Talk:Natural selection/Archive 008

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[edit] Extinction and NatSel

Ok, this is getting interesting. Basically, when a species goes extinct because it is maladapted, it is not Natural Selection that is causing it, but other processes. That is just to weird... What are those processes and how are they different from NatSel. -- Kim van der Linde at venus 04:27, 9 July 2006 (UTC)

If we have a few children left in a sexually reproducing population and they are all eaten by a predator before reproductive age I fail to see how this could be drift. Since the predator didn´t select any particular children to eat, it just ate them all, it is arguably not selection either. — Axel147 14:00, 9 July 2006 (UTC)

Basic problems: 1) whatever natural selection is, we'll all agree that there is no intelligent designer behind it. So how does natural selection "know" whether there are only a few individiuals or just one individual or many? 2) how do we know the predator didn't select children to eat? Who should we ask? Gleng 21:26, 9 July 2006 (UTC)

Just some points of clarification. The reason I´m ruling rule out drift is because this occurs as a result of sampling. Extinction happens to an entire population rather than a sample of it so (unless we widen our use of drift to include random processes that can happen to entire populations) extinction cannot be explained by drift. Natural selection would also be ruled out if the children had equal fitness. However, I don't see any of this as problematical as (in a sense) extinction is not evolution: gene frequencies do not change from one value to another. Instead they 'change' to an undefined value, no value at all.
So drift cannot result in extinction of species, but it can lead to extinction of traits within a species. The liklihood of a trait surviving extinction (given enough time) depends on its starting frequency and its relative fitness. With a low population size starting frequency (drift) is the dominant factor, whereas in a large population relative fitness (natural selection) dominates. Despite this drift is still directionless in that it doesn't alter the expected frequency of the trait. (In a similar way I can still run out of money playing a gambling machine even if it is biased in my favour, but this is less likely to happen if I bet the same amount of money in small stakes.)
Just to check we are all on the same page here let's see what happens in this case. We have 10 phenotypically different individuals with equal fitness. 5 get eaten. Gene frequencies change and our population has evolved. Now is this caused by drift or natural selection? It has to be drift. Sure, some traits are preserved while other equally fit alternatives are eliminated. But differences in traits did not influence the process. Natural selection is the extent to which reproduction is biased by trait differences. But in this case there are no fitness differences, no traits have additonal use or confer additional benefits on our individuals. There is nothing for nature to choose between and therefore no natural selection. — Axel147 16:15, 10 July 2006 (UTC)

How do you know there are no fitness differences except by who survives?Gleng 20:23, 10 July 2006 (UTC)

Well biologists may only be able to measure fitness differences by looking at who survives. That doesn't mean we have to define natural selection in terms of a result. It is more like a starting point: is there anything to select? We can imagine a predator looking for 5 who were easier to kill. They all happened to have the same overall fitness even though some were slower but stronger. So it just ate 5 at random. (I´m simplifying a bit here to illustrate the point.)
Let´s do an analogy with artificial selection. Imagine I have a bag of 10 balls: 6 red and 4 green. If I pick 3 green balls this is artifical selection. But instead if I pick 3 ´red or green´ balls each ball has equal chance of coming up. This is no longer artificial selection but random sampling. (And either colour may become ´extinct´.) — Axel147 23:16, 10 July 2006 (UTC)
Maybe. But we don't know and can't know. You're trying to define natural selection as some separate force, some metaphysical power that has a special meaning, in ways that can be taken as ascribing it some purpose reflecting some underlying direction or intelligence, or as something that only some overseeing intelligence can divine. There is a clear danger of "deifying" concepts, and this brings problems. Look at your answer above. Fitness is not an absolute, because it depends on the environment in its most broad sense - an organism might be well fitted to one environment but badly adapted for another, and the environment constantly changes - not least because in part the environment includes all the other individuals present within it. So it isn't really true to think of fitness as something independent of survival and reproduction - in the end that's all there is - fitness is survival and reproduction, there's nothing else. We can measure the contribution of a trait to fitness by estimating its value for survival and reproduction if we have enough representatives of the trait and enough data. But if we can't measure these because we don't have enough data or the sample is too small, it doesn't make sense to say that there is some different force acting or some different explanation. Gleng 09:54, 11 July 2006 (UTC)
I am most certainly not trying to define natural selection as a 'metaphysical power that has special meaning'! And I think you are wrong about fitness. Fitness is certainly different from actual survival and reproductive success. It is like life expectancy. I may have a high life expectancy but could get hit by a bus tomorrow. In terms of measurement I said the definition of natural selection should hold firm irrespective of man´s difficulties in measuring it. — Axel147 12:17, 11 July 2006 (UTC)
Don't mean to imply that you're intending this consequence. I was just trying to brutally exclude any sense in which there is someone or something that "does" the natural selecting. The argument about fitness is a universal problem about defining probabilities. Probabilities are estimates of the likelihood of an event happening. After the event there are no probabilities, only certainties. So we can talk about the probability of Italy winning the world cup only before; after, it no longer makes sense. So if you see fitness as probability, like life expectancy, it is an imperfect estimate based on incomplete knowledge; with complete knowledge, there would be no probability, only certainty, the certainties of who will survive and who won't. So I don't think you can ever separate fitness from how it's measured; the estimate will change depending on how much knowledge is available to you.Gleng 13:16, 11 July 2006 (UTC)
Italy´s probability of winning the world cup before the final was 56% (according to the odds on betfair). That still has explanatory value even now: it helps explain why they won. (It is just that it no longer has predictive value.) — Axel147 14:43, 11 July 2006 (UTC)
This is what I'm trying to avoid; it doesn't explain anything - as an explanation it is irretrievably circular; it reduces to they won because they were more likely to win Gleng 15:57, 11 July 2006 (UTC)

I think you're trapped in endless paradoxes. If you want to keep natural selection as meaning the theory of evolution by natural selection, OK, this may be common useage. But if so, don't try to use this also as a rigorous definition that you can use coherently in a logical argument. Gleng 09:54, 11 July 2006 (UTC)

I´m not trapped in endless paradoxes and, believe it or not, I´m no longer arguing the definitional point about inheritance. In these arguments I am using standard phenotypic defintions e.g. non-random differential reproduction.
Returning to my example I was suggesting the predator was performing the role of nature single-handedly. Let's imagine instead that 5 of the individuals get fat. Say this increases their risk of predation but at the same time reduces their susceptibility to disease, in such a way that overall fitness is unchanged. The predator decides to have a meal of 3 individuals, while disease kills 2. Now of course it is true to say the predator was likely to have eaten fat individuals. Similiary the disease is more likely to have killed thin ones. We could say the predator selected fat individuals, while disease selected thin ones. (Observation lifted from Sober.) But overall there were no fitness differences. 'Nature' overall did not do any selecting with respect to overall reproductive success. There was nothing to choose between these individuals in terms of overall reproductive success. As such there is no natural selection.
Note that unless we have this distinction it does not make sense to think of natural selection as one mechanism of evolution and genetic drift as a different one. Natural selection would subsume genetic drift. (I wouldn't have a problem with this view but genetic drift would then become the stochastic component of natural selection. I don't think this is standard?) — Axel147 12:17, 11 July 2006 (UTC)
Seems to be how drift is defined in WP.Gleng 13:16, 11 July 2006 (UTC)

[edit] Adam and Eve

Suppose we have two individuals in a population. Let´s call them Adam and Eve (just to play devil´s, or god´s, advocate). The point I am making is whether they go extinct or have children has nothing to do with natural selection. But is what happens next caused by divine intervention? No, of course it isn´t, I haven´t suddenly turned creationist — it is just evolution by a different mechanism!

If Adam and Eve´s reproductive success are tied together they must both have equal fitness. (And this is true even if Eve is susceptible to disease and Adam is not.) So Adam and Eve and all their traits have equal chance of reproducing. Natural selection cannot occur because it has no material to act on. There is no variation. It is only when they have children that fitness differences arise.

Clearly what happens does depend on the liklihood of Adam and Eve surviving the prevailing conditions and that is influenced by their characteristics. But we are not forced to label all natural processes that impinge upon reproductive success as 'natural selection'.

Natural selection requires variation in fitness before it can occur at all (and it requires heritable variation in fitness for it to produce evolutionary response).

(I realise we could puncture this argument by trying to 'force' fitness differences. We could argue Adam is fitter because he might reproduce with his daughter. Or we could talk about cheating during meiosis. But this is not the point. The point is that we are not obliged to name all process that influence reproductive success as 'natural selection'. Natural selection is the set of processes that bias reproductive success according to phenotypic differences.) — Axel147 14:43, 11 July 2006 (UTC)


"Natural selection requires variation in fitness before it can occur at all" I think we've probably played this out, and it has been fun :) As a last riposte I'd just say that natural selection requires that some survive and some do not, and fitness simply is an estimate of who is likely to survive. If we had enough knowledge, our estimate would be certain knowledge of who would survive and who would not; as our knowledge is imperfect we can only make a guess, and the value we give to fitness depends on how much knowledge we have. For Adam and Eve, their fitness is 1 or 0. Now while we know that there are only Adam and Eve, they don't know that - and for all we know there might have been a million others who somehow failed to find the apple. Now my only point is that you cannot sensibly describe what happens to them as natural selection or not depending on whether there are many others or no others. What happens and why it happens to them is the same, unless you think that the mere existence of other people somewhere in the garden alters the nature of the influences on A and E. So I'd say that natural selection is simply to be understood as the absence of artificial selection - it is the assertion of the absence of a God, a designer, a directing intelligence. It is whatever happens that means that some die and some don't. And that this, this absence of direction, this absence of intelligence, when there is variation and heritability, is enough to result in adaptive evolution, is the radical ideaGleng 15:57, 11 July 2006 (UTC)


The problem we are faced with (Kim´s orginal concern I think) is that if a lion happens to eat one of a species it probably doesn't know or care whether it is the last one. From its perspective the process is the same whether or not it causes extinction. The only answer I have to this is that from nature´s perspective there is no choice to make between individuals of the same species if we are down to just one. (One way round this problem is by dropping the word 'process' out of the definition as many authors do e.g. Futyama. But I not suggesting we do this.)

I agree that in a sense nature is still 'selecting' whether an individual survives or not, but I don´t think this is what we mean by 'natural selection'. The distinction is important to separate (conceptually at least) processes that may change gene frequencies in a predictable direction, from those which alter gene frequencies at random. Here is a published view on this. Once again Sober p159:

Natural selection is one mechansim that can destroy variation. For it to act at all, there must exist variation (in fitness). But once a selection process begins it gradually destroys the conditions needed for its continued operation. Selection eliminates variation in fitness, and thereby brings itself to a halt.

— Axel147 17:42, 12 July 2006 (UTC)

"Selection eliminates variation in fitness, and thereby brings itself to a halt." This may be Sober´s POV and perhaps there is a place for it but I do not think it is a fair representation of mainstream theory of evolution and natural selection. Sober´s claim (in this one sentence at least) is simply not true. It would be true only if one sees variation as a precondition and in static terms. This is consistent with creationism (we start out with all the species God created, though some are now extinct). Darwin understood that there must be constant forces generating variation, even though he did not know that they are. in other words, variation is not a chronological precondition for NS, it is a conceptual precondition. Variation itself is not static, it is not that there "was" variation at one point in time. Just as there are selective forces, there are mechanisms that lead to variation. it is a constant process. user:Slrubenstein
For particular alleles, and for everything but balancing selection, this is true, though. Selection produces fixation, which means there is no variation in fitness any more - at that particular site. Graft 19:16, 14 July 2006 (UTC)
Just to clarify the context, Sober is talking about natural selection acting by itself here. Of course in practice there are mechanisms to generate and preserve variation. For a population of phenotypes (and this applies at site level in Graft´s balancing selection case)
  1. Natural selection does its job of eliminating the unfit
  2. ´Mendelism´ does the job of re-creating variation (in fitness)
So although in theory natural selection could grind to a halt (if everyone had equal fitness), it would soon be up and running again as soon as individuals reproduce (assuming no extinction). — Axel147 23:22, 14 July 2006 (UTC)

For completeness I suppose I should add that if one considers 'differential speciation' to be a form of higher level natural selection e.g. Gould, extinction could count as a form of natural selection after all! (If we have more than one species.) — Axel147 13:19, 22 July 2006 (UTC)

Yeah, you know, there are reasons why Gould's reputation among theorists is not quite the same as it is in the general public/popular press. Pete.Hurd 18:10, 22 July 2006 (UTC)

[edit] Inconsistent use of the word 'trait'

According to Wiki 'In biology, a trait or character is a genetically inherited feature of an organism.' If this is true why does the article say 'but only the heritable component of a trait will be passed on to the offspring'. Which is correct? Is a trait by definition heritable? — Axel147 20:20, 16 July 2006 (UTC)

We are using trait in the original, normal language definition, as a distinguishing characteristic of an individual rather than the more recent more specialised sense. Science has a way of hi-jacking common words; I've no idea what we should do - maybe a footnote?Gleng 20:42, 16 July 2006 (UTC)

I corrected the article on that point. [1] Traits can be fully or partly genetic, or not at all. Hence QTL analysis - carried out when the inheritance of the trait is not exactly known. - Samsara (talk • contribs) 08:17, 17 July 2006 (UTC)
Good, what corrrection are you putting in? Maybe I'm just being pedantic here, but we also using trait inconsistently in another respect: in the opening line but not elsewhere it refers to the state or 'value' of a feature (e.g. brown vs. eye colour). Also suggestions traits can be decomposed into parts some of which are heritable - different saying a trait as a single entity has different extents to which it is heritable. — Axel147 08:32, 17 July 2006 (UTC)
Unfortunately, trait is not used consistently in genetics. The Stansfield dictionary equates trait with character and phenotype (as does Hartl's old Human Genetics text). Some people use trait to refer to carriers, such as talking about individuals who are sickle-cell carriers as having sickle trait (vs. sickle-cell disease for homozygotes). Still others (myself included) use trait to refer to the gene, with characteristic as the phenotype (I don't know when I picked that up. It may have been when I was teaching Human Genetics). TedTalk/Contributions 13:47, 18 July 2006 (UTC)


[edit] Fitness and Ecological Selection

In the fitness entry we still have the other use! 'If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This process is called natural selection.'

Similarly, ecological selection (commonly natural selection excluding sexual selection)....

  1. Ecological refers to strictly ecological processes that operate on a species' inherited traits without reference to mating or secondary sex characteristics (Ecological selection)
  2. Ecological selection occurs when organisms that survive and reproduce increase the frequency of their genes in the gene pool over those that do not survive. (Evolution)
  3. Ecological selection covers any mechanism of selection as a result of the environment. (Natural selection)

These articles seem stable but inconsistent. I am no longer fresh to this so don't know if it's important to do anything about it! — Axel147 17:08, 10 August 2006 (UTC)

Natural selection does not include sexual selection, they are different. Slrubenstein | Talk 19:10, 10 August 2006 (UTC)
This has always been my understanding as well. The relevant WP entries have always (since I started paying attention, [2]) maintained that SL is a subset of NL. It would be nice if someone went and fixed and referenced this. Pete.Hurd 19:38, 10 August 2006 (UTC)
Darwin considered sexual selection a subset of natural selection, and this distinction would seem to be upheld in that we do not count sexual selection as a fifth microevolutionary force. Mate choice and competition evolve as a result of natural selection, so how can they not be a subset? - Samsara (talk • contribs) 20:19, 10 August 2006 (UTC)
Samsara, perhaps I was mistaken. I thought Darwin made this distinction in OoS. I defer to you and others, Slrubenstein | Talk 20:27, 10 August 2006 (UTC)
Hmmm, my students seem to have wandered off with the best references, but they've left Helena Cronin behind, and she has a chapter on this, Ch 10. It begins "For classical Darwinism sexual selection was an oddity, entirely different from natural selection and generally opposed to it..." then documents the shift, over time, to incorporating intraspecific selection pressures (ie sexual selection) into "natural selection". Like I say, it's my impression that the view that SS is not a subset of NS is quite common (at least among the sexual selection crowd). Cronin seems to suggest that this view is still held by a minority. I'd be happier with references documenting the taxonomy of selection types presented in the selection article than an argument by pure logic. Because, even if this is the current view, it was not always that way. Pete.Hurd 21:29, 10 August 2006 (UTC)
Well, you're right about the direction of effects and the feeling that it was an oddity, but from my reading of The Descent of Man, and Selection in Relation to Sex (note the title!), there was no doubt in Darwin's mind that at the bottom of it was natural selection. - Samsara (talk • contribs) 21:35, 10 August 2006 (UTC)
Hmmm, ok, I'm wrong(ish) yet again. Endler's Natural selection in the wild replicates the taxonomy, but calls what selection has as "ecological selection" "narrow sense natural selection" and then has "narrow sense NS" + SS = NATURAL SELECTION. Endler presents a nice review... Endler says "Darwin (1859, p88; 1871) made a careful distinction between natural selection and sexual selection ..." my copies are (of course) elsewhere but it would be interesting to see what he says. Pete.Hurd 21:41, 10 August 2006 (UTC)
Sorry, Sexual selection is a integral part of Natural selection. The contrast is with artificial selection. -- Kim van der Linde at venus 03:44, 11 August 2006 (UTC)
Yes, I agree sexual selection is a subset of natural selection. But this is not in contrast with artificial selection: artificial selection (providing we are talking about differential reproduction of some biological entity) is also part of natural selection. I think this selection diagram is wrong. — Axel147 12:52, 11 August 2006 (UTC)
Sigh, artificial selection is not natural selection, even darwin makes that clear. -- Kim van der Linde at venus 17:24, 11 August 2006 (UTC)
Hmm... that distinction is a bit out of date, isn't it? I'm thinking about artificial selection in the 19th century, which wasn't much different from humans being domesticated from the savannah (EEA) into the metropolital environment, vs. today's breeding efforts using best linear unbiased predictors (BLUP) and QTL analysis, which is an entirely different kettle of fish. I see it more as something that Darwin used as an explanatory tool. Darwin, of course, saw the two as closely analogous, enacting the same process. I would say that sexual selection can be as much affected by bringing creatures into captivity as "ecological selection" can. I do wonder what textbooks say on this. - Samsara (talk • contribs) 13:31, 11 August 2006 (UTC)
The only difference between artificial selection and natural selection is that one is performed by something with a sense of agency (i.e. humans) while the other is not. Darwin invoked artificial selection because its powers of transformation over the form and function of the organism were well known in his day. The "natural" element is just Darwin saying, "OK, you know how people breed horses, right? Well imagine that nature is breeding the horses, just by the fact of its day-to-day existence!" It is, in the end, a difficult distinction in many cases, I think, for the same reasons we have trouble in identifying what exactly separates human agency from non-human agency (i.e. a human builds something and we call it agency; a beaver builds something and we call it an extended phenotype). --Fastfission 17:40, 11 August 2006 (UTC)
Yes. No need for Kim to sigh! Artificial selection is a special case of natural selection as long as biological entities are differentially reproducing. Sober agrees anyway...
Artificial selection is not selection that takes places outside of nature, but selection that occurs within a particular niche found in nature. The [Waddington] experiment involved the interaction of members of one species with members of another. The fact that one of those species happens to have been Homo sapiens sapiens does not affect the point. Artificial selection is a variety of natural selection; the relation is one of set inclusion, not disjointness.
— Axel147 18:12, 11 August 2006 (UTC)
So *all* selection is natural selection, and the word "natural" means nothing? Endler (1986) flat-out contradicts Sober's view. Pete.Hurd 20:07, 11 August 2006 (UTC)
Oh. What does Endler say? (I've only got the 1st chapter that you sent before.) What happens if I'm hungry and I go into the woods and shoot an animal. Is it natural selection? Does it make a difference if I eat it, kill it for fun or as an experiment? — Axel147 20:30, 11 August 2006 (UTC)
"Does it make a difference if I eat it, kill it for fun or as an experiment?" Hairsplitting sophistry. You're saying that there is no such thing as articifial selection, that the term is meaningless? Pete.Hurd 20:45, 11 August 2006 (UTC)
He's just saying that if you treat humans as part of the natural world then there is technically no ontological difference, since the distance between humans and the natural world is just a story humans like to tell themselves. But the term clearly has meaning, even if that meaning is largely as a heuristic—emphasizing the difference between active intelligent agency and un-guided natural operations maintains its usefulness in thinking about evolution, even if one doubts that human intelligence is removed from the sphere of the natural. --Fastfission 20:56, 11 August 2006 (UTC)
I agree with Fastfission here. — Axel147 21:00, 11 August 2006 (UTC)
The word 'natural' means something in that 'nature' is, metaphorically, the agent doing the selection. This from Wallace to Darwin is partly relevant and interesting anyway (abridged)...
My dear Darwin,— I have been so repeatedly struck by the utter inability of number of intelligent persons to see clearly, or at all, the self-acting and necessary effects of Natural Selection, that I am led to conclude that the term itself, and your mode of illustrating it however beautiful to many of us, are yet not the best adapted to impress it on the general naturalistic public...I think [the difficulty in understanding] arises almost entirely from your choice of the term Natural Selection, and so constantly comparing it in its effects to man's selection, and also to your so frequently personifying nature as 'selecting', as 'preferring', 'as seeking only the good of the species', etc., etc. To the few this is as clear as daylight, and beautifully suggestive, but to many it is evidently a stumbling block....I wish, therefore to suggest to you the possibility of entirely avoiding this source of misconception in your great work (if now not too late)...by adopting Spencer's term viz. 'Survival of the Fittest'. This term is a plain expression of the fact; 'Natural Selection' is a metaphorical expression of it, and to a certain degree indirect and incorrect, since, even personifying Nature, she does not so much select special variations as exterminate the most unfavourable ones.
— Axel147 21:00, 11 August 2006 (UTC)

There is a critically important distinction between artificial selection and natural selection, which I think is fundamental and essential not to blur. In Darwin's sense artificial selection involved design, directed intent. Natural selection does not. Gleng 19:22, 12 August 2006 (UTC)

Yes, I know what you mean. But I'm saying (like Sober) artificial selection is a special case of natural selection. It is still important to clearly identify what that special case is. 'Design, directed intent' do seem to be at the heart of it. I'm struggling with the exact boundary. Maybe one animal can kill another through intelligence or 'directed intent'? Maybe all selection involving man should be classified as 'artificial'? The more serious point is I think the opening sentence 'Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce' is correct. But you seem to be suggesting we need to qualify this with 'except if they are killed in experiments by man!' — Axel147 21:58, 12 August 2006 (UTC)

It's not ideal. It was and is obvious that artificial selection can change the form of species, but this process involves intent to produce that particular change, it is a deliberate exploitation of variation and inheritance by an intelligent designer. Darwin's insight was that no intent is necessary. I agree with you, that in the broad scheme of things then artificial selection is just part of our extended phenotype, but appreciating this requires both subtlety and rewriting Darwin even more extensively. I thnk it's important not to blur Darwin's insight that no designer is needed, so the distinction between artificial selection and natural selection is a useful one. Take away this and you don't need the word "natural" . Gleng 07:57, 14 August 2006 (UTC)

Yes, fair enough. I wonder if anyone thinks the Wiki artificial selection has got it wrong by including 'unintentional modification of a species through human actions'? If artificial selection is not a form of natural selection, this suggests an absurdity: man has evolved principally through artificial selection rather than natural selection! (In my view the meaning of natural selection does not depend how we define artificial selection so I'm not so concerned!) Maybe I'm just being pedantic, if so sorry! — Axel147 16:45, 14 August 2006 (UTC)

I hadn't seen that, and I agree with you; I think it is just silly not to see humans as part of the natural world.Gleng 14:08, 15 August 2006 (UTC)

I think insisting that artificial selection is a subset of natural selection through some reducio ad absurdium argument about humans not being "natural" airbrushes out a long, historically relevant, debate about teleology in selection. Pete.Hurd 17:22, 15 August 2006 (UTC)
i agree, hadn't meant to imply that I agreed that artifial selection should be classed in this way, but was agreeing that the definition of artificial selection appeared to be too broadGleng 17:37, 15 August 2006 (UTC)
Whups, I've made a misunderstanding, the comment was directed more at Axel, my bad. Pete.Hurd 18:48, 15 August 2006 (UTC)
No such airbrushing is needed! It is perfectly coherent to say artificial selection is a special case that requires intelligent manipulation, whereas natural selection in general does not. The point is natural selection can occur without intelligent manipulation, not that it always has to. Isn't the antibiotic resistance example to some degree artificial? — Axel147 12:09, 16 August 2006 (UTC)
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