Natural Selection

[Mokele]

the minor is assumed; it is not logically self-evident that this is true, unless one looks at the "conclusion first.

 

Actually, it is, for precisely the reason lucapsa said: geometric population increase. Since all animals have more offspring than are needed to replace them, yet we aren't knee-deep in cockroaches, we must conclude that not all of those offspring survive, ergo the stuggle for existence.

 

Mokele

[lucaspa]

Look, I'm not going to argue philosophy, but the major premise is observed, the minor is assumed; it is not logically self-evident that this is true, unless one looks at the "conclusion first. Anyway, drop it, it's not important.

 

Now you've made it important. By saying the minor premise -- struggle for existence -- is "assumed". It's demonstrated. Darwin spent an entire chapter on the "Struggle for Existence". http://pages.britishlibrary.net/charles.darwin/texts/origin_6th/origin6th_03.html

 

What you are saying is "assumed" is that the "fittest survive". The problem there is that you are using the soundbite of natural selection to be all of natural selection. It's not. "Survival of the fittest" is the soundbite version. Like all soundbites, it is not totally accurate.

 

What natural selection says is that variations useful to the organism will do better in the struggle for existence and those variations will be passed to the offspring. Over time, the accumulation of new variations will change a population and transform a population.

 

One says that x is false in general if one can't find a proof that rigourously exludes not-x. And proofs do not, in general, come from data, these are merely probabilities. But, if you conclude from the data that there are exceptions to x, you are entitled to say "not true (i.e. false) in general" (As it happens, Lewontin is my hero, so I would agree with him, wouldn't I?)

 

Can you please find me a source for that "one says that x is false in general"? I've never come across that phrase in all my reading. Also please find a source that "not true in general" = "false". I've always seen "false" associated with being "totally wrong", not with "not true in general".

 

Thank you.

[lucaspa]

Actually, it is, for precisely the reason lucapsa said: geometric population increase. Since all animals have more offspring than are needed to replace them, yet we aren't knee-deep in cockroaches, we must conclude that not all of those offspring survive, ergo the stuggle for existence.

 

Mokele

 

Thank you. Darwin also, in Chapter 3, documented many cases of the struggle for existence. The geometric increase is the cause of the struggle, but the struggle itself can be observed, often directly.

[Xerxes]

Whatever you say.

[Xerxes]

What natural selection says is that variations useful to the organism will do better in the struggle for existence

Please don't lecture me on evolutionary genetics. What you say is manifestly true (by observation), but, logically, "useful" is predicated on "do better". It's not a syllogism (but it's still true, empirically)

 

 

Can you please find me a source for that "one says that x is false in general"? I've never come across that phrase in all my reading. Also please find a source that "not true in general" = "false". I've always seen "false" associated with being "totally wrong", not with "not true in general".

 

Thank you.

You don't need a source, it's elementary. x is false = x is not true. x is "false in general" means that, whenever x might be true, it may sometimes not be true, what's so radical?

[john5746]

Human manufacture. We turn out hammers, screws, etc that are identical. The degree of how identical depends on the precision of the machine tools.

 

You can quibble that there is variation at the molecular level, but at the level of "trait", each hammer or screw is identical.

 

Manufacturing has variation above the molecular level, but of course, you won't get a sledge hammer from a normal hammer line.

 

Humans do seem to be obsessed with fighting variation, which seems to be the natural order. So, if you see life with no variation, exact replicas, maybe that is a sign of intelligent design, or manufacture?

[lucaspa]

Manufacturing has variation above the molecular level,

 

Please tell me an assembly line that turns out hammers or screws with variations above the trivial molecular level.

 

Humans do seem to be obsessed with fighting variation, which seems to be the natural order. So, if you see life with no variation, exact replicas, maybe that is a sign of intelligent design, or manufacture?

 

It can't be, because you see the organisms replicate and there is no intelligence in sight, is there?

 

Creationists often try to say that some organisms -- horseshoe crabs, sharks, for instance -- are "unchanged" for tens of millions of years. However, this is untrue. Altho the basic shape is similar, the species of horseshoe crab that exists today is different than those in the past.

 

Also, there are actually 3 forms of natural selection:

1. Directional. This is the one that is usually considered, and the one that changes a population as the environment changes.

2. Stabilizing selection. Once a population reaches a "fitness peak" and is well-adapted to the environment, further changes will actually make the individual LESS fit for that particular environment. So, as long as the environment remains constant, natural selection will actually act to prevent change to the population and instead narrow the variation to the optimum for that particular environment.

3. Disruptive selection. This happens when a population has wide geographical range and faces different environments in different parts of the range. There is a tendency for directional selection to adapt the sub-populatoins to the particular environments, but gene flow between populations counters that. If for any reason, there is a disruption of gene flow, you get separate species. This is what happens in "ring species".

 

As we both noted, human manufacture does have minor variation: each hammer is not identical if you would analyze it in enough detail. There would be irregularities at the level of the atom between different hammers. Because of the second law of thermodynamics, complete replication is not possible. It's just that the differnces between hammers is too minor to matter. Also, because hammers don't replicate over generations, it is not possible to accumulate differences to turn a claw hammer to a sledge hammer.

 

BUT, in living organisms you have an amplification mechanism -- development -- that turns the molecular variations at the DNA level to macro differences at the level of the organism. Also, differences accumulate because of natural selection. So the minor changes are additive, not isolated like they are in manufactured artifacts, and you can get the equivalent of turning a claw hammer into a sledge hammer.

 

Manufactured artifacts are isolated. You make one hammer. Then you make another. But in biological organisms you have the equivalent of one hammer making another hammer. The hammers are connected by ancestor-descent.

[lucaspa]

Please don't lecture me on evolutionary genetics. What you say is manifestly true (by observation), but, logically, "useful" is predicated on "do better". It's not a syllogism (but it's still true, empirically)

 

The logical chain is still a syllogism. It's just that the syllogism has been confirmed by empirical observation. There are objective criteria that can be used to assess "do better". Since you don't want me to lecture you on evolutionary genetics, I will only remind you of Hardy-Weinberg.

 

 

You don't need a source, it's elementary. x is false = x is not true. x is "false in general" means that, whenever x might be true, it may sometimes not be true, what's so radical?

 

Yes, I do need a source. Because we have the situation x is not true in some situations. That is not the same as saying "x is not true". That statement has the implied "x is not true AT ALL"

 

"false in general" is not the same as "sometimes may not be true. "in general" refers to a majority of the time. "sometimes" refers to much less than 50% of the time.

 

So I also need as source that equates "in general" with "sometimes'.

 

Thank you.

[lucaspa]

Whatever you say.

 

The evolutionary geneticist hadn't read chapter 3 of Origin of Species? You could be a bit more gracious in admitting that the idea was in error.

[Xerxes]

You are obviously getting cross, but then....

The logical chain is still a syllogism.

Then give me a definition of "useful" that doesn't entail the definition of "do better" (or v.v.) A syllogism is a logical chain, as you put it, which admits of no possible exceptions. Empirical data, of course, always do.

Since you don't want me to lecture you on evolutionary genetics, I will only remind you of Hardy-Weinberg.

I fail to see what H-W has to do with it. Are you suggesting, perhaps, that as all alleles at a locus are not in H-W equilibrium, selection must have occurred? We know this to be "false in general". Which brings me to this extremely uninteresting point.

 

 

 

 

Yes, I do need a source. Because we have the situation x is not true in some situations. That is not the same as saying "x is not true". That statement has the implied "x is not true AT ALL"

 

"false in general" is not the same as "sometimes may not be true. "in general" refers to a majority of the time.

Sorry, but that is the way I and my mathematically inclined colleagues use the term "in general". You don't need to like it, but that's how it is. x is true in general if there are no exceptions ("always" is not usually used as it implies a time dimension).

 

x is not true in general means that x is not always true. x not-true = x false. How about this, as a slightly less jargonistic compromise?

 

The statement that x is true is not true in general, therefore the statement that x is true in general is false i.e. the statement that x is false in general is true? Any better?

[CanadaAotS]

I love the hammer analogy:

 

"Manufactured artifacts are isolated. You make one hammer. Then you make another. But in biological organisms you have the equivalent of one hammer making another hammer. The hammers are connected by ancestor-descent."

 

Now, how often do you get to say "The hammers are connected by ancestor-descent"

[bombus]

I love the hammer analogy:

 

"Manufactured artifacts are isolated. You make one hammer. Then you make another. But in biological organisms you have the equivalent of one hammer making another hammer. The hammers are connected by ancestor-descent."

 

Now, how often do you get to say "The hammers are connected by ancestor-descent"

 

Mmm, yes but a new design of hammer is usually based on older designs of hammers. In fact the development of all tools follows a similar pattern to evolution, with slight refinements taking place over time, and sometimes by pure chance. Obviously it is Intelligent Design that makes tools 'evolve' not natural selection (unless one considers tools to be natural as man makes them and is himself a product of nature!), but for obvious reasons the parallels are there.

[Mokele]

Look, the logic is *not* that complex.

 

Premise one: heritable variation exists. (obviously true)

Premise two: some of this variation affects the ability of the organism to replicate and pass on that variation. (obviously true)

Premise three: not all offspring survive (see malthus).

 

So, if you have variants, and some make more copies of themselves than others, the relative proportions of the variants in the population will change. Simple as that.

 

Attempting to define "fitness" in a way that doesn't inclue "surviving better" is flat out silly; it's like asking to define "morphology" without using "shape" or "form". Genetic variation can lead to variation in phenotype (bigger, faster, more toxic venom, more efficient digestion, higher sperm count, etc), which in turn leads to the organism producing more offspring relative to one which does not possess the trait, or who possesses an alternative form of the trait.

 

However, that's really all superfluous. All you need to say is that in a population whose members vary in their total number of lifetime replications, the proportion of the variants in the population will change.

 

You don't even need limited populations; if you've got two bacteria, one of which can replicate one every hour, and the other of which can replicate 5 times an hour, even if the populations grow without limit, the proportions will change.

 

It all boils down to varying ability to replicate.

 

Mokele

[john5746]

Please tell me an assembly line that turns out hammers or screws with variations above the trivial molecular level.

 

 

http://www.americanfastener.com/fasteners/machine.asp

 

I think these tolerances are far larger than the molecular level.

 

Creationists often try to say that some organisms -- horseshoe crabs, sharks, for instance -- are "unchanged" for tens of millions of years. However, this is untrue. Altho the basic shape is similar, the species of horseshoe crab that exists today is different than those in the past.

 

It seems that variation or "mistakes" happen naturally. We have to go through considerable effort to minimize it. So, if we were to find a species where all the life forms had identical DNA, both past and present, that would seem more intelligence driven then all the diversity we see in nature, IMO.

 

 

As we both noted, human manufacture does have minor variation: each hammer is not identical if you would analyze it in enough detail. There would be irregularities at the level of the atom between different hammers. Because of the second law of thermodynamics, complete replication is not possible. It's just that the differnces between hammers is too minor to matter. Also, because hammers don't replicate over generations, it is not possible to accumulate differences to turn a claw hammer to a sledge hammer.

 

Yes, we know hammer manufacturing isn't biological, but it does have variations. Some hammers are larger than others, some are thinner than others, some are longer than others. Nevertheless, it is easy to see that if they were selected based on usage and then used as a blueprint to produce more hammers, you would have very different tools after some time.

 

I don't want to beat this idea to death, just seems that variation is natural in processes. Intelligence would have been required to maintain that first living goo exactly the same as it was when life on earth began.

[Xerxes]

Look, the logic is *not* that complex.

 

Premise one: heritable variation exists. (obviously true)

Yes

Premise two: some of this variation affects the ability of the organism to replicate and pass on that variation. (obviously true)

Forgive my editing your quote (bold and italic), but this is an important point. It's not "obviously" true, but an assumption. But that's OK, as it is a premise, in your words.

Premise three: not all offspring survive (see malthus).

Malthus? Not, I think, a reliable source, but again a valid premise.

 

So, if you have variants, and some make more copies of themselves than others, the relative proportions of the variants in the population will change. Simple as that.

Well, I don't like that way of putting it, but fine. How does this imply natural selection?

 

Look, I know this thread is supposed to be about natural selection, so maybe I'm spoiling the party. But it is an important point.

 

Evolution was once described by a man whose name I always have trouble spelling, as the change in allele frequency over time. Which is roughly what you were saying.

 

But....we know of at least three classes of mechanism that explains this phenomenon: natural selection, drift and allopatry. So at least two out of three evolutionary engines are nothing to do with selection.

 

There was in the 1960's, I think, a heated debate as to whether selectively neutral mutations could ever become fixed in a population. We now know (isozyme polymorhisms, DNA sequence polymorphisms in non-coding regions) that they can and do become fixed, and this is therefore evolution in the absence of selection.

 

Oh, and by the way. If others don't like my use of terminology, that's fine with me. However, I reserve my right to use it as long as I am willing to explain it when asked (which I always am).

[CanadaAotS]

Whats allopatry?

[Mokele]

Forgive my editing your quote (bold and italic), but this is an important point. It's not "obviously" true, but an assumption. But that's OK, as it is a premise, in your words.

 

Well, it's been experimentally verified, so it's a moot point; there's loads of literature about heritable variations in the performance of numerous ecologically relevant tasks.

 

Malthus? Not, I think, a reliable source, but again a valid premise.

 

How is he not a reliable source? His work on demographics and the inherent limitations of populations forms the basis of all ecology.

 

Well, I don't like that way of putting it, but fine. How does this imply natural selection?

 

That *is* natural selection. Natural selection is defined as "unequal propagation of genotypes", meaning that if different genotypes which result in different phenotypes in turn result in different lifetime reproduction (due to differences in survival, reproduction, reproduction of kin, survival of offspring, etc), the proportions of those genotypes will change. There's all sorts of sub-sets of natural selection, such as kin selection, sexual selection, and the issue of quantity of offspring versus quality, but all of them boil down to heritable variation in the ability to make copies of oneself.

 

But....we know of at least three classes of mechanism that explains this phenomenon: natural selection, drift and allopatry. So at least two out of three evolutionary engines are nothing to do with selection.

 

True, but as you said, the thread is about natural selection. Nobody denies drift and various other factors, they're just not the main point of this thread.

 

I'm not entirely sure what you're getting at here. Is it the issue of how do we know a trait is there because of selection rather than because of drift? If so, I'd recommend a work simply called "Adaptation" by Lauder (possibly part of a book; I've only got a photocopied version, and that's back at lab), which details the various methods one can undertake to make the case for a trait being an adaptation and the shortcomings/limitations of each.

 

The definition you posted, change in allele frequency over time, is AFAIK the common one, with selection, drift etc being the various mechanisms occuring either separately or together to produce evolution. However, well, to be blunt, selection is just plain more interesting, and thus gets more attention. I know in my work, though I try not to be adaptationist, there's a strong emphasis on selection, since it's pretty easy to see how variation that affects the locomotor performance of an animal can be selected for (running away from predators or chasing down prey).

 

Oh, and by the way. If others don't like my use of terminology, that's fine with me. However, I reserve my right to use it as long as I am willing to explain it when asked (which I always am).

 

I'm not sure what terminology you're using. So far it doesn't seem that different. The only term I find odd is allopatry, which literally means having non-overlaping ranges. While I'm familiar with allopatric speciation, I usually only see the term alone in descriptions of biogeography or species ranges.

 

For reference, I'm using the 'change in allele frequency' definition of evolution, the 'unequal propagation of genotypes' definition of selection, and 'change in allele frequency due to population-size-related sampling error' for drift.

 

Mokele

[Xerxes]

I'm not entirely sure what you're getting at here.

Neither am I any longer!

Is it the issue of how do we know a trait is there because of selection rather than because of drift?

Ah yes, that was it. I do not like, on purely theoretical grounds, the bland assumption that only selection drives evolution.

However, well, to be blunt, selection is just plain more interesting, and thus gets more attention.

You see, I happen not to agree. But that's cool (no doubt you don't find cricket as interesting as I do?)

The only term I find odd is allopatry, which literally means having non-overlaping ranges.

Yes, well more literally, different father(land). But I use it, I don't think too abusively, to refer to any situation that restricts interbreeding. Not all of these, of course, are geographic.

For reference, I'm using the 'change in allele frequency' definition of evolution, the 'unequal propagation of genotypes' definition of selection, and 'change in allele frequency due to population-size-related sampling error' for drift.

 

Mokele

And I would use exactly the same definitions. Maybe I started off being too snooty, for which I apologize. I'm afraid I am a bit of a pedant in these matters. Ho hum.

[insane_alien]

Ah yes, that was it. I do not like, on purely theoretical grounds, the bland assumption that only selection drives evolution.

 

the only people i have seen make this statement are people who don't fully understand evolution. sadly the majority of these seem to be on the crazy side so when an innocent comes along people might be a bit hostile to them.

[Dak]

neutral evolution can be an inportant factor of evolution, but, without a mechanism to propogate advantageous traits above disadvantageous traits, i dont think it's a 'driving force' behind evolution as such.

[Xerxes]

Dak I am sorry to say this, as we had what I thought of as a good rapport on poor old WiSci, but....

neutral evolution can be an inportant factor of evolution, but, without a mechanism to propogate advantageous traits above disadvantageous traits, i dont think it's a 'driving force' behind evolution as such.

This is precisely the sort of loose language I was originally objecting to!

 

Define "advantageous". You cannot do it a priori, but only by looking at the way in which your "traits" are propagated over generations. This in no way implies selection, for the reasons I gave. Of course selection is an agent of evolution, but it is by no means the only one.

 

As to what you mean by "neutral evolution" I can only guess. Not heading towards some perfect future, perhaps? Maybe you meant neutral variation? But we know it exists stably as all sorts of polymorphisms on all sorts of sexually reproducing organisms.

[Mokele]

Ah yes, that was it. I do not like, on purely theoretical grounds, the bland assumption that only selection drives evolution.

 

I doubt many are assuming it's only selection, but that it's the original definition, the one aspect most people are familiar with, and the one responsible for adaptations. When talking with those who aren't really familiar with evolution (basically 99% of the US population), it's usually the best place to start, especially since it can be grasped fairly easily (while drift and such require more extensive familiarity with genetics, sampling, stats, etc).

 

You see, I happen not to agree. But that's cool (no doubt you don't find cricket as interesting as I do?)

 

Not as anything more than lizard food, at least. But to each their own...

 

Yes, well more literally, different father(land). But I use it, I don't think too abusively, to refer to any situation that restricts interbreeding. Not all of these, of course, are geographic.

 

Ahh, I'm used to using it only for geographical range disjunction, while other forms of genetic isolation that occur in sympatry being termed things like non-random mating, pre- and post-mating barriers, etc.

 

I'm afraid I am a bit of a pedant in these matters.

 

Oh, I know how that goes; I chastize people for applying the term "run" to locomotion that doesn't precisely fit the definition of running gaits.

 

neutral evolution can be an inportant factor of evolution, but, without a mechanism to propogate advantageous traits above disadvantageous traits, i dont think it's a 'driving force' behind evolution as such.

 

Well, it can contribute or even drive speciation, fixing different genes in different populations so that, say, pattern changes enough that they no longer recognize each other as viable mates, or changing various genes that result in hybrid embryos being inviable.

 

Define "advantageous". You cannot do it a priori, but only by looking at the way in which your "traits" are propagated over generations.

 

Well, that depends. I'd argue that some traits *can* be defined, a priori, as advantageous. A good example would be more potent venom for a snake; it kills the prey faster, thereby reducing the risk of injury to the snake or the risk of loss of prey, but doesn't actually cost any more to make, as it's just a differently-shaped protien (same amino-acid number would mean same metabolic cost to manufacture a given quantity). This one-sided selection is likely why many snakes have venom so much more potent than what they need to kill the mice and rats they eat.

 

Also, I don't think Dak's language was loose. He was using "advantageous" as simply another word for "higher fitness".

 

Mokele

[Dak]

no need to appologise xerxes

 

a thought experiment:

 

imagine we have a population that is only experiencing nutral evolution (genetic drift, alopatry etc). over time, their allele frequency would change, perhaps due to random fluctuation (gene drift), sexual isolation (allopatry), and other factors. they would hardly ever adapt to fit their environment as the changes would be, essentially, random. occasionally, the species would, by pure chance, experience changes that did make them more suitable to the environment, but as 'neutral evolution' has no mechanism for maintaining certain alleles over others, this advantage of suitability would eventually be lost by the mechanism that caused it in the first place.

 

now imagine a population that experiences only natural selection. in this population, bereft somehow of alopatry, gene drift etc, would still, over time, adapt and become more suitable to their environment due to natural selection.

 

in other words, if:

 

evolution = change in allele frequency over time

f = allele frequency

n = number of alleles in population

p = population

and, for simplicity, f = n/p

 

then genetic drift changes n and thus f (is evolution); allopatry changes both n and p, and thus f (is evolution); natural selection changes n (and thus is evolution).

 

however, if we consider evolution to be a change in allele frequency over time that is generally accompanied by either an increase in average suitability to the environment or a maintanance of the average suitability to the environment, then, imo, NS is the only mecanism that can explain the bold phenomena above; neutrual evolution (gene drift, allopatry etc) has no mechanisms to 'guide evolution to improving suitability to the environment', hence 'neutral' evolution (im aware that im being quite loose with my wording in this post, btw, but only because i think it improves brevity without sacrafising clarity ).

 

so... i guess what im saying is that:

 

Ah yes, that was it. I do not like, on purely theoretical grounds, the bland assumption that only selection drives evolution.

 

if we take evolution to be change in allele frequency over time, then i'd agree; NS in all it's forms (natural selection, selfish gene etc), and neutral evolution in all its forms (gene drift, allopatry), along with artificial selection etc, all contribute to a change in allele frequency over time (i.e., 'drive evolution').

 

however, if we include the interesting repercussion of evolution into the definition (improvement over time), then only natural selection 'drives' it; gene drift et al contribute, and, i think, probably speed it up crap loads, but do not 'drive' it as such.

 

might just be a semantic thing, i.e. the definition of 'drives'. (actually, having read mokele's responce, i think it is.

 

Well, it can contribute or even drive speciation, fixing different genes in different populations so that, say, pattern changes enough that they no longer recognize each other as viable mates, or changing various genes that result in hybrid embryos being inviable.

 

i agree. but, reguardless, none of that will reliably result in an improvement over time without the help of NS, which is what i consider to be 'driving' evolution-by-which-i-mean-improvement-over-time.

 

as for your (xerxes) actual question, mokele answred it more-or-less as i would; 'more suitable' being 'confurs an increase in fitness to reproduce.'

[Xerxes]

Well, that depends. I'd argue that some traits *can* be defined, a priori, as advantageous. A good example would be more potent venom for a snake;

Then you and I have different notions of what a priori means. To me, if a venom kills more efficiently, then it is more potent. This is an a posteriori argument. Id est, observe the effect of the venom and conclude it is more potent. Anyway, this is getting silly - look at the conclusion of my respone to Dak, folowing.....

[Xerxes]

over time, their allele frequency would change, perhaps due to random fluctuation (gene drift), sexual isolation (allopatry), and other factors.

Yup, we all agreed that as a definition of evolution.

but as 'neutral evolution' has no mechanism for maintaining certain alleles over others, this advantage of suitability would eventually be lost by the mechanism that caused it in the first place.

Then you are insisting that neutral alleles are not maintained at "polymorphic" levels? We know that they are. This is a change in allele frequency, but you are right in a sense you don't possibly realize (aren't I cheeky!). It is not (necessarily) a continuous change (I can show you the calculus if you want)

 

 

however, if we include the interesting repercussion of evolution into the definition (improvement over time),

Use the word "improvement" again, and I'll find out where you live and come round and eat your goldfish.

 

Anyway, to Mokele and Dak: We don't really disagree in substance, merely language, which is not that interesting. I have a proposition to put to you clever guys, which you might well find controversial. We'll see.

Meantime, Saturday night, I'm going out to get hammered. Care to join me?