Stochastic Events and Natural Selection (Split thread)

[overtone]

 

 

Right. All kinds of contingencies can be responsible for differential reproductive success. So, what are the necessary and sufficient conditions whereby we attribute the success to natural selection? Or to luck? Or to genetic drift?

Luck, genetic drift, rocks falling out of the sky, etc, are all perfectly good mechanisms of natural selection. They fit into standard Darwinian theory smoothly and easily.

 

 

If we can't discern causality there, then the theory is that much weaker for it How so? As no particular cause is implied by the theory, and root causes of differential reproductive success are often obscure and complex, being able to pin down a specific cause for a specific example is hardly to be expected in general.

[CharonY]

 

Luck, genetic drift, rocks falling out of the sky, etc, are all perfectly good mechanisms of natural selection. They fit into standard Darwinian theory smoothly and easily.

 

No, these are aspects are stochastic events that contrast with natural selection. The latter is specifically tied to differential reproductive success under given conditions. I.e. selective forces act on a given gene pool and shapes the resulting frequency.

[overtone]

 

 

If luck and falling rocks are perfectly good mechanisms of natural selection, then we should try to correct the widespread misunderstanding that natural selection has to do with the particulars of phenotypes and the fitness contributed to them by their various traits.

You seem to think the existence of some kinds of selection events excludes others. Why? For example: How does the occasional asteroid killing everything on land over 40 pounds prevent an incremental improvement in reproductive survival odds via camouflage, in between asteroids?

 

If the theory doesn't imply particular causes, then it's not much of a scientific theory. That is, it's merely descriptive, but not explanatory.

And yet it has explained so much. How do you explain that?

[overtone]

Assume now that mutations create more variations, but none of them are functional or have reduced functionality under the given environment. The result is that selective sweeps stabilizes the existing genotypes. All things considered equal, the three will keep a steady frequency in the population.

They probably won't. The rarer ones will probably random walk to extinction - and new ones are rare. Which is why very few of the hundreds of mutations each individual human carries that neither of the parents carried have much chance of characterizing the human species in a thousand years.

 

Which is one reason stochastic outside events, like large meteorite impacts, are included as selection events - surviving them is easier for some organisms than others. Widespread vs localized, high r vs low, small vs large, mobile vs immobile, etc.

Edited November 17, 2014 by overtone

[CharonY]

They probably won't. The rarer ones will probably random walk to extinction - and new ones are rare. Which is why very few of the hundreds of mutations each individual human carries that neither of the parents carried have much chance of characterizing the human species in a thousand years.

 

Which is one reason stochastic outside events, like large meteorite impacts, are included as selection events - surviving them is easier for some organisms than others. Widespread vs localized, high r vs low, small vs large, mobile vs immobile, etc.

 

This is either wrong or a misunderstanding of principles. You are apparently comparing different species. However, if within a population there is differential reproduction (surviving is not even precisely relevant here) due to an event, that event is not stochastic.

[overtone]

However, if within a population there is differential reproduction (surviving is not even precisely relevant here) due to an event, that event is not stochastic.

What is the term for unpredictable events without local cause or history - such as meteor strikes, or earthquakes, or large jokelhaups - that by chance dramatically alter allele frequencies within a population (as most catastrophic events do) ?

 

 

 

 

EVERY trait has been selected, sayest thou. I think probably not

You have overlooked the nature of selection pressure - it does not necessarily work first or directly on whatever aspect of a trait you have decided to label. White bones have been selected, first because they are easily available in the trait space as byproducts of selected strength and so forth, and then because any other color would incur a cost without a benefit. All the other colors would have been been selected against, in other words, as and when they occurred. Edited November 17, 2014 by overtone

[MonDie]

 

What is the term for unpredictable events without local cause or history - such as meteor strikes, or earthquakes, or large jokelhaups - that by chance dramatically alter allele frequencies within a population (as most catastrophic events do) ?

 

Where would that fall into the Hardy-Weinberg principle if not "selection"?

 

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Returning Dec 19

[CharonY]

 

What is the term for unpredictable events without local cause or history - such as meteor strikes, or earthquakes, or large jokelhaups - that by chance dramatically alter allele frequencies within a population (as most catastrophic events do) ?

 

 

 

It seems to me (but I may be wrong) that you are conflating two elements. The event itself may be random in terms of being unpredictable (i.e. meteor strike). But to assess whether selection is going on, the important bit is whether the effects of the events on allele frequencies is random or not. You mentioned above that have better survival after that event.

i.e.

 

surviving them is easier for some organisms than others.

 

This would point at a selective pressure of sorts. If the genotype has no influence on survival of the event (I am here also conflating survival with reproductive success for brevity's sake) , then it is truly stochastic and in small populations (or huge events) genetic drift would be the result.

 

I would also like to point out that not all traits are selected for, but it is probably more accurate to state that this is true only for small set. The majority probably has not been selected against.

[overtone]

 

 

But to assess whether selection is going on, the important bit is whether the effects of the events on allele frequencies is random or not.

It's going to favor some alleles over others, by chance, with near certainty.

[Ten oz]

It seems to me (but I may be wrong) that you are conflating two elements. The event itself may be random in terms of being unpredictable (i.e. meteor strike). But to assess whether selection is going on, the important bit is whether the effects of the events on allele frequencies is random or not. You mentioned above that have better survival after that event.

i.e.

 

 

Sometimes no species are able to survive. A event like a meteorite strike may kill everything in a locality. What follows is a chain reaction as then other species outside that locality may become impacted based on things like changed migratory patterns. Natural selection will continue amongst the groups outside the destoryed locality. Survival after an event and surviving an event being two different things. Natural disasters influence natural selection. And sometimes there is just no surviving.

[CharonY]

It's going to favor some alleles over others, by chance, with near certainty.

Then what follows is selection and the mechanism is not stochastic anymore. The actually killing by the strike can be seen as stochastic (i.e. if directly hit there is no selective advantage of any genotype of a given population (and again, the concept is only valid if we look at populations separately). But the resulting environmental landscape provides a change in selective pressure for the survivors. At this step selection kicks in.

[overtone]

 

 

Then what follows is selection and the mechanism is not stochastic anymore.

So selection by random chance is not stochastic.

 

Note that large disasters afflicting populations are almost certain to favor some alleles over others, by chance. So very few events are going to be stochastic, and you won't be able to tell very easily which few are.

[CharonY]

 

Note that large disasters afflicting populations are almost certain to favor some alleles over others, by chance

 

This is again confusing chance of chance events with the mechanism. If something is favored, it does not happen by chance. There is a mechanism that favors it.Depending on the type of population and effects we are talking here, it can be quite possible to distinguish those. You have to keep in mind that these studies are done by looking at events that took place over time and is not based on singular sampling directly after something happened, for example.

[overtone]

This is again confusing chance of chance events with the mechanism. If something is favored, it does not happen by chance.

Why not? It's a near certainty that in any major catastrophe leading to a genetic bottleneck, the population is going to lose a lot of alleles by chance (and the same pattern will hold at every taxonomic level). That's just statistics in action. Rare alleles tend to random walk to vanishing - more ordinary probability theory. There are even names for some of the circumstances of this: founder effect, genetic drift.

 

 

 

 

Oh, brother. Here we go: What's qualifies as a "trait" and what qualifies as merely an "aspect" of a trait? You're just making it up as you go. Can you give me some logic, and not just a list of assertions?

If you don't know what "trait" means, or "aspect", look them up in the dictionary. I just write in English, using a simple vocabulary most native speakers of English should be familiar with.

 

Do you deny the existence of traits? Aspects? If not, whatever you take them to be, in agreement with a decent dictionary, will do.

 

 

 

But the putative mechanism of natural selection is just plain incoherent.

Nobody's having much trouble with the basics of it except you. The word "selection" is clear, right? - from a set, certain members continue and others don't, so that later on you have lost some members you used to have, the remainder is "selected". Is the word "natural" difficult for you to comprehend? It means anything following the laws of the physical universe but not directly constrained or caused by human beings.

 

So we start with a population, we don't do anything to it, and later notice that some kinds of members have disappeared while others become more common. Natural (we didn't do it) selection (some gone, some still in the population) has happened.

Edited November 19, 2014 by overtone

[CharonY]

 

Why not? It's a near certainty that in any major catastrophe leading to a genetic bottleneck, the population is going to lose a lot of alleles by chance (and the same pattern will hold at every taxonomic level). That's just statistics in action. Rare alleles tend to random walk to vanishing - more ordinary probability theory. There are even names for some of the circumstances of this: founder effect, genetic drift.

 

This is precisely what I try to tell you. If the shift is independent on the alleles and their properties (i.e. allele A is not more likely to be eliminated as allele B, unless by pure chance) this is non-selective, but stochastic. Genetic drift is a stochastic effect. Founder effect is more a starter position that affects the spread of alleles and both, stochastic and selection events can be involved. As I said, you got the right things in your mind, you just need to separate them a bit more.

[overtone]

If the shift is independent on the alleles and their properties (i.e. allele A is not more likely to be eliminated as allele B, unless by pure chance) this is non-selective, but stochastic. Genetic drift is a stochastic effect. Founder effect is more a starter position that affects the spread of alleles and both, stochastic and selection events can be involved. As I said, you got the right things in your mind, you just need to separate them a bit more.

The point that I'm making is that random, stochastic event is one of the agents of natural selection as the term is employed (i.e. comprehensively) in Darwinian theory, starting from its very first employment in explanation of -> the origin <- of the species. Populations are subject to selection by chance, chance is natural. Darwinian theory includes all the mechanisms of natural selection, all the evolutionary change we see, not just the subset analogous to definite human choice via observed criteria or human defined trait.

 

The problem you would face by trying to exclude it, somehow set up a different branch of your theory to incorporate the workings of probability, is that almost all differential reproduction involves a large component of random chance. Naturally selective events graph on a sliding scale of probability involvement vs cause/effect, at all levels of selection - almost none are free of randomness, and many are dominated by their stochastic aspect. Chance does not separate out cleanly from the other components of natural selection. Most advantageous traits get their advantage in the first place through biasing the odds.

 

The human propensity to project design and intent into the natural world simultaneously projects cause and effect - or maybe that should read the other way around; at any rate, the founder effect is a pretty clear example of selection by chance, at least of most of the variant alleles from their population the founders happen to be carrying.

Edited November 19, 2014 by overtone

[CharonY]

 

The point that I'm making is that random, stochastic event is one of the agents of natural selection as the term is employed in Darwinian theory, starting from its very first employment in explanation of -> the origin <- of the species. Populations are subject to selection by chance. Darwinian theory includes all the mechanisms of natural selection, all the evolutionary change we see, not just the subset analogous to definite human choice via observed criteria or human defined trait.

 

I hoped that I made it clear in the last posts but unfortunately I see that you are confusing things again. I will provide a simple example and leave it at that as I feel I am just repeating very simple things over and over again.

 

Imagine a population in which 100 individuals of allele A, B and C exist (each). Now a rockslide kill 25 individuals, independent of their alleles. As the population is small we will see a change in allele frequency (drift) but it is absolutely independent of what the alleles the 25 killed individuals had. Thus we have a stochastic and not a selection event (and you cannot handwave that distinction away).

In a large population the same event may leave no trace over generations and will be hard to impossible to spot.

 

Second example: Imagine some organisms in deep sea with a distinct life cycle: Sessile and motile. The amount of time they spend in either form is determined by two alleles, A (mostly sessile) B (more motile). According to various influences there is a given frequency distribution in the population we are looking at. Now suddenly a thermal vent opens. This releases compounds that are harmful to the organisms. Directly at the vent, both alleles are killed similarly (i.e. stochastically) but toward the edge, B has an advantage, they start moving slightly further away and have a higher reproductive rate than A that just tend to stick around and need to invest much more energy into their stress responses than into reproduction.

Now if we sample the population over time, we will see a continuous increase of allele B in the give population.

 

In the second example a catastrophic event created two effects, a stochastic one, that changes frequencies randomly but then exerts a selective pressure that shapes the allele distribution in distinct way.

The reason why have to distinguish (not dismiss) these different mechanisms is because they create different outcomes. As we can see these differences it makes not sense to claim that they are indistinguishable.

 

I honestly get the feeling that you kind of see that but have a hard time to admit to ignorance and need to explain those things away. Which is a pity as it is another lost learning opportunity.

[Delta1212]

 

I hoped that I made it clear in the last posts but unfortunately I see that you are confusing things again. I will provide a simple example and leave it at that as I feel I am just repeating very simple things over and over again.

 

Imagine a population in which 100 individuals of allele A, B and C exist (each). Now a rockslide kill 25 individuals, independent of their alleles. As the population is small we will see a change in allele frequency (drift) but it is absolutely independent of what the alleles the 25 killed individuals had. Thus we have a stochastic and not a selection event (and you cannot handwave that distinction away).

In a large population the same event may leave no trace over generations and will be hard to impossible to spot.

 

Second example: Imagine some organisms in deep sea with a distinct life cycle: Sessile and motile. The amount of time they spend in either form is determined by two alleles, A (mostly sessile) B (more motile). According to various influences there is a given frequency distribution in the population we are looking at. Now suddenly a thermal vent opens. This releases compounds that are harmful to the organisms. Directly at the vent, both alleles are killed similarly (i.e. stochastically) but toward the edge, B has an advantage, they start moving slightly further away and have a higher reproductive rate than A that just tend to stick around and need to invest much more energy into their stress responses than into reproduction.

Now if we sample the population over time, we will see a continuous increase of allele B in the give population.

 

In the second example a catastrophic event created two effects, a stochastic one, that changes frequencies randomly but then exerts a selective pressure that shapes the allele distribution in distinct way.

The reason why have to distinguish (not dismiss) these different mechanisms is because they create different outcomes. As we can see these differences it makes not sense to claim that they are indistinguishable.

 

I honestly get the feeling that you kind of see that but have a hard time to admit to ignorance and need to explain those things away. Which is a pity as it is another lost learning opportunity.

Really, I think the issue is simply that overtone is using "selection" to refer to any event that has an impact on the allele frequency of the population (by removing some members). I think it's simply an issue of using the term overly broadly rather than a misunderstanding of how the whole thing actually works.

[CharonY]

I would say that these things go hand in hand, if you realize that you are talking about different things, you more likely to use different terms or otherwise try to distinguish them, If you conflate concepts, you have missed the finer points.

[Robittybob1]

 

...

Imagine a population in which 100 individuals of allele A, B and C exist (each). Now a rockslide kill 25 individuals, independent of their alleles. As the population is small we will see a change in allele frequency (drift) but it is absolutely independent of what the alleles the 25 killed individuals had. Thus we have a stochastic and not a selection event (and you cannot handwave that distinction away).

In a large population the same event may leave no trace over generations and will be hard to impossible to spot.

 

 

I just want to focus on this paragraph please. Did you mean independent or dependent?

I think the gene frequency change is dependent on the alleles lost by the 25 killed individuals.

Can you explain why you say "absolutely independent" please?

[Delta1212]

I just want to focus on this paragraph please. Did you mean independent or dependent?

I think the gene frequency change is dependent on the alleles lost by the 25 killed individuals.

Can you explain why you say "absolutely independent" please?

Which alleles were lost is independent of what the lost alleles were, since none of the alleles in the population would have been any more or less likely to avoid being removed in that situation. Edited November 19, 2014 by Delta1212

[CharonY]

Yep, That is what I meant. Under that circumstance the type of the allele (and any traits they may have conferred) had no influence on whether they survive that particular event.

 

I would like to stress that while I also used survival as an example, the real thing to look at is reproductive success. If the catastrophe had hit only a group of non-reproducing individuals (e.g. animals that are past their reproductive age who also do not social activity that may influence reproductive success of relatives) the effect would not be the same if it hit a potentially reproductive group,

[Robittybob1]

Yep, That is what I meant. Under that circumstance the type of the allele (and any traits they may have conferred) had no influence on whether they survive that particular event.

 

I would like to stress that while I also used survival as an example, the real thing to look at is reproductive success. If the catastrophe had hit only a group of non-reproducing individuals (e.g. animals that are past their reproductive age who also do not social activity that may influence reproductive success of relatives) the effect would not be the same if it hit a potentially reproductive group,

I see it now, you are using the word "dependent" based on whether the trait enabled survivability or not, and you are assuming none of the alleles improved survivability and therefore the result was "absolutely independent" of the alleles.

The way I thought you wrote it means that the final gene frequency, post landslide, is dependent on what alleles were lost in the disaster. i.e. if they were all the one type and 25 of them perished that particular allele frequency is hit severely, in fact it could have completely wiped out that type.

Which alleles were lost is independent of what the lost alleles were, since none of the alleles in the population would have been any more or less likely to avoid being removed in that situation.

You have made an assumption there. Do you know if or how the alleles benefited the individuals? Just as a suggestion one allele might have given them better eyesight or hearing and they got out of harms way quicker and safer.

Edited November 19, 2014 by Robittybob1

[Delta1212]

I see it now, you are using the word "dependent" based on whether the trait enabled survivability or not, and you are assuming none of the alleles improved survivability and therefore the result was "absolutely independent" of the alleles.

The way I thought you wrote it means that the final gene frequency, post landslide, is dependent on what alleles were lost in the disaster. i.e. if they were all the one type and 25 of them perished that particular allele frequency is hit severely, in fact it could have completely wiped out that type.

 

You have made an assumption there. Do you know if or how the alleles benefited the individuals? Just as a suggestion one allele might have given them better eyesight or hearing and they got out of harms way quicker and safer.

The assumption was implicit in the example. You could replace "landslide" with "nuclear bomb" if you wanted. The point is that we're assuming some event that would have killed anyone involved, regardless of physical characteristics and genetic makeup.

 

A mini Pompeii maybe.

[Robittybob1]

The assumption was implicit in the example. You could replace "landslide" with "nuclear bomb" if you wanted. The point is that we're assuming some event that would have killed anyone involved, regardless of physical characteristics and genetic makeup.

 

A mini Pompeii maybe.

OK I'll give you that one. "Now a rockslide kill 25 individuals, independent of their alleles." That does make it clear there was no selection advantage, but I'll be a Devil's Advocate and ask how was this determined that it was just random and independent in the first place?

Think of the Chicxulub Meteor it wiped out a high percentage but some animals survived.