, there are 120 possible paths, and findings showed that only 18 ever obtain all five mutations.

Now, Gore has used this information to see whether bacteria could become resistant to cefotaxime and then lose it if placed in a new environment. 

To do this, Gore first looked at the genetic states. If genetic states differ by only one mutation, they are always reversible if one state is more fit in a certain environment and the other state is more fit in another environment. With this knowledge, the MIT team used computational models and a series of experiments to figure out how often and under what conditions two states' rate of reversal decreased as the number of mutations between them increase. 

According to Gore, complex adaptations can be reversed, but they're just more difficult to reverse. In fact, the study showed that a very small percentage of these evolutionary adaptations can be reversed if they have four or more mutations. 

"This is the first case where anyone's been able to say anything about how reversibility behaves as a function of distance," said Gore. "What we see in our system is that once the system gets four mutations, it's unable to get back to where it started."

Gore was able to show the reversibility between "every possibly intermediate state" in the researchers' fitness landscape, which not only shows how likely it is for an evolutionary adaptation to be reversed in certain environments, but also explains why unneeded organs like the appendix do not disappear. 

"You can only ever really think about evolution reversing itself if there is a cost associated with the adaptation," said Gore. "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."  

This study was published in Physical Review Letters.