Study shows evolutionary adaptations can be reversed, but rarely

May 11, 2011 by Anne Trafton
Graphic: Patrick Gillooly

(PhysOrg.com) -- 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.

Jeff Gore, assistant professor of physics at MIT, says the critical question to ask is not whether 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 with experiments on the evolution of in bacteria, Gore and his students have, for the first time, calculated the likelihood of a particular 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 . 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 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.

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

Daniel Weinreich, assistant professor of biology at Brown University, says the study's most important contribution is its analysis of the reversibility between every possible intermediate state in the fitness landscape.

"What Jeff has done is show that there's another layer of mathematical complexity that enters when you ask questions about reversing environmental pressure," says Weinreich, who was not involved in this research.

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. 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 also helps explain why organs no longer needed, such as the human appendix, do not readily disappear. "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."

In a follow-up study, the researchers are looking at how the rate of environmental change affects the reversibility of evolution. In the study, they assumed an immediate switch between two environments, but they believe that more gradual changes might alter the rate of reversal.

Explore further: Human faces are so variable because we evolved to look unique

Related Stories

How bacteria evolve into superbugs

Jul 27, 2007

Researchers at McGill and Oxford Universities have applied ecological and evolutionary theory to demonstrate how bacteria become resistant to antibiotics in hospitals.

Recommended for you

Life on Earth still favours evolution over creationism

Sep 11, 2014

Charles Darwin's Theory of Evolution, first published in 1859, offered a bold new explanation for how animals and plants diversified and still serves as the foundation underpinning all medical and biological ...

Non-dominant hand vital to the evolution of the thumb

Sep 10, 2014

New research from biological anthropologists at the University of Kent has shown that the use of the non-dominant hand was likely to have played a vital role in the evolution of modern human hand morphology.

User comments : 18

Adjust slider to filter visible comments by rank

Display comments: newest first

RobertKarlStonjek
not rated yet May 12, 2011
...reversible..."
...I want my tail back!!! ...wagging my finger just isn't the same...
DavidMcC
not rated yet May 12, 2011
What Dollo must have meant is that the genome can't retrace its steps, but sometimes, natural selection favours convergence with a previous phenotype, at least for one trait or other. This phenotype invariably occurs with a different combination of genes, because beneficial mutations, occuring at random, cannot be expected to retrace their steps.
pauljpease
not rated yet May 12, 2011
I happen to know Jeff Gore, we went to grad school together. Way to go Jeff!
Johannes414
1 / 5 (6) May 12, 2011
Evolution is not science, because it cannot be observed or tested in a controlled environment. Darwinian evolution is at best so called historical science, a speculation about what might have happened in the distant past, where no one was there to observe how some slime grew legs. It should be classified in the faith section along with UFO's, big bang and other pseudo-science.
pauljpease
4.8 / 5 (4) May 12, 2011
Evolution is not science, because it cannot be observed or tested in a controlled environment. Darwinian evolution is at best so called historical science, a speculation about what might have happened in the distant past, where no one was there to observe how some slime grew legs. It should be classified in the faith section along with UFO's, big bang and other pseudo-science.


I doubt there's any physical evidence that your great-great-great-great-great-great-great-great-grandmother existed, but somehow we know that she did. Weird. I do agree that evolution is an historical SCIENCE like you said though, science being the operative word.
kaasinees
5 / 5 (4) May 12, 2011
Evolution is not science, because it cannot be observed or tested in a controlled environment.


Wrong, microbiologist do it all the time.
Also people that work with animals with a fast reproduction rate can tell you otherwise.
Not to mention the decades long russian study.

Some species are VERY sensitive to their environment , some species are able to evolve faster than others, but the evidence is there.

As for a controlled environment, we can make changes in the environment and see how species react... i think that is enough research to prove that evolution exists (together with historical science).
DavidMcC
5 / 5 (3) May 13, 2011
I see Johannes is "at it" again, posting his set piece in different threads on the subject of evolution. Question is, do we just ignore him, like the shoe adverts?

Paulipease, evolutionary science is similar in many ways to forensic science, in which you look for clues as to "who dunnit", only, in evolution, it's more "what dunnit".
Ethelred
5 / 5 (2) May 13, 2011
Evolution is not science, because it cannot be observed or tested in a controlled environment.
Gee two lies in one sentence. Evolution is a science that studies the history of life on Earth AND it can tested in labs. Indeed more than one Creationist has demanded lab tests and then whined that the lab tests had humans involved because it took place in a lab. I suspect that YOU are one of those whiners.

Darwinian evolution is at best so called historical science,
Telling the same lie twice doesn't make it true. YOU have been here long enough to have seen many lab and field experiments that showed evolution occurring. And yes since February is enough to have seen more than one.

a speculation about what might have happened in the distant past
Reasoned interpretations interpolating between sets of fossils is not mere speculation.

where no one was there to observe how some slime grew legs.
No one ever claimed that slime grew legs.>>
Ethelred
5 / 5 (2) May 13, 2011
Although slime molds do grow structures. Of course no was there to see the Earth created 6,000 years or so ago. Oh, there were LOTS of people around but they didn't see a world being created because it already 4.5 billion years old.

It should be classified in the faith section along with
So we should go on YOUR faith in a disproved book with a Flood that never happened and vegetarian dinosaurs with 9 inch serrated teeth. I have this suspicion that what you think is real is simply pure fantasy written by ignorant men a couple thousand years ago or so.

By the way when was that Flood? Where is the evidence that the world is young. Both should obvious if it was true. Surely you have some evidence to support you beliefs. Something besides a book that can't even get the first two chapters to agree on when men were created.

Do you have the guts to answer or are you just another hit and run Creationist without the courage to discuss this?

Ethelred
Johannes414
1 / 5 (3) May 13, 2011
I challenge any evolutionist: give me the very best scientific observation by scientists of Darwinian evolution in the past 150 years.

My greatgreatgreatgrandmother indeed lived. 17th century church records show it. But no sign of the so called missing link yet. Your theory lacks proof.
DavidMcC
5 / 5 (2) May 13, 2011
TalkOrigins has quite a nice summary @ http://www.talkor...omdesc/, Johannes. I'm sure you'll ignore it, though.
BTW, there is no such thing as "proof" of a hypothesis in science. There is only relatively strong evidence. In the case of evolution, this evidence is so strong that no non-religious scientists, and even few religious ones challenge it.
You can, of course, prove that you are naive, as you have done.
Johannes414
1 / 5 (2) May 13, 2011
"BTW, there is no such thing as "proof" of a hypothesis in science"

Yes there is. The theory of gravity has been proven because space ships have been observed to fall back to earth and the apple always drops down, not up.

You may disagree on the validity of certain evidence for evolution, but you are not at liberty to determine what evidence actually is.
DavidMcC
5 / 5 (2) May 13, 2011
Johannes, of course the law of gravity is well enough established to be accepted as fact. However, the current philosophy of science does not allow for absolute proof. One reason is that the range of validity of a law can remain unclear. For example, it was only a few years ago that it was shown to work at distances less than a mm.
Javinator
5 / 5 (3) May 13, 2011
Gravity is not a law, but a theory supported by observation.

My greatgreatgreatgrandmother indeed lived. 17th century church records show it.


Keep adding "greats" until there are no more records and you might get the point that was being made.

But no sign of the so called missing link yet. Your theory lacks proof.


So because the exact path from ape to human is unknown right now, the whole demonstrable process that is evolution is invalidated?

Please use logic in your arguments.
Ethelred
5 / 5 (2) May 15, 2011
I challenge any evolutionist: give me the very best scientific observation by scientists of Darwinian evolution in the past 150 years.
You did that before and were given plenty of evidence. So go read the stuff you ignored the last time.

Megatons of fossils showing evolution occurring over hundreds of millions of years is more than good enough of course.

You could read this site, as the one we are on as it has had ample evidence for evolution since you arrived.

Or this site
http://www.talkor...ogy.html
Or this
http://pandasthumb.org/
Or this
http://humanorigins.si.edu/
Or the Dover trial record
http://www.aclupa...ipts.htm
Or this
http://afarensis....olution/
Or this massive paleobiology database, something that could NOT exist without evolution as there would be no paleobiology
http://paleodb.or...ridge.pl

More
Ethelred
5 / 5 (2) May 15, 2011
More blatant human evolution
http://www.archae...cies.htm]http://www.archae...cies.htm[/url]
The reality of biology in the form of cladistics
http://www.archae...cies.htm]http://www.archae...cies.htm[/url]

All against your NOTHING that supports you in anyway. Clearly you are aware of that massive lack of data since you consistently refuse to support yourself or answer questions.

Nevertheless just when was that Great Flood anyway?

More
Ethelred
5 / 5 (2) May 15, 2011
The theory of gravity has been proven because space ships have been observed to fall back to earth and the apple always drops down
That is not proof of a theory. It is evidence IF DONE CORRECTLY in support of a theory. That things fall is FACT. Why and the details of how is theory. Evolution is a fact as can be seen in the fossil record and lab experiment but there are theories of how it occurs. All modern theories of evolution are are related to Darwin and Wallace's theory of Natural Selection. Any study of genetics quickly leads to the conclusion that evolution by natural selection is something that MUST occur and cannot not happen since both mutations and selection do happen.

Now its your turn to answer questions. How about a smidgen of proof instead of another rant. And I will post this exact same post on every thread where you post questions based on ignorance but refuse to debate or try to answer the more than reasonable questions that I and others have asked.

Ethelred
DavidMcC
not rated yet May 16, 2011
Javinator, actually, Newton only propounded a power law of gravity, one which has been confirmed to hold from distances of <0.1mm up to astronomical values (how great these are depends on whether you accept dark matter as real). Thus, it was never a theory as such, which would have to explain the attraction, rather than just predict its value.