Hi,

i was wondering wether evolution was really a linear path,but occured by sudden cataclysmic events over a period of time? which were noticable,like a meteor strike etc.

What do you mean by "linear path"?

For example, monkeys, cats and human beings all share a common ancestor, but humans did not evolve from monkeys, and they didn't evolve from cats, either.

i mean in terms of graduality, was evolution phenomenon not exponential due to some cataclysmic events occuring over a period of time?

I would say that evolution is in general gradual, but major events can cull a large part of and variation within species. It can have a huge influence on the nature of the collective biosphere or local biosphere, but does it shape new form? I would say that gradual evolution works more efficiently in this area.

Maybe,...

There's a lot of debate. Evolution/descent with modification/survival of the fittest happens continuously of course. There is a theory of "punctuated equilibrium" [google] that argues that major evolutionary changes only occur now and then when there are drastic environmental changes causing mass extinctions. We- the human race- seem to be one of those changes, so, if we survive, we may see proof of the theory.

We have punctuated equilibrium because species are stable in large populations for long periods. Evolution is an adjustment to pressures that make a specie less than ideal for its environment. The correlary is that if the specie is well adapted then there's no reason for it to evolve. The specie changes when its environment changes.

Often that change is continuous, like a predatory cat getting bigger and faster while its lunch does the same. However, it may also be that the species are stable as they currently are. In that case you con't get any change for a while, and then when you do, both may change relatively rapidly.

i mean in terms of graduality, was evolution phenomenon not exponential due to some cataclysmic events occuring over a period of time?As usual, I arrive late for the party. Anyway, here's my bottle.

This is an excellent question, with no simple answer. Others have told you that sudden cataclysmic environmental change can somehow lead to a corresponding "cataclysmic" evolutionary change. It is not at all obvious how this can be.
We assume that mutation rates are not, in general, affected by environmental change. Let's assume a uniform rate of mutation of...er... 4 x 10^-9 per generation. Let's also assume that a species is perfectly adapted to its ecological niche (that's bold!), then mutations will be detrimental or neutral. Providing the ecological niche reamains the same, the detrimental mutations will be quickly removed by selection, and if they are neutral, some may become "fixed" (by a process I can describe if you're interested) but most won't.
The point I'm trying to make is that it is very difficult to see how a resevoir of diversity can stably exist in a population of such a form and in such a way that, given cataclysmic environmental change, there will be a genetic pool capable of exploiting it.

Nevertheless, as others have said, punctuated equilibria seem to to be the way evolution works (think of a staircase, but don't take the "upness" of it literally!). There are models, I have one which I expounded here not long ago. But all the evidence, fossil and other, says that this is the way it goes.

So, the answer to your question is that, there may have been periods of gradual evolutionary change, mostly these changes have been rather abrupt.

could you please ellaborate a little more for better grasp of the same...?

thank you.

could you please ellaborate a little more for better grasp of the same...?

thank you.Well, I'll try, but it might be better if you just asked questions.
Let's start with a hypothetical population perfectly adapted to its ecological niche, and let's say that niche is unchanging at the moment. We must assume that new mutations are occurring at a certain rate. As our population is perfectly adapted, these mutations cannot improve fitness in this niche - they must either reduce it or be neutral. Those mutations which reduce fitness will be removed from the population by selection. The question is, how do the neutral mutations become fixed. Classical theory says they are removed by dilution except where they are subject to a process called "fixation by random genetic drift". Ask if you want that explained.

Now let there be a sudden change to our population's niche. In its present form, our population is ill-adapted to its new environment. So where are the individual members who carry the mutations which will allow them to survive in their new circumstances? We have explicitly eliminated all those except the minority that was fixed by drift. It is not at all obvious, under classical theory, that this process is sufficient to allow the accumulation of enough genetic variation for a sub-population to survive sudden and dramatic evironmental change.

I hope that at least explains the problem.

OK, so could it be possible that as the world got smaller and more people of more cultures became more in contact with each other, that humans as a whole made greater strides in our own evolution?

It is not at all obvious, under classical theory, that this process is sufficient to allow the accumulation of enough genetic variation for a sub-population to survive sudden and dramatic evironmental change.

I hope that at least explains the problem.Hmm...I'm replying to myself - is this a first?

There is something I didn't make clear because it's obvious to me, but may not be to others. So....

Selection acts only on the phenotype. This means that the only effect that selection can have is to change allele frequencies in our population, and only ever in the generations that follow that which has had selection operating upon it.
And second, for selection to have any effect there must be - must - pre-existing phenotypic and genotypic diversity in the population. That's why I asked the question "where does the genetic diversity come from?"

So the rate of mutation and rate of evolution are different. The rate of mutation will be fairly constant. During periods of equilibrium, any beneficial mutations will be mixed into the population and evolution will proceed rather slowly. But during a period of stress on the population, the beneficial mutations have a strong advantage over the non-mutated and the rate of evolution increases. Am I on the right track here?

That's why I asked the question "where does the genetic diversity come from?"A couple of points, not as absolutes, but as plausibles.
The process you have described in simplified form relating to punctuated equilibrium is that an environmental change occurs in which different genetic characteristics will afford an advantage. However, where did those changes come from?
I suggest that many did not arise till after the envionmental change. (Don't panic - I'm not going down the Lamarckian route.) The magnitude of change in organisms seems roughly porportional to the change in environment. That change is often associated with not only a reduction in species numbers, but seemingly a reduction in population numbers. When the change is small the reduction in number is small, when the change in large the reduction is large. What seems to me to occur is that within this reduced and stressed population, that is no longer in balance with its environment, mutations occuring at the normal rate now have an opportunity to produce a change that is beneficial in this new enviroment. Secondly, you noted that
.....our population is perfectly adapted
I suggest that this is rarely the case, and that there are always the opportunities for small changes that provide minor enhancements. In some settings the differences between two options may be so small that neither comes to dominate, yet in a changed environment one or other may deliver a substantial survivial benefit. These cryptovariants then provide a further source for the genetic diversity required for selection pressures to operate on within a changed environment.
Together these two mechanisms seems to me to solve the problem.

So the rate of mutation and rate of evolution are different. The rate of mutation will be fairly constant. Yes. Mutation occurs at a constant rate (as far as is known), evolution requires phenotypic diversity and evironmental change or migration (plus selection)During periods of equilibrium, any beneficial mutations will be mixed into the population and evolution will proceed rather slowly. Hmm...our population is ideal- it is perfectly adapted- so "beneficial" mutations (don't like that term, but we all use it) don't exist. Of course if our population were not perfectly adapted, then beneficial mutaions will confer a survival advantage, and our population will become better adapted. But during a period of stress on the population, the beneficial mutations have a strong advantage over the non-mutated and the rate of evolution increases. Am I on the right track here?You are, but my point was, in a perfectly adapted population, where is the genetic variation required for rapid evolutionary change?

The magnitude of change in organisms seems roughly porportional to the change in environment. Well, either you are stating the obvious - that change in the environment gives rise to selection pressure- or you are indeed being ever so slightly Lamarkian! That change is often associated with not only a reduction in species numbers, but seemingly a reduction in population numbers.Yes, well. I was being sloppy here. I was using the terms population and species interchangeably. Large-scale population reductions are called a "bottle-neck", and give rise to some quite interesting effects. We Europeans, for example, are almost certainly the result of some sort of bottle-neck. Hmm. I'd like to say more on that, but won't just now. .... mutations occuring at the normal rate now have an opportunity to produce a change that is beneficial in this new enviroment. Yes, dear boy, that's the standard model.
I suggest that this is rarely the case, and that there are always the opportunities for small changes that provide minor enhancements. Yes, my population was hypothetical. The point I was addressing was that, in gradually changing environments, it is possible to imagine that a mutation rate of 4 x 10^-9 is sufficient to keep pace with slow environmental change. But it may not be so when environmental change is abrupt.These cryptovariants then provide a further source for the genetic diversity required for selection pressures to operate on within a changed environment. But you haven't explained how these "cryptovariants" (ugh) stably co-exist with the rest of the population.

Evolution is humans reason for questioning itself as a collective conciousness.

I believe that the fact we recognise our evolutionary path provides reason to share and create with other entities. The fact that the "known" universe is apart of our lives shows just how far ahead of ourselves we could be, but not just for human reasons.

Yes, dear boy, that's the standard model.
I was looking to engage in an interesting discussion not in a game of patronisation.
But you haven't explained how these "cryptovariants" (ugh) stably co-exist with the rest of the population.I have to my satisfaction "I suggest that this is rarely the case, and that there are always the opportunities for small changes that provide minor enhancements. In some settings the differences between two options may be so small that neither comes to dominate, yet in a changed environment one or other may deliver a substantial survivial benefit. " You seem to imply that a phenotype of restricted character always reflects an equally restrcited genotype. I await the opportunity to be educated.

Let's also assume that a species is perfectly adapted to its ecological niche (that's bold!)

That's bold indeed. But even if we assume that such a perfect state exists, one mustn't forget that even detrimental mutations are unlikely to be eliminated completely, if the given pehnotype is recessive.
That is, the detrimental allel might survive very long, even if it is selected against. Thus diversity in the gene pool is likely to exist even in the perfect state,possibly providing the basis of rapid adaptive radiation.

I was looking to engage in an interesting discussion not in a game of patronisation.Hey! I'm sorry if you found my "dear boy" offensive. Here in the UK it is regarded as a (rather dated) form of paliness.
I have to my satisfaction "I suggest that this is rarely the case, and that there are always the opportunities for small changes that provide minor enhancements. In some settings the differences between two options may be so small that neither comes to dominate, yet in a changed environment one or other may deliver a substantial survivial benefit. "And I agreed that I was talking hypothetically, and that in practice, perfect adaptation is unlikely to occur (and not only for the reasons you gave). I'm trying to build a model, not describe the real world! You seem to imply that a phenotype of restricted character always reflects an equally restrcited genotype. But I'm afraid I don't understand this comment. What is a restricted genotype? Are you in fact saying that in the population genetic homogeneity may not result in a corresponding phenotypic homogeneity? Or maybe that just because a population is phenotypically homogeneous that doesn't mean it is genetically homogeneous? Yes, that looks better. And if so, you are right, of course. But I don't see how it helps. Remember that selection acts on the phenotype, and it's only affect on genotypes is to change allele frequencies in the population.

That's bold indeed. But even if we assume that such a perfect state exists, one mustn't forget that even detrimental mutations are unlikely to be eliminated completely, if the given pehnotype is recessive.
That is, the detrimental allel might survive very long, even if it is selected against. Depends what you mean by "very long". Remember a population (mine here is, at least) is always in Hardy-Weinberg equilibrium, even when the recessive homozygote is lethal (or strongly selected against - it amounts to the same thing). This pushes the allele frequency down rather rapidly, in evolutionary terms.Thus diversity in the gene pool is likely to exist even in the perfect state,possibly providing the basis of rapid adaptive radiation.Look, I'm not trying to be controversial, I am simply trying to point out that yes, punctuated equilibria are the name of the game, but they are not that easy to accomodate in the standard neo-Darwinian model.

Well actually according to most models those recessive allels won't be eliminated at all, as there is no selection against the heterozygote form. Of course the percentage will be considerably small, but be remain stable at that level.

In addition according to a paper I read recently the so-called puctuated equilibrium problem has been basically settled, in the context of modern synthesis. I have to admit that I didn't look and read the relevant papers, though.

I do think that it might have been still problem in the old neo-Darwinian model, though.

Ref:
Kutschera U, Niklas KJ.The modern theory of biological evolution: an expanded synthesis.

Well actually according to most models those recessive allels won't be eliminated at all, as there is no selection against the heterozygote form. Of course the percentage will be considerably small, but be remain stable at that level. I just came back to correct an error in my last, and here you are! I said populations are alway "in" H-W equilibrium. This, of course cannot be the case where the recessive homozygote is lethal. What I should have said is populations always tend to H-W equilibrium.
Anyway, you're right that full selection against the recessive homozygote by itself will not completely eliminate the heterozygote (any more than repeatedly dividing by 2 ever gets you zero), but once it is below a certain frequency we expect drift to finish the job (usually)

Everybodt speaks of "perfectly adapted" organisms: isn't it more often the case that living beings are adequately adapted to their environment? Often when a foreign competitor is introduced it proves to be function better in an environment than the native species which have been there for a very long time.

Well this was only a thought experiment (or at least I understood it as that). In this hypothetical model there is a perfect genotype and every aberration results in lowered fitness. In other words, identical clones of this assumed perfect model would have the highest overall fitness, and it is clear that this model does not follow nature.
Of course there is never a state a perfect equilibrium (including perfect adaptation).
And in fact, overspecialization quite often lead to extinction in reality.

Well and yes, genetic drift could eliminate allels with low frequency, but this is of course a purely stochastic process. Depending on the process, it could by chance also locally increase the occurence of detrimental allels. That is, there is no direction of elimination those allels that did not by themselves lead to detrimental phenotypes.

Well and yes, genetic drift could eliminate allels with low frequency, but this is of course a purely stochastic process. Depending on the process, it could by chance also locally increase the occurence of detrimental allels. Didn't I tell you mine is a perfectly outbreeding population? No? silly me! Anyway if it is, here are no local effects. You are of course right though in general, that's why I said "(usually)"That is, there is no direction of elimination those allels that did not by themselves lead to detrimental phenotypes.But we would not expect drift to be a significant factor on an allele whose frequency is not very low, for which I'll let selection take the credit.

It is in consideration of this that bottle-necks and founder effects are so interesting, because they can be thought of as special cases of fixation by stochastic means.

Everybodt speaks of "perfectly adapted" organisms: isn't it more often the case that living beings are adequately adapted to their environment? Often when a foreign competitor is introduced it proves to be function better in an environment than the native species which have been there for a very long time.I like that term - adequately adapted. I'm going to use it myself, if you don't mind.
You are right. But I'm modelling. What we do, in all sciences, is we knowingly make unrealistic simplifing assumptions, get our model, see if it fits the real world, then see which of our assumptions needs to be fiddled with so the model matches reality.
Think of the perfect blackbody in physics, the ideal gas in chemistry etc.

Hum well, I partly agree. Though in this case of course so many basal factors are neglected or not specified that it is not really usable to derive hypotheses. Especially in biology simplification is a big problem, as we always deal with open, complex systems here, as compared to other natural sciences.

Hum well, I partly agree. Though in this case of course so many basal factors are neglected or not specified that it is not really usable to derive hypotheses. Especially in biology simplification is a big problem, as we always deal with open, complex systems here, as compared to other natural sciences.Well, my Ferryman friend, you seem determined to disagree with me about something! But this isn't important or interesting enough to argue about. But let me just say that it is precisely because of the complexity of biological systems that simplified models are so useful as a guide.

If you want a real argument, try this: the key to understanding punctuated equilibria (and possibly a lot more besides) is gene families. Think about it, then draw your sword!!

Well, if you want, I'll disagree :)

the key to understanding punctuated equilibria (and possibly a lot more besides) is gene families
Actually I think I don't quite get what you mean, care to elaborate? I suppose I missed some important points here.

I suppose I mainly don't see how the proposed models do relate to the PE theory (as as far as i I see it) try to envision a situation in which PE probably won't happen. But as they are not reflected in nature, the conclusions cannot be tested.

But more to PE:

If I recall the papers correctly, in which PE were stated (that is the papers and book sections by Gould somewhere in the 70s), it was clearly stated that PE was clearly a consequence of the allopatric speciation model.
This is, at best a further addition to the (neo)-Darwinian model (and has been integrated into the synthetic model). As such, there is no serious problem of PE per se.
So, my main point is that I don't clearly see PE as something special within evolutionary theories. The major problems at the time when the PE thory was formulated were mainly some deductions that Gould and colleague proposed as a direct effect of PE, but PE itself is well funded within the allopatric speciation model.

Actually I think I don't quite get what you mean, care to elaborate? I suppose I missed some important points here.Later. Let's deal with this first.


....PE was clearly a consequence of the allopatric speciation model.Well yes. That allopatry may lead to speciation is self evident. But look - let's remember we may not be alone. In case there are people out there who think we are a couple of experts using funny words, let me put some flesh on these bones.
A population where all possible matings are potentially realizable is called sympatric. A population where this is not the case is called allopatric - there may be any number of reasons for this, let's assume for now it is geographical (which makes it easier to think of local environmental conditions i.e. different selection criteria for allopatric populations).
This is, at best a further addition to the (neo)-Darwinian model (and has been integrated into the synthetic model). As such, there is no serious problem of PE per se.I suppose if two allopatric and genetically different populations merge, then anything might happen, all in a rush, resulting in some sort of punctuation. But my point would be that, while allopatry self-evidently may lead to speciation according to the gradualism of the classical model, it cannot, without further assumptions, result in a punctuation. And the evidence that links one sub-species to another is not there. That's what PE means!

Anyway, more on gene families later, if you are still interested.

I suppose if two allopatric and genetically different populations merge, then anything might happen, all in a rush, resulting in some sort of punctuation. But my point would be that, while allopatry self-evidently may lead to speciation according to the gradualism of the classical model, it cannot, without further assumptions, result in a punctuation.

Actually it was argued the other way round. Isolation and genetic drift can, as some phylogenetic studies imply, lead to allopatric speciation. As these effects do not need to be gradual, it may in fact lead to punctuation. As such subspecies are a non-issue here.
So, what Gould and Eldredge simply proposed is that PE is a special case of allopatric speciation. In their view large populations are usually stable (and do not even need to be perfectly adapted as in the model you proposed).
However small isolated groups can evolve more rapidly, due to smaller genetic variation (in conjuncture with the given environmental effects). That is, basically due to the founder effect.
So, small populations are usually evolutionarily more unstable and evolution rates increase, until the conditions for stasis are met again.

This was more or less what I remembered regarding PE. As I am working mostly on microbial genetics (where slightly different rules apply), and not that much in evolutionary biology, I may have overseen some stuff, though.

Regarding gene families, sure, I am eager to hear about that.

In their view large populations are usually stable .......
However small isolated groups can evolve more rapidly, due to smaller genetic variation ......basically due to the founder effect. So, small populations are usually evolutionarily more unstable and evolution rates increase, until the conditions for stasis are met again..What?? Smaller gene pools allow for more rapid evolutionary change? How so, Mr. Boatman? The founder effect severely restricts the genetic variation in a population. It's a simple equation: variation + selection = evolution. Reduce either term on the LHS and the RHS correspondingly decreases!


Regarding gene families, sure, I am eager to hear about that.Yes...I'm thinking of starting a new thread. But I wonder who else would be interested (other than you) ? In spite of my argumentative tone, I am enjoying "talking" to you.

Yes! In smaller population evolutionary effects act faster (that is, in less generations).

Evolution is nothing but a change in allel frequency in a population, right? Now imagine a limited population size. Here stochastic events (genetic drift for example) have a much higher influence on the allel frequency than in large population. As such, large populations have a far more stable distribution of allel frequencies than small ones.

In addition, if a smaller variation exists selection can act quicker. In a large population the frequency reduction of a given detrimental allel is far slower than in a slow one, as one can easily imagine (e.g. if in a smaller population only one individual is carrying the allel, the loss of it is far higher compared to a population where 100 are carrying the given allel).
A higher variation only means that there is a higher potential of adaptation, not a difference in the evolutionary path.

Well and if you think that the given talk is not of interest to anyone else (wouldn't be the first thread more or less highjacked by two peeps) we can always continue in pm.

Evolution is nothing but a change in allel frequency in a population, right? Yes - nicely stated.Now imagine a limited population size. Here stochastic events (genetic drift for example) have a much higher influence on the allel frequency than in large population.Mmmm...not sure. I'll say yes for nowAs such, large populations have a far more stable distribution of allel frequencies than small ones.Why no!! Remember Hardy-Weinberg. p² + 2pq + q² always.

...if a smaller variation exists selection can act quicker. This is where we part company. I'm not going to be too dogmatic, because I have been wrong before, but I simply do not see this. Take an extreme example - no variation. No selection, right? How do you translate that into less variation, more selectionIn a large population the frequency reduction of a given detrimental allel is far slower than in a small one, as one can easily imagine (e.g. if in a smaller population only one individual is carrying the allel, the loss of it is far higher compared to a population where 100 are carrying the given allel).ah! Are you confusing "variation" in the general sense of "genetic diversity" with "variation about the mean"? I need to get back to you on that, but not tonight.
A higher variation only means that there is a higher potential of adaptation, not a difference in the evolutionary path.I agree with the first part of that (who would not), but simply do not understand the second!

Well and if you think that the given talk is not of interest to anyone else (wouldn't be the first thread more or less highjacked by two peeps) we can always continue in pm.High-jackers?? Pfft! I seem quite capable of giving offence unintentionally. See me when I mean it!

High-jackers? Mmm..maybe hi-jackers.

Why no!! Remember Hardy-Weinberg. p2 + 2pq + q2 always.

Ah, no. Hardy Weinberg explicitly does not apply to small populations!
One of the basic tenets of H-W is that the population is of inifinte size.
What it states is basically that in the given large population even genes with no present selective values will be retained.

These are the basic prerequisites for H-W to apply:


- mutation is not occurring
- natural selection is not occurring
- the population is infinitely (or very) large
- all members of the population breed
- all mating is totally random
- everyone produces the same number of offspring
- there is no migration in or out of the population
- no change in selective pressure


As one can easily see the above given effects, especially genetic drift and possibly natural selection act harsher on the gene pool of a small population.
Again the example, imagine a given allel is present in 1% of the population. Now envision a population of 100,000 individuals and one of 100. In the first case 1000 individuals carry that allel, in the second only one.
Now due to a randome event in both cases one of the carriers of the allel dies. in the first case there are still 99 and given something like h-w equlibrium it will come near to the basic distribution again. In the second case the allel is completely lost, resulting in a change of allel frequency for this allel from 1% to 0%, with no chance of going to equilibrium conditions again (barring gene influx).

Ah, no. Hardy Weinberg explicitly does not apply to small populations!
One of the basic tenets of H-W is that the population is of inifinte size.As I thought, we are at cross-purpose. Note that when I used the term Hardy-Weinberg, I deliberately did not include the term "equilibrium". Why? Because if p and q are allele frequencies, I am free to interpret (p + q)² = 1 as a probability distribution. Which means I can apply H-W probability distribution to a population of any size ( given the other constraints you mentioned).
As one can easily see the above given effects, especially genetic drift and possibly natural selection act harsher on the gene pool of a small population.Sure, but that's not all you said earlier. You saidIn addition, if a smaller variation exists selection can act quicker.Which is clearly not true. I got confused whether you meant "variation about the mean" or "genetic variation".
Look. Suppose I have two similar but not identical objects which I can measure in some way. From those measurements I can extract a "mean" (oh dear!) and two tails. If I take one of those objects at random and ask what is the probability that it lies within a certain arbitrary (small) distance from the mean, the answer is of course zero, because each object is itself a tail, and thus is as far from the mean as it's possible to be.
So, for these two objects, variation about the mean is about as big as it gets. But if I then ask - what is the actual variation within my sample, the answer is, of course, either one of two values. In other words, the variation defined this way is small.

Oh well. I'm sure we agree, really. And in any case, it is hardly a controversial subject.
Pax!

Ah yes, I see what you mean. I suppose I should have been more precise about relative and absolute numbers. When I said natural selection acts faster I meant of course that given a stable selective pressure the given time frame in which it will lead to a change in allel frequency in a smaller population will be smaller than in a large one.

And yes, it ain't controversial at all. as the whole PE-discussion. It was only controversial when it was proposed because both, proponents and opponents of that time initially misunderstood each other (hey, what does it remind me of?), but as of now it has been clearly resolved.

Nonetheless, it was somewhat fun trying to remember old stuff that one learned when one was student, wasn't it? Ah the olden days ;P

We are all talking gene frequencies again. Evolution is also (or totally?) about form though.

Is the evolution of form gradual? I would say yes in principal. No true new form can arise overnight.

Is the speed of evolution variable? Yes, so form can remain constant for a long time and then change quickly.

We are all talking gene frequencies again. Yes, we are. As Charon so elegantly put it, evolution is all about about changing allele frequencies. C'est tout.
Is the evolution of form gradual? I would say yes in principal. No true new form can arise overnight. Or would you rather...

Yes, so form can remain constant for a long time and then change quickly.Huh?

For some groups of people yes, evolution will be gradual.
But,
hopefully the laws of "affirmitive action" will help speed up evolution for some?