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Author Topic:   Evolutionary momentum
Mammuthus
Member (Idle past 6476 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 3 of 33 (26725)
12-16-2002 9:25 AM
Reply to: Message 2 by [xeno]Julios
12-16-2002 8:19 AM


quote:
Originally posted by [xeno]Julios:
I guess another way of asking the question, or perhaps a related question, is whether there are any "stable" states for genes to be in. I know hardly anything about mutational hotspots, let alone the mechanisms of mutation, so I'm groping in the dark here.

++++++++++++++++
Hi XJ,
I am not sure what you are trying to get at since your example is a behavioral trait (for which the underlying genetics is currently not known) but your second post is a genetic question. There are different types of stability if I grasp what you are asking in the second post. A specific allele of a gene can become fixed in a population i.e. the sequence is identical in most individuals, by selection or genetic drift. There are also more complicated examples where heterozygosity is maintained or balancing selection occurs which is also a stable state (unless the environment changes and one allele is favored over another or a genetic bottleneck occurs and one allele is lost by chance).
An example of a mutational hotspot (or hotspots) is the mitochondrial control region or hypervariable regions I and II. There are a few conserved transcription domains in this segment but the rest is free to vary without being selected against. Coupled with an about 10 fold higher mutation rate in general in the mitochondria due to a relatively crap DNA repair system, some of the sites are mutational hotspots. They are free to vary with little or no selection against them. This is seen by the relatively high amount of variation within a species i.e. you and I are probably different in our hypervariable regions as compared to coding sequences like mitochondrial cytochrome b where you and I would likely be identical.
I am not sure if this answers your question(s)
As to the specific behavior question...schrafinator is our resident expert I believe.
cheers,
M

This message is a reply to:
 Message 2 by [xeno]Julios, posted 12-16-2002 8:19 AM [xeno]Julios has not replied

  
Mammuthus
Member (Idle past 6476 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 7 of 33 (26949)
12-17-2002 5:21 AM
Reply to: Message 6 by [xeno]Julios
12-16-2002 10:49 AM


Hi Xeno
So in a really really simplistic model - let's say that a gene exists like this:
A-A-A-A-A
now a mutation occurs:
A-A-A-A-G
this causes a small fear response to be associated with the image of a snake.
Let's say that A-A-A-G-G codes for a more intense fear
A-A-G-G-G is even more intense
and finally:
A-G-G-G-G is really intense, and also poses the best survival advantage for the host organism.
Now obviously, left to nature, if any of the subsequent mutations arise, then A-G-G-G-G will be the best genotype and will probably be the end result of the evolutionary pathway.
M: Actually, this is not necessarily the case. Evolution does not always lead to the absolute best outcome. If one phenotype has an increased chance of reproductive success over the others, it will be more successful. Thus in your scenario, if most individuals are A-A-A-A-A and the mutation A-A-A-A-G occurs, the latter could become fixed and you may never reach the optimal A-G-G-G-G.
X:
My question, is this:
All other things being equal (ie no selective pressures), will there be a tendency for the following mutations:
A-A-A-A-G ----> A-A-A-G-G
A-A-A-G-G ----> A-A-G-G-G
A-A-G-G-G ----> A-G-G-G-G
so that A-G-G-G-G would be a "mutational basin"
I'm not asking if it would definitely mutate to that state, but rather if it is possible that mutational pathways exist that are likely to occur irrespective of the survival factor - ie - this tendency would be purely a matter of chemistry on the genomic level.
M: I would say the answer is a qualified yes. Qualified because there is no directed pathway that would guarantee that A-G-G-G-G would ever appear in the population. The yes comes from the chemical bias for transitions over transversions i.e. C-T changes are more likely than C-A for chemical reasons...thus, some types of mutations are more likely than others.
X:
There may indeed be mutational pathways that exist, but my question is even more tantalizing - do such pathways exist that also happen to result in an expression of a gradually magnified phenotype as the pathway is "travelled" along.
M: You seem to be interested in neutral evolution. For example, without selective constraints, some genomes grow and grow in size because of repetitive DNA. So at some level this is your gradually magnified process.
cheers,
M

This message is a reply to:
 Message 6 by [xeno]Julios, posted 12-16-2002 10:49 AM [xeno]Julios has replied

Replies to this message:
 Message 8 by [xeno]Julios, posted 12-18-2002 3:42 AM Mammuthus has replied

  
Mammuthus
Member (Idle past 6476 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 9 of 33 (27147)
12-18-2002 4:02 AM
Reply to: Message 8 by [xeno]Julios
12-18-2002 3:42 AM


Hi Xeno,
I still don't think I get the question. Increase in size of a bird wing without selective constraints would unlikely reach a steady state. It would increase or decrease randomly within the population without consequence. A specific size might be reached and remain stable for a period of time if there were a genetic bottleneck. But if the population expanded again the trait would also begin to vary.
As to mutational load or a mutational basin, I guess you mean heterozygosity? Most diploid species have some level of heterozygostiy including recessive lethal mutations (why inbreeding is such a bad idea). But this may not be what you are getting at either...sorry to be so dense.
cheers,
M

This message is a reply to:
 Message 8 by [xeno]Julios, posted 12-18-2002 3:42 AM [xeno]Julios has replied

Replies to this message:
 Message 10 by [xeno]Julios, posted 12-18-2002 10:53 AM Mammuthus has replied

  
Mammuthus
Member (Idle past 6476 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 12 of 33 (27213)
12-18-2002 11:11 AM
Reply to: Message 10 by [xeno]Julios
12-18-2002 10:53 AM


Hi Xeno
That's why I'm stressing the factor of the mutational pathway being feasible through chemical processes alone.
M: Ok, the mutational process is by chemical processes alone in any event i.e. C to T transitions are a chemical change. There are other types of chemical mutation that do not change the DNA sequence called imprinting.
X:
If anything, the survival consequences of these mutations would reinforce the basin, but alone would not account for the stastical likelihood of these mutations becoming widespread.
M: However, if it has a survival consequence then it is going to be selected for or against and is neutral.
X:
Do you understand what I mean by a mutative basin?
M: I am not sure. I have never heard this term before.
X:
The increase in wing surface area would not spread randomly without selective constraints if there was such a neutral basin underlying the phenotype's progression.
M: However, mutations will occur at random and under neutrality will be lost or fixed in the population at random so in this model, the trait would vary randomly. You would probably end up with a very large distribution of wing surface area since no one phenotype would have an advantage over the others.
X:
That is the whole point of the basin! If such a basin existed, then it would justify the speculation that I posed at the beginning of this thread (in the quote out of the essay I was writing) about "evolutionary momentum" - momentum being the key word here, since chemical processes add directional force to the survival trend.
M: The chemical processes generate the novel mutations upon which slection will act if it is directional. Otherwise it is neutral and will be more free to vary...of course it could be in a non-recombining part of the genome etc and not mutate very much but there is nothing constraining it. There is also no intrinsic amount of variation one can expect in a given population. This will be determined by environmental stress i.e. radiation, repair enzymes, effective population size and a host of other factors. So I don't really see directional momentum without selection i.e. an intrinsic tendency for a trait to consistently change in one direction without any advantage.
cheers,
M

This message is a reply to:
 Message 10 by [xeno]Julios, posted 12-18-2002 10:53 AM [xeno]Julios has replied

Replies to this message:
 Message 14 by [xeno]Julios, posted 12-19-2002 1:44 AM Mammuthus has replied

  
Mammuthus
Member (Idle past 6476 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 16 of 33 (27326)
12-19-2002 5:16 AM
Reply to: Message 14 by [xeno]Julios
12-19-2002 1:44 AM


Hi XJ
That's coz i just made it up - here's a crude analogy that might help: Think of a bowl or a basin. Now put a marble into it and swirl the bowl for a few seconds. The ball will start to roll around the sides of the basin until it settles in a stable state, most likely at the bottom of the bowl. This is because of the way gravity, and the curve of the inner surface of the basin, interact. Similarly, by mutative basin, I mean a genetic state that will arise because of mutational hotspots (the surface of the bowl), and the laws of physics and chemistry (gravity). It has absolutely nothing to do with survival consequences. Survival consequences will determine which mutations are allowed to persist, while mutative basins will determine which mutations are likely to occur in the first place!!
M: The problem in this analogy is that gravity would be selection and is the selective force that tends to make the marble travel in a specific direction. Also the curve of the bowl would be a selective force. Quetzal did a great job of pointing out the issues of mutation so I won't go into it since I agree with what he said. I would only add that the mutation rate and amount of mutations or polymorphism in a population will depend on the rate of error of the various polymerases and the size of the effective or breeding population.
cheers,
M

This message is a reply to:
 Message 14 by [xeno]Julios, posted 12-19-2002 1:44 AM [xeno]Julios has not replied

  
Mammuthus
Member (Idle past 6476 days)
Posts: 3085
From: Munich, Germany
Joined: 08-09-2002


Message 20 of 33 (27457)
12-20-2002 4:28 AM
Reply to: Message 17 by [xeno]Julios
12-19-2002 2:37 PM


Now - think of the entire pathway as a conceptual "basin" - the stable centre of which is simply a point of probability. Thus, there is only a higher probability of the genetic state settling in that basin, as compared to any other arbitrary state.
M: However, this is contrary to what is expected. You would expect a normal distribution of mutations among individuals if it is a stochastic and random process and this is what is observed. Thus, you would not expect to settle in any one state.
X:
Forget about survival pressures for now; the whole point of this thought experiment is to conceptually dissociate the environmental pressures and the ensuing survival consequences from the mutations that occur - remember, this basin would determine the statistical likelihood of a mutation actually occuring while the environment would influence which mutations "survive" and are propogated.
M: Again, there is only counter evidence for such directional mutations so I don't think the basin model is viable.
Now, my question in part is whether these pathways could incidentally be expressed by a gradual change in a specific phenotype. By this, let me try to illustrate: imagine a mutational pathway exists, by virtue of the successive hotspots that arise after each mutation. Now, in some cases, these very mutations will be expressed in a certain way in the organism. Sometimes each of these successive mutations will express completely unrelated "traits" - for example the first mutation would code for a protein X, and the second one would code for a completely different enzyme. Or even more abstractly, mutation A could alter the eye colour, while mutation B would influence production of haemoglobin, and mutation C would sprout an extra limb!
M: Unfortunately, almost all of the "hotspots" are in non-coding sequences as those that do have an impact on a trait/protein are immediately selectively advantageous or disadvantegeous.
X:
However, I am asking whether it is feasible to postulate pathways that express a gradual change in a specific phenotype. For example: mutation A would express itself as a longer finger, mutation B would give rise to an even longer finger. And of course if this was the case, then all things being equal (for example say this set of organisms evolved in a completely (hypothetical) neutral environment) then there would be an increased statistical likelihood that over the eons, longer fingers would become the norm.
Now back to the real world: let's say that longer fingers actually was a huge survival advantage. In this scenario, we have two independent "pressures" that direct the growth and survival of longer fingers: on the one hand, the survival advantage would facilitate an increased likelihood that longer fingered organisms would survive and reproduce more, etc... on the other hand, there is the mutative basin that guides the necessary mutations. In this case, the mutational basin would give momentum to the evolution of longer fingers.
M: I could envisage a trait that is not selected on becoming fixed in a population by chance i.e. by genetic drift you end up with an overall averge small size in the population though larger sizes would be equally supportable. The environment changes suddenly and small size is a huge advantage and maintains the state or causes average size to decrease further. However, this is not a directional mutation. This is plain evolution. Chance mutations that in a given environment are either advantageous, neutral, or detrimental. The larger the population (the population could be seen as your mutation basin) the more possible mutations exist (since a diploid organism only has two copies of any given nuclear gene), and the more possible genotypes available on which selection can work. That is why genetic bottlenecks can really suck for a species. If you remove almost all the variation from a population, the environment changes and the given phenotype or few phenotypes are not suitable to the changed environment, then there are no individuals that may have a better phenotype that can survive to reproduce and extinction is a possible outcome.
Another scenario is that a highly selected trait is genetically linked to a completely neutral trait. The non-selected trait will get fixed in a particular state because of linkage with the selected trait. But this is not the same thing as what you are proposing because the non-selected trait is hitchhiking to fixation because of selection on a different trait.
But without selection I don't see your directed mutations happening as you envisage.
cheers,
M
[This message has been edited by Mammuthus, 12-20-2002]

This message is a reply to:
 Message 17 by [xeno]Julios, posted 12-19-2002 2:37 PM [xeno]Julios has replied

Replies to this message:
 Message 21 by [xeno]Julios, posted 12-20-2002 2:23 PM Mammuthus has not replied

  
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