Evolutionary Adaptations Can Be Reversed, but Rarely:
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.
Here are the primary results:
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.
The paper is not online yet. I’ll definitely check it out when it comes out, at least if the math isn’t too difficult. The reason is that these sorts of models always rely on simplifying assumptions, and they may not have insights which are robust to the chaotic nature of reality. This is hinted at in the last section of the press release:
In a follow-up study, the researchers are looking at how the rate of environmental change affects the reversibility of evolution. 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.
Razib Khan’s degrees are in biochemistry and biology. He has blogged about genetics since 2002, previously worked in software development, is an Unz Foundation Junior Fellow and lives in the western US. He loves habaneros.
May 12th, 2011 at 5:22 am
well, there’s DDT resistance in mosquitos…that reversed following cessation of pesticide application. Although I’m not sure how many mut’s are involved in any given case of resistance…there are several possible, but not sure how many tend to all occur at the same time in a given population/individual.
May 12th, 2011 at 9:39 am
I think that blind cave vertebrates have been shown capable of switching eyesight on and off over long periods of time. IIRC the vision genes were still there, but had been overridden by another gene which could be reversed.
May 12th, 2011 at 10:00 am
My first red flag was at “physicists.” Physicists who work on biology quickly learn to avoid generalizing rules from a small number of observations (which often works in physics and never works in biology). Maybe these guys haven’t had that epiphany yet.
May 12th, 2011 at 11:58 am
#3, the publication in physicals letters makes me wonder. i’ll check the paper out.
May 12th, 2011 at 1:06 pm
Speaking of evolution’s gears reversing, but on a more meta level, I’ve always wondered why land animals evolve to be ocean-dwelling so frequently in the fossil record, but ocean animals have only colonized land a couple of times. Is there been a good explanation for this that the biologists who read this blog can point to?
My intuition tells me that it would have to do with competition for ecological niche or lack thereof, but it’s not clear to me why the oceans have so many ecological niches available for imperfectly adapted air-breathing animals to fill.
John Emerson,
here you go: http://news.nationalgeographic.com/news/2008/01/080108-cave-fish.html
May 12th, 2011 at 2:47 pm
Haven’t read the article, but my first impression from the verbiage is that much may lie in the exact meaning of “reversing” an adapted trait.
If it simply means that it’s unlikely that the exact same DNA sequence (and/or protein) will be used after “reversal” as before, then I can see how that might be a rare event. But there’s more than one way to skin a cat, and a trait that’s useful once again can, I should think, readily evolve, even if the underlying DNA and/or proteins involved are not identical. After all, why should evolution care whether the DNA and/or proteins are the same as before? It cares only about function.
May 12th, 2011 at 3:21 pm
It’s only generalizing qualitatively if you move from “”complex adaptations are harder to reverse, but in a sense that you can quantify”* (surely a valid quantitative generalization) to ‘sufficiently complex adaptations will never occur’.
* Based on “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.” Not zero, not never.
@ Rob:
The physicist’s view would be that original adaptation to land is more difficult, and that re-adaptation is facilitated by what remains of earlier functionality.
As for niches, I’m sure these are a factor too. Isn’t most niches concentrated to surface waters? But I’ll leave that for biologists.
May 12th, 2011 at 6:26 pm
re mosquitoes and DDT, there were always mosquitoes that never developed resistance. When the pressure was gone, these may have been better adapted to a DDT-free environment, and recolonized from the edges. I expect the same to occur with bacteria and antibiotics, if we ever get to the point where we stop using the ones that attack the same weaknesses, eventually that resistance may be lost, if keeping it has a cost.
May 13th, 2011 at 3:44 am
The big simplifying assumption is that none of the original alleles exist in the population. If even a single copy still exists – it might be statistically irrelevant – but it’s definitely not evolutionarily irrelevant!
May 13th, 2011 at 2:34 pm
Rob: I don’t think the switch from water-breathing to air-breathing has happened often in either direction, but air-breathing animals have a lot easier time moving to water-dwelling than the reverse.
May 13th, 2011 at 3:26 pm
To add to Tom Bri’s comment, mosquitos would become susceptible to DDT again by having the resistance bred out of them, not through reversing a mutation. Just as Africans brought to America lost the sickle-cell allele. Bacteria don’t have two copies of every gene, so they would have to go through reversal, although David Boxenhorn has a point.
May 13th, 2011 at 3:30 pm
Going from living in air to living in water happens more often because air breathers can hold their breath. Water breathers would be too burdened by the weight of water to carry with them, especially because of the low oxygen density in that water. Not to mention water breathers are already built not to carry their own weight. Whereas a land-dweller going into water is relieved of that burden.
May 16th, 2011 at 1:37 pm
I’m not actually aware of any lineage that went from obligate water breathing to obligate air breathing (losing water breathing in the process), and then lost air breathing while re-evolving water breathing. I know of a few examples where some secondary water breathing adaptions appeared, but air breathing remained the primary method of obtaining oxygen.
But it should be noted that for complex macro-traits, like terrestriality, or wings, there’s more than one genetic way to skin the cat, so to speak. Evolution does not have to go wholly in reverse. The second time around somewhat different genetic mechanisms might be co-opted. One could think about it as actually examples of convergent evolution happening in the same lineage, separated in time. (So for example, within the lineage that led from Urbilateria to Orca, a torpedo shaped body for swimming with fins for stabilization and direction control evolved more than once, but each time it was put together differently).
This would be different from a genetic re-evolution, wherein the same set of specific mutations is reversed.