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Can Genetic Variation Revert Back?

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Can Genetic Variation Revert Back?

Postby PhillipusRex » Wed Dec 30, 2009 3:46 am

I have a question regarding evolution:

I realize that external pressures can cause an organism to evolve. For instance, if a group of insects are sprayed with a pesticide, than if some of the insects survive they can pass on their genetic resistance to their offspring. If the offspring continue to be exposed to the same pesticide, and if those offspring continue mating, than after a certain point, the insects will be immune to this particular pesticide.

However, something has confused me recently. I have read, that if a situation like this occurs, than the outside pressure ( in this case, the pesticide) must remain constant in order for the genetic change to remain true. In other words, if the pesticide is removed for a long enough period of time, than the insects will revert back to their previous state. They would be vulnerable to the insecticide once again!

To me, this makes absolutely no sense. Is this correct? Can an organism pass on genetic material that is "contingent" on environmental conditions and will revert back to the previous state?

I appreciate any information you can provide.
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Postby biohazard » Wed Dec 30, 2009 9:02 am

This can indeed happen.

It is very often seen in bacteria, where continuous exposure to antibiotics forces bacteria to develop resistance towards it. If the antibiotic is removed from the environment, eventually non-resistant strains emerge. This is pretty much similar to the situation with insects, but bacteria usually develop the resistance via plasmids, which allows them to react much faster.

The reason why organisms may revert back to their previous state is quite simple: resistance requires the expression of additional enzymes or non-optimal forms of some essential proteins (which are unaffected by the drug/pesticide). This increases the metabolic load of the organism, and thus organisms carrying these mutations do not normally survive the competition with their non-mutant neighbours. However, if a drug or a pesticide wipes out all the non-mutants, the few organisms carrying a protective mutation survive.

If the pressure caused by the drug/pesticide/etc is removed, sooner or later some of the organisms mutate back to the previous state and their offspring soon overgrow the resistant, but metabolically disadvantaged mutants.

This pattern is possible with all organisms from bacteria to insects and even humans, but the faster generation time and (in bacteria) single genome make these simpler organisms much more likely to experience these changes during a relatively short time span.
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Postby PhillipusRex » Thu Dec 31, 2009 4:39 am

That is fascinating to me.

To me, it doesn't seem like it would be over exerting its metabolic load. After all, they seemed to be expressing these enzymes or proteins to begin with - since they had survived the pesticide and/or antibiotic.

Let me ask then: If genetic mutation is such a stress on the survival of an organism, how the heck did anything survive long enough to evolve to complexity???
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Postby biohazard » Thu Dec 31, 2009 12:57 pm

For bacterial antibiotic resistance the organism needs an additional plasmid, which is a considerable metabolic burden.

For mutated enzyme or other protein the mutated version does not usually function as well as the original one in its "normal" environment, thus the mutant is in disadvantageous position - until the environment changes to some that makes the normal product dysfunctional or worse than the mutated one.

And these were just two examples, the big picture is of course overwhelmingly complex, but these examples give quite a good view to this "big picture" :)
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Postby PhillipusRex » Tue Jan 05, 2010 11:03 pm

Thanks for the information biohazard.

I have to assume then, that since the variation can revert back, it must be a temporary adaptation. I suppose this would mean that it is not actually affecting the organisms genetic code (new copies of the organisms genes would not necessarily have this same variation). Or is it?

I guess my question is: When is the point at which an adaptation becomes a change in the genetic code, thereby pushing the organism into a new notch of the evolutionary continuum?
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Postby biohazard » Thu Jan 07, 2010 7:40 am

I don't think such point can be determined. Basically any sequence in a genome can change, but of course many of those changes are detrimental for the organism and get eliminated from the gene pool. The more conserved or important the gene, the less likely it is that it can "revert back" or mutate somehow and that these mutants would survive and eventually replace the original version of the organism.

There are many genes that have been virtually unchanged for millions of years (these are called evolutionary conserved genes). Examples of these are genes that take part in certain crucial functions of a cell, such as cellular respiration, stability or basic metabolism/biomolecule synthesis.

So, I would say that an adaptation becomes a permanent change when there has been a long lasting change in the environment (like emergence of atmospheric oxygen) and the organisms have developed mutations to utilize it, and these organisms depend on it so much that they cannot any more live without it. (Naturally, the atmospheric O2 was a huge change - smaller but sustained changes would do the trick as well)

In other words, a change in the genetic code is often pretty much as long-lasting as is the change that allowed it to emerge and persist in the first place. With atmospheric oxygen you can assume that the mutations depending on it are pretty much permanent, but for example cases of industrial melanism (where previously light-coloured insects become dark-coloured ones due to soot and air pollution darkening the surfaces of their environment) disappear when the air quality increases, thus being only "temporary adaptation". But a genetic change and its degree of permanence can be anything between these two extremes.

Of course, even these "permanent" mutations related to oxygen respiration would disappear if all atmospheric oxygen was lost at some point during the planet's life span. The advantages of utilizing O2 were so huge that O-dependent organisms took over almost all eological niches. However, if atmospheric O2 suddenly disappeared, there would be a mass extinction of O2-breathers and the non-O2 dependent organisms would take back what was theirs some billions of years ago. If the atmospheric O2 disappeared only very very slowly, (some of) the now O2-breathing organisms would gradually evolve back towards anaerobic respiration. Nothing in biology is everlasting...
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Postby JackBean » Thu Jan 07, 2010 6:53 pm

I don't think that metabolic enzymes are so much conserved. The other story are e.g. histones...

If the mutation occurs, than it's permanent. But that doesn't mean, that it can't be reverted back. But if is it passed through generations, than is the DNA involved for sure...
http://www.biolib.cz/en/main/

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Postby biohazard » Fri Jan 08, 2010 8:01 am

Uh okay I was a bit in haste, so I didn't have time to specify what metabolic enzymes I meant. My bad. Naturally, there are hundreds of enzymes that can be categorized as metabolic enzymes, of which many are mutated often enough. In my post I meant conserved enzymes that are essential for basic metabolical functions, such as respiration. Take cytochrome c, for example.

And now you're playing with semantics, Jack. Of course everything can be said to be permanent as long as it remains unchanged and one does not know when or if it's going to change...

About mutations being permanent or not, I think it is quite obvious that in this context we are interested in knowing whether a given mutation is likely to revert back whenever the external pressure is removed (e.g. antibiotics resistance, industrial melanism, etc.) as opposed to something that is likely to to remain unaltered (on a species-wide scope) for the next couple of millenia, or so.

Edit: Looking back to my previous post I noticed I actually did specify that I meant basic metabolsim. Oh well...
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Postby JackBean » Fri Jan 08, 2010 8:44 am

Well, yeah, with CytC you're right, but histones are still more conserved ;)

OK, that was written probably a little wrong :lol: My point was, that your (your and PhillipusRex's) posts sounded a little, like if you took these mutations not being in DNA and just after long time, they actually are 'incorporated' into DNA, so the mutation becomes permanent...

I wished to say, that all these mutations are already in DNA and the chance, they will be lost by some reversion is the same, as that they emerged before...
http://www.biolib.cz/en/main/

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Re:

Postby biohazard » Fri Jan 08, 2010 9:49 am

JackBean wrote:
OK, that was written probably a little wrong :lol: My point was, that your (your and PhillipusRex's) posts sounded a little, like if you took these mutations not being in DNA and just after long time, they actually are 'incorporated' into DNA, so the mutation becomes permanent...

I wished to say, that all these mutations are already in DNA and the chance, they will be lost by some reversion is the same, as that they emerged before...


Yep, I agree with you. Once a mutation is there it is there and in that sense it is permanent. And reverting back needs similarly a chance event to turn it again to the old version. Or to yet another new one :)
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Re: Can Genetic Variation Revert Back?

Postby jwalin » Sat Jan 09, 2010 3:10 am

okay guys i have another example an easier one.
i am talking about the tropics in this whole case
we have rabbits. rabbits are usually brown as its an adaption for thier camouflage and to stay safe. we also have white rabbits that would be killed easily by a predator.
suppose it snows (which it won't unless like something that occurred once or twice in history). then the selection pressure is towards the white rabbits. the brown ones will decline. over a period of time majority are white. now after sometime the snow melts away. then the situation is like what was earlier. after a while brown will make up for the majority.

a reveration is only possible in such cases when the selectiopn pressures change in such a manner. :roll:

@biohazard
the metabolism pressure is a good point. but i don't know and nor think it will work in this case. am not sure :twisted:
if it is true then what we could do is stop using penicillin for say a decade and then reuse it. it would so lot more effective. :P this is just an idea i don't know how feasible it is.
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Postby JackBean » Sat Jan 09, 2010 10:34 am

Well, I guess the India won't be under snow for several years in a piece ;) But you can see white animals in the north ;)
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