Natural Limitation to Evolutionary Processes (2/14/05)

quote:

---

As for mutation, Mark24 suggested that mutation increases evolution potential, balancing out the effect of the loss of genetic variability through these processes, and claimed that mutation occurs at an amazingly rapid rate. I need to look into that, but for the moment I would just consider the possibility that beneficial mutations are occurring in all populations. What happens then? Either the population absorbs the new characteristics, homogenizes and remains relatively stable over time as in the case of immigration of new genetic material, or it is subjected to "processes of evolution" that reduce its genetic variability as in all the above cases.

---

I think you need to check your reasoning on this point.If you accept for the sake of argument that new variation is entering the population through mutation then what happens depends on the rate at which new variations appear against the rate at which variation is lost through selection. Only if the rate at which variation is lost is greater than the rate at which new variation enters the population will there be a net decrease in variation.

No, I don't agree with everything you said - I CERTAINLY don't agree that genetic drift reduces variation (quite the opposite). Nor do I agree that the cheetah represents a normal outcome (the cheetah suffered a SEVERE genetic bottleneck - a population reduction so bad that extinction would be the normal outcome plus lesser bottlenecks trhough more recent hunting). I focussed on the one point I did since it was crucial to your argument and needed to be rethought.Let us also be clear that I am not necessarily proposing a high rate of beneficial mutations. The rate of mutation requied only needs to be igh enough to maintain variation and neutral mutations will do - so long as they have a phenotypic effect. In neutral mutations that remain neutral or neutral mutations that become deleterious - or even mildly deleterious mutations will do, since their removal still accounts for part of the loss.The real problem here however is that you are not addressing my point. Your whole argument depends on the assumption that the rate at which variation is lost must exceed th rate at which variation is introduced by mutation. You've offered no reason to beleive that the former rate is very high (although you claim that it is) or that the rates cannot balance. So I ask you again to reconsider your claim - and if you cannot support it accept that your argument lacks a foundation.

Genetic drift is NOT a selective process. So what is "absolutely known" about selective processes simply does not apply.And you're simply wrong that genetic bottlenecks represent a form of selection. A bottleneck is caused by a redution in the population to a low number - whether it is selective or not. The YEC view of Noah's Flood represents a massive bottleneck for all affected species.And it is absolutely daft to insist that species start out with a huge amount of potential for variation. More likely they start out with comparatively little variation and variation accumulates as the population expands.And this is also badly wrong:

quote:

---

If variability were being introduced at any observable rate whatever, there would never be a genetically stable population

---

This essentially denies that genetic drift happens at all ! Alternatively you could consider the alternative that stabilising selection generally removes new variations INSTEAD of existing variability.BTW perhaps you would like to clarify what you mean by "pre-existing potentials". It appears to refer to the variations already present - which would be a clear misrepresentation of my point since I expressly refer to mutation as adding new variations.

Genetic drift (aka the Neutral Theory) was originally proposed to explain why there is more genetic change than could be reasonably explained by selection.Genetic drift relies on the chance spread of neutral mutations. Statistically it is inevitable that some will become fixed in the population, explaining the observed genetic differences.However it implies an overall increase in variation compared to a scenario without Genetic drift. The process is as follows:1) A neutral mutation appears, entering the gene pool
2) Some of these will be eliminated quickly, but others will spread to a significant proportion of the population
3) A proportion of these will eventually become fixed.So at any one time there will be a number of neutral mutations present in a significant portion of the population. Some of these will eventually become fixed and others will not. However their very presence itself represents a pool of variation. And since the whole theory requires that this pool of variation is being constantly topped up it directly contradicts your idea that the net rate of change must be an inexorable decrease in variation.

Yes. The evaluation of a mutation as beneficial, neutral or deleterious is relative to the environment. I did not raise the issue mainly because it was an unnecessary complication. Also some mutations are almost always neutral (e.g. silent mutations) and some are almost always detrimental (e.g. cystic fibrosis is detrimental because it is often fatal at a young age - and it is only because it is recessive that it is not severely detrimental).

Well you started in post 1 by stating that you intended to SHOW that there was a definite limit to evolution.We now find that the heart of your argument is the assumption that relevant mutations do not happen. But as yet we see no support for such a claim - and your call of a "time out" suggests that at present you don't have a case on this point.On the other hand evolution has always assumed a continual supply of new variations (Chapter V of Darwin's Origin deals with this issue). So your claim to the contrary is simply untrue.So ultimately all you ahve done is assume that the new variations required by evolutionary theory do not appear and your whole argument essentially boils down to this assumption.

quote:

---

You're going to have to show the math on that one. You could just as easily claim that the variability caused by mutation will always outweigh the variability removed by selection, and that therefore variability will always increase. (Which appears to have been the case during the history of life on Earth.)

---

Actually we CAN argue that given a stable situation the degree of variation is likely to remain constant on average.Assuming a fixed population size, and ignoring alleles that are not passed on even once (for BOTH rates) :1) The number of new variations entering the population will be constant on average.2) The number of variations removed from the population will depend on the number of variations within the population - the lower the number of alleles the lower the rate of removal (drift should be directly proportional, while selection can only work against alleles that are present in the population)3) Therefore the situation will tend to reach a dynamic equilibrium because a net removal of alleles produces negative feedback on the rate of removal, while the rate of increase is unaffected.The only way to refute this argument is to show that the rate of new variations entering the population is too low for variation to recover from selective events.

Genetic drift is a general term for non-selective change in allele frequencies. It includes things like bottlenecks (where the cause is NOT selective) and the founder effect (which needn't be due to "destruction" at all - the classic example is colonising a new habitat, like an island). But drift doesn't need a reduction in population - statistics guarantees that some degree of drift will happen whatever the populations does.As for mutation let me stress the reason that it is referred to as only one of the mechanisms of evolution is because it acts only as a source of variation for selection (and drift). I find it odd that you weren't aware of that - or even aware that scientists recognised that there was a need for a source of new variation.On the other hand I haven't noticed anyone claiming that there is an improbable "great rate" of mutation - just a rate high enough to compensate for losses (which as I have pointed out can be qute low, due to feedback effects). So it seems that your claim relies on an implicit assumption that there is a "great" rate of loss of variation - but I haven't seen any support for that claim.As to your talk about "alleleic variation" the term as I understand it refers to variations already present. If that is what you meant you've already seen examples refuting that as an explanation of all variations.THe fundamental problem of your argument is that it relies on comparing two rates but estimates neither. Rather it just assumes that one rate must be "great" without any support or without any estimation of the actual rate (or even a theoretical look at the issue of negative feedback leading to dynamic equilibrium which I raised). Yet this assumption is the core of the argument. At this stage your argument is not so much refuted rather, it is shown to be an unsupported assumption

The "founder effect" specifically refers to the founding of a new population. Migration is more general - IIRC you yourself used it to refer to new alleles being brought into an existing population.The Noachic Flood would be a severe bottleneck for the affected species since (depending on the reading) even "clean" species are reduced to not more than 14 individuals and "unclean" species to 2.The difference between selection and non-selection is in fact very important in evolutionary theory. And as I pointed out the slower pace typical of drift actually leads to a consequence of greater variation than selective change would. So even the effects are different altough it requires some analysis to see that.Drift requires some variation to be present and therefore requires mutation to replace lost variation. Aside from that there is no specific cause. As I said it's a general term and ordinary statistical fluctuations are included. The "mitochondrial Eve" is largely a consequence of drift (i.e. matrilineal lines are interrupted if a woman has no daughters - and this is largely a chance effect at the appropriate level of analysis).You are however correct to note that evolution requires a source of new variations - and as I have pointed out Darwin devoted a chapter of his work to dealing with the issue (he was largely wrong but not unreasonably so given the state of scientific knowledge at the time).But you are badly wrong to insist that there must be a huge number of mutations based on the number of processes reducing variation and even more wrong to insist that rate is unimportant. In this discussion everything BUT the rates is peripheral. Either the rate at which new variation enters a populations is sufficient to balance out the loss or - as you claim - it isn't. That's it. And I don't see how you can make your point without dealing with actual rates - your only hope would be to make a theoretical argument along the liens of the one I made earlier pointing to equilibrium as the expected state, but I don't think that such an argument is available to you.You're even wrong to claim that the Hardy-Weinberg equation always points to declines in variation - it can also point to equilibrium situations (as it does in the case of sickle-cell anaemia).Finally I think you've missed the point that the experiments with bacteria started with a clonal population. Any genetic variation has to be the product of mutation. That it occurred before the selective influence was added doesn't help your position at all - because your position is that that could not happen.

As I've pointed out the rate of mutation is almost certainly in a dynamic equilibrium with the rate at which alleles are lost. The rate of mutation is typically constant for a given population size while the rate at which alleles are lost is subject to negative feedback.To the best of my knowledge there is no evidence that the rate of loss is so high as to force the level of variation down to near zero. On the face of it even the examples discussed so far indicate that this is false - clonal bacteria cultures are able to gain new variation, and the cheetah's - as well as being an unusual case - have also recovered a degree of variation (this is how we can estimate the date of the severe bottleneck).

quote:

---

Well, I do believe the Flood explains the facts, certainly far better than the Geologic Time Table does, but my main argument is that the Geologic Time Table is such a silly idea on the face of it, just looking at the strata it supposedly explains, it should embarrass scientists to take it seriously. No need for all the minute investigations of particular bits of data as the basic picture is already ridiculous. Don't know if I care to slog through all that again, but we'll see.

---

The real situation is exactly the reverse. The Flood as an explanation for geology is very, very silly which is why geology abandoned it quite early. Mainstream geological views - while incomplete - are far more sensible.

Yes, Geology abandoned a "wrong view" of the flood - the idea that it accounted for an appreciable part of the Geological record.And quite frankly before condemning modern geological views you should actually learn what they are, instead of beating up on straw men.

quote:

---

That's not how it is presented on umpteen biology websites and evolution glossaries.

---

Please support this claim.

Doing a quick Google search to locate a few sources I have so far found none that insist on evolution consisting of selection without mutation:http://www.fossilmuseum.net/...m%20Scientific%20Glossary.htm http://www.stanfordalumni.org/...says/Natural_Selection.html Evolution and Natural Selection http://anthro.palomar.edu/synthetic/Default.htmhas a section on Mutation and the section on Natural Selection also mentions mutation as the source of an allele used in an example

Well, defining evolution as "change in the frequency of alleles" is hardly restricting evolution to natural selection.As to the talkorigins.org definition essay (http://www.talkorigins.org/faqs/evolution-definition.html ) it says rather moreThe Futuyama definition includes the following The author's expansion of the "change in allele frequency" definition also refers to mutation: and also states But why refer solely to the definition essay - when we have a more complete treatment expressly labelled as an "Introduction to Evolutionary Biology" ? http://www.talkorigins.org/faqs/faq-intro-to-biology.html It's even first in the list of evolution FAQs (The Talk.Origins Archive: Evolution FAQs )There we find: and: Well I can certainly believe that you didn't do much research. But if you hadn't why claim to be familiar with "umpteen" sources ?