The video is generally very good, but gives the impression that we have DNA repair mechanisms that have "adapted" to natural background radiation levels.
That's really an oxymoron, since we wouldn't have evolved ("adapted") if the fidelity of our DNA was perfect.
It's not. Radiation causes mutations that help drive species evolution AND cancer evolution.
We do have good repair mechanisms, but they're not perfect.
That's why we get cancer. That's why species evolve.
Greetings, Bob!
ReplyDeleteAgain, as per our previous discussions, I take issue with the majority of these critiques, which I will hope to clarify:
First, adapting to DNA-damaging processes is not an oxymoron, nor is the process static as the critique implies. (To say we've adapted (mutated) to a mutation-causing process is only an oxymoron if one assumes a fully-competent DNA repair process spontaneously arose instead of the repair process itself being subject to evolution. The latter is the sensical view of the evolutionary process.)
Certainly early versions of the DNA-repair process (early adaptations) were less effective at DNA repair, and as mutations/evolution occurred, the repair mechanisms improved in a dynamic adaptive process, (which based on the ubiquity of ionizing radiation on Earth one imagines has been an adaptive process occurring within cells for the last ~3 billion years!).
One could easily and logically envision a scenario where an ideal organism eventually develops a perfect defense against damage-derived DNA mutation. Far from being an oxymoron or contrary to evolution, this hypothetical organism would represent a very pinnacle of evolution (with respect to a specific threat), and genetic transfer and/or mutation would simply be inspired by another mechanism.
So, the view of evolution you promote, (perhaps because I have a background as a geologist), is I would argue fairly misdirected. Evidence suggests evolution would still occur even in the complete absence of spontaneous mutations (see ontogeny). Genetic alterations are witnessed as a result of selective pressure during embryonic development that are *not* a result of ionizing-radiation-induced point mutations. Simply accelerating or delaying the introduction of an enzyme during an embryonic organism's growth, for instance, is itself enough to enforce evolution, which may be easily triggered by stresses on the parent organism (lack of food, glut of food, overactive adrenal glands, response to injury, etc.)
So as a geoscientist, I take issue with your statement that imperfect repair mechanisms are "why species evolve" - in that way using the mere existence of evolution to infer LNT, which the modern understanding of evolution does not support.
Two cents.
Cheers,
Ben
Hi Ben:
ReplyDeleteWoW! You seem to be missing the BIG picture on evolution. There are 4 fundamental factors in evolution:
1. Natural selection
2. Genetic drift
3. Gene flow
But none of those matter without the most important:
4. Mutations
Maybe you've heard the term "selfish gene". DNA is selfish. It doesn't care about you (that's why you die). It doesn't care what body type it builds (that's why there are flies, humans, fungi, etc.)
It only cares about perpetuating itself. It has "discovered" that by keeping itself inherently mutable, it can create a vast number of body types which will increase its chances of being passed on to another generation. Body types and bodies come and go.
If DNA had allowed itself to be non-mutable, then this greatly increase its chance of extinction (ie, if DNA only produced sabre-toothed tigers, there would be no DNA today).
LNT & evolution are very similar...an agent (one being radiation)can cause DNA to mutate. If the mutation happens in sex cells, the organism offspring are subject to macro-environmental natural selection processes and the species waxes or wanes depending on its environment. If the mutation happens in a tissue cell (somatic), the cellular offspring are subject to micro-environmental natural selection processes.
When the genomes of a group of organisms becomes so changed that it doesn't have reproductive success with existing others, we say a new species has evolved.
When the genome of a tissue becomes so changed that it no longer behaves like the original tissue, we say a new tumor has evolved.
It only takes about 30 eV to ionize DNA. A Cs-137 photon has about 662,000 eV. It can easily ionize a bunch of DNA either in sex cells or somatic cells, thereby contributing to the evolution of species and tumors.
This may help: http://en.wikipedia.org/wiki/Population_genetics
Bob,
ReplyDelete(First, I am remiss in not yet thanking you for welcoming me to your blog!) To the point - I appreciate the above info, but frankly, this seems to be a bit of a non-sequitur. I fail to see how your reply indicates that I've somehow missed the boat on evolution - you've explained to me in a very thoughtful way the value of mutation, yet this is something with which I don't disagree, (nor do I believe I ever did...?)
Further, I would say that your example of LNT & evolution is a bit misclassified (I would say you describe evolution & evolution, not "LNT" & evolution). The difference between a LNT and non-LNT view relates only to the rate of mutation accumulation and whether or not repair/transcription mechanisms can keep up.
At the end of the day, I don't argue the effects of point mutations, which is also something it seems you feel I have disagreed(?). The difference in our opinions with respect to LNT/non-LNT seems to only be the time and time rate that it would take for "the genome of a tissue [to become] so changed that it no longer behaves like the original tissue."
So, it seems you missed the point of my reply, which was regarding *one specific mechanism* of mutation, and further, specifically of *damage-induced mutation* - i.e., gamma-rays. (I should also add that I believe it is right to call out y-rays specifically from other forms of ionizing radiation, which at low levels are bound to express very different risk statistics with respect to one-another due to LET, intrinsic properties, etc.)
My salient point was with respect to ontogeny, which is where I feel you've really missed the big picture on evolution (if I may in turn be so bold). Selective pressure on an organism *without need for damage-induced mutations* was the apparently little-considered point I was trying to stress. So, my example was used merely to illustrate a point. If we assume that ionizing radiation is a threat to DNA propagation (which is de facto the reason that there are DNA repair mechanisms in the first place), my example was just a hypothetical point taken to an extreme. (The alternative in a blindly selfish DNA scenario would be to have no DNA repair mechanisms in order to maximize the number of available mutations. Clearly something else is afoot, and we can agree that the selfish DNA sees some benefit in consistency – the question would really be, how much?)
I was attempting to stress that ionizing radiation is not the only means to mutation, and the elimination of one mode of mutation is not contrary to evolution, as you suggest, particularly if the benefit of isolating more directed or more efficient mechanisms of adaptation (mutation) from the damage of competing methods outweighs or even just out-paces the potential benefits of preserving those competing adaptive pathways (i.e., ionizing radiation, which includes unpredictable DSBs, etc.). We see life picking these sorts of chemical pathways all the time in the paleontological record - look at aerobic versus anaerobic respiration, C3 versus C4 photosynthesis, and on. (cont’d)
(cont'd) So, having said all of that, I think it'd be helpful to take a step back:
ReplyDelete1. I claimed that we are adapted to one specific threat to DNA replication (ionizing radiation).
2. You claimed that this suggestion was contrary to evolution (and indeed that gamma ray mutations are required for it to occur).
3. I rebutted that the presence of repair mechanisms indicates that, far from being contrary to evolution, resistance to damaging mutation is a *result of* evolutionary processes. I then offered a hypothetical scenario where an organism has completely removed the threat of ionization-induced mis/non-function in favor of a more 'pure', environmentally-selected adaptive pathway, such as ontogenological stress (which we have measured to be hundreds if not thousands of times more rapid than random, ionization-induced point mutations or mistranslations after strand breaks).
4. You countered again with the point that mutations are important. I never argued the point. However, "deleterious" versus "beneficial" mutations (note: from any perspective, not preserving *human* life) really translates to "successful" versus "unsuccessful" mutations. Perhaps I should clarify, then, (and perhaps if you re-read my reply you will recognize), that my point was simply that it is not at all contrary to the concept or process of evolution to see life mitigate less successful biological processes in favor of more successful ones.
--And recent advances in the understanding of ontological evolution provide both a justification and a pathway for enhanced defense against and replacement of ionization radiation-induced mutations.
Therefore, you could view ionizing radiation as a relic evolutionary partway - one that is slowly in the process of being evolutionarily closed off/abandoned in favor of more successful (ontological) ones, which would definitely support a non-LNT view of dose-response.
(This is in contrast to, if I'm hearing you correctly, the idea that selfish DNA/cellular processes are "leaving the door cracked" so that ionizing radiation can supply a mechanism for mutation.)
Progress? (At least in clarifying positions?)
We've evolved to communicate with eye contact, facial expressions, hand gestures, etc. not by computer!
ReplyDelete1. You wrote: "Evidence suggests evolution would still occur even in the complete absence of spontaneous mutations (see ontogeny)". As my last link describes, that is NOT true. Evolution requires mutations of some sort (spontaneous, non-spontaneous doesn't matter).
2. You are correct IR is not the only means to mutation. I never said it was, I wrote: "an agent (one being radiation)" to imply there are many.
3. What you're stating about ontogeny is confusing. Ontogeny deals with the development of an individual. Evolution deals with the development of different populations.
You wrote: 'Genetic alterations are witnessed as a result of selective pressure during embryonic development that are *not* a result of ionizing-radiation-induced point mutations".
Again, I never said only radiation produces mutations. I never limited the discussion to point mutations.
But what you are calling "genetic alterations" are mutations...mutations of some sort are necessary for evolution as I've stated. (Unless you mean something else by "genetic alterations"???)
Now there are epigenetic effects too (ie, molecules outside the genome can affect how genes are switched on or off). But let's not go there!
I see you're continuing...but I have to work on something else for a bit....I hope the above helps!
OK, moving to your second post:
ReplyDelete1. We are adapted to ionizing radiation. We have imperfect DNA repair mechanisms which help ensure that we don't get cancer before our reproductive age (not everyone enjoys this, but most of us do). A species which had poor DNA repair mechanisms such that most individuals got cancer before their reproductive age would go extinct pretty quickly.
2. I never said gamma rays are required for mutations. I said mutations are required for evolution (and cancer). Gamma rays cause mutations.
3. I think I responded to this in 1.
4. Again your use of ontogeny is confusing, so I won't respond until you've had a chance to clarify.