Is there more than one definition of natural selection?

Hold on a minute. I saw a three-legged dog the other day and it never occurred to me that he is a bearer of "the three-legged dog trait." I really don't think he is. I think all "traits" are genetically determined characteristics. If not, then, by your reasoning, a dead dog suffers from "the dead-dog trait."”HM

Percy writes: Yes. Percy, what leads you to say "...but selection pressures were present just like they always are, and so the process of natural selection was still in force"? There is nothing in your scenario that means or implies that "selection pressures were present." How would you know this if there is no selection? Unless there is differential reproductive success amongst individuals, there simply is NO natural selection. Period.”HM

I've printed out Millstein's paper but I still haven't read it (weather's been too nice today). But I will read it to see what I can learn, and comment later. Good question. NS seems to have a zero or rest state, and it seems to have an active state with variation of activity, but I don't know if a scale of effect would be possibly to draft or measure. Still, philosophically, at least, anything that has a 'differentiality' should be measurable somehow. (I guess many of us have already argued at length on another thread about where we should insert the probe of an NS-detection meter, if there were one.)*”HM*I expect crashfrog will drop by soon to tell me where to insert the probe. Edited by Hoot Mon, : typo

Good grief, Hoot Mon, are you as thick as a log? I gave you the argument, I added three analogies, and all you can do is restate your initial premise? How many ways do people have to explain things to you before they run off screaming in frustration, because that's going to happen long before you understand anyone's argument. You're like a Buzsaw-light - you have actual facts but there's still no comprehension.Hi Hoot Mon,It will difficult to give you a reply because you're just restating your initial position. I gave you the argument and added three analogies, it would be really helpful if you could try to pick up the thread of at least something I said and directly address it. I'll take a stab at this anyway.Think of Quetzal's marble analogy and consider the case where all the marbles are the same size. The marbles still fall through selection holes, but their size causes them to all fall through the same selection holes and end up in the same place. Selection still happened, it just had equal results because of equal size.Or consider an argument that assumes your premise is correct and follows it to its logical conclusions to see if it holds up. Your premise is that if the differential reproductive success was 0 that there was no selection. Consider the example of three creatures, A, B and C, who all mate and contribute progeny to the next generation. You compare A to B and discover that they both left an equal number of progeny, there was no differential reproductive success, so you conclude there was no selection. Now you compare A to C and discover that C left more progeny, and so now you conclude A was selected against because he had negative differential reproductive success as compared to C. Since it is contradictory that A wasn't subjected to selection pressures when compared to B but *was* subjected to selection pressures when compared to C, the only possible conclusion is that your initial premise that differential reproductive success of 0 means no selection was occurring was incorrect.--Percy

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Hoot Mon wrote:
I think all "traits" are genetically determined characteristics.

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If only things were as simple. Unfortunately, even genetically identical organisms in identical environments can have different phenotypes. For example, the lac operon possessed by some bacteria includes a heritable epigenetic switch that determines whether or not the organism in which it resides will be able to utilize lactose (i.e. the trait) as a source of energy. If you have the (intact) lac operon you have the potential to metabolize lactose. Thing is, your ability to do so will depend on whether or not your "parent" left the switch in the on position or not (i.e. it's heritable). So, you could find yourself in the situation where you are in a lactose rich environment and you still won't be able to metabolize lactose EVEN THOUGH your genetically identical "sibling" sitting 5 micrometres to your left might be able to.PLEASE NOTE: the lac operon is bacterial and when I wrote "you" above it implies that "you" are a bacterium (and I'm not claiming that you are Hoot Mon). Confused yet?Edited by Hawks, : possessor of lac operon added.

I hope you don't mind me jumping in as well.
I'm sorry if I'm stating what you already know, I would just like to clarify what you meant when you said 'NS seems to have a zero or rest state, and it seems to have an active state with variation of activity'.Anyway in both this and the previous thread, natural selection seems to repeatedly be referred to like an entity in itself, deciding what to smite and what not to smite. I've always regarded it as a collective term for the various obstacles organisms have to overcome in order to survive and pass their genes to the next generation, for example the availability of food/water, predation or disease.Now if the organism fails to overcome these obstacles it will die, effectively ending it's opportunity to pass it's genes to the next generation. If it fails to attract a mate, it won't necessarily die, but the outcome is the same in that it has failed to pass on it's genes.
Now which factors have the greatest selective pressure depends on the organism and the environment it exists in i.e. not all selective pressures apply equally and some may be non-existant for a particular scenario. In a harsh environment, a desert for example, the main selective pressure may be lack of food/water and the majority of organisms in a generation will die before getting a chance to mate. In this case attracting a mate (i.e. sexual selection) is a low selective pressure due to fewer individuals surviving. Is this what you meant in the other thread when you defined sexual selection as something separate from natural selection?Now consider a scenario where other selective pressures are reduced, for example plenty of food or less predation. Under these circumstances competition to attract a mate increases, so this becomes the predominant factor in natural selection as to who passes their genes onto the next generation.Have I got this right? I know it's a bit over simplified, I've just not looked at this for a while.

Percy, although I don't want to screw things up similarly to what happened on the other thread on NS, I'm not sure this is an accurate description. Possibly I'm misunderstanding what you're trying to say.Almost every definition of natural selection I've read emphasizes that in order for selection to occur, there must be some difference for selection to act upon. Whether these differences are allelic (as Mod, Justin and the other Defenders of Evolutionary Purism prefer), phenotypical traits carried by individuals, or even characteristics posessed at population or species levels, there has to be some consistent difference. As Douglas Futuyma defines it (in the glossary to Evolutionary Biology),

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The differential survival and/or reproduction of classes of entities that differ in one or more characteristics; the difference in survival and/or reproduction is not due to chance, and it must have the potential consequences of altering the proportions of the different entities, to constitute natural selection.

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As you can see, the key focus here (whether at the genetic level or the species), is that there must be some kind of differences for natural selection to operate. What you described with the marble example above would therefore not be natural selection - unless there was another group of marbles within the overall population that didn't pass through the holes for some reason.The conditions you describe could, of course, exist at least theoretically. It would be the ideal case of HW equilibrium. I can imagine a population of invariant organisms (at least invariant in NS terms) exposed to absolutely equal environmental pressures simply continuing to exist and reproduce ad infinitum, until and unless the pressures change. Whether such a population could exist in the real world, with all its constant microvariations is arguable, IMO.

You can define traits in that fashion if you want - but it leads you to difficulty in discussing how Lamarckism compares to Darwinism. I think it easier to simply say that Darwinism is about genetic (and thus inheritable) traits, whereas Lamarckism implies that acquired traits can be inheritable (such as a stretched giraffe neck).Just for extra confusion (as Hawks mentioned) - we have epigenetics. Then we can include acquired traits which are hereditary.My hair colour is a trait - but I can change it using dye. I can wear clothes to blend in to the background. These things can convey a survival advantage but these traits are not heriditary traits.If you'd rather - mentally replace the word trait with 'characteristic'.Edited by Modulous, : No reason given.

Hi Quetzal,I'm actually trying to accommodate myself to something closer to the way you and Modulous were looking at natural selection in the other thread. Though there were obvious differences in your preferred perspectives, there were also commonalities that I found I didn't share, and so I'm trying to adjust.My view of natural selection was very focused and individualistic. For example, when an organism dies during a period of environmental stress, to me that was natural selection. But looking carefully through several definitions on the web I found that that is more commonly just referred to as selection. When talking about natural selection these definitions tended to be addressing a higher level view of the process, a multigenerational view encompassing changes in allele frequency over time due to selection pressures and descent with modification (AbE: which appears to be the same as the definition as evolution, but I included everything that's going on in my description of the higher level perspective - natural selection still only refers to the selection aspects in this higher view).And so in my version of your marble example I referred only to selection, not natural selection, and it was intended to be analogous to only a single generation. Even though the marbles had identical phenotypes with respect to size, that is no guarantee that the next generation would all be the same size. The marble drop represents the selection part of the process, not the reproduction part. In the next generation some of the marbles may be of different sizes. Since selection pressures are operating in each generation, sometimes choosing more, sometimes less, sometimes all, sometimes none (extinction), the process of natural selection is going on. The fact that in one generation every marble survived doesn't mean natural selection suddenly stopped or was absent.I am trying to work toward a common definition, but as I've said before, the more finely the definition of a concept is sliced, the more different ways people will find to slice that concept into different definitions, giving the appearance of disagreement when everyone actually agrees on the specifics of what is happening in the real world that we're trying to describe.--PercyEdited by Percy, : Clarification.

Correct, if you observe it occurring across a population. Percy, you've spun me around with this one. It's your "negative differential reproductive success" that is troubling me. But I'll soldier on. This flies south on me. Are you saying a population can host three states or charges of natural selection: positive, neutral, and negative? I'm not comfortable viewing NS this way. NS can be either active or not; there is no negative activity that has any meaning in the context of differential reproductive success amongst individuals across a population. The trouble with your example is that it assumes NS can be analyzed amongst three individuals. I would be more comfortable looking across a population to do that. If NS can do what you say it does on a population of three individuals, I wonder what role random genetic drift would play in changing that population's allele frequencies. Probably quite large, as compared to the role of NS.”HM

Mod wrote: Yes, I would rather, because "trait," in my lexicon, implies heritability. Otherwise, we're going to struggle with a "three-legged-dead-dog trait," where neither Darwin nor Lamarck would have anything relevant to say about it.”HMEdited by Hoot Mon, : added subtitle

Then you are going to be very uncomfortable. Positive selection, negative selection, purifying selection, disruptive selection and frequency-dependent selection are all frequently discussed forms of selection.I think the main problem is that your view of what NS encompasses is too simplistic.TTFN,WK

There's nothing complicated here. If our unit of measure for differential reproductive success of two individuals were the difference in the number of progeny, what is the differential reproduction success of A with respect to C if C produced more progeny. If we call the number of progeny p, then: Since p_a < p_b, this will be a negative value. That's all it means. If you feel more comfortable with positive numbers then just reverse it and say you're measuring the differential reproductive success of C with respect to A, and then it's a positive number. No, of course not. I'm saying that any population can be divided into sub-populations. If you consider the sub-population consisting of A and B, then you conclude no natural selection, yet if you instead consider the sub-population consisting of A and C, then you conclude there was natural selection. How could whether A was subjected to natural selection or not vary simply by how you divide your population into sub-populations? It can't without being contradictory. Therefore it is incorrect to conclude that A was not subject to natural selection.--Percy

I think the basic difference between what I and Mod actually agreed with on the previous thread, and what you are trying to say here, is that natural selection per se doesn't include the kinds of chance events that you describe. Chance events obviously have an effect on individual survival, and may even change the proportion of alleles/genotypes/phenotypes existing in subsequent generations. However, NS is inherently "anti-chance". Let me try and explain this point using a hypothetical population.1. Chance: Assume a population containing 15 individuals. Ten of them have characteristic A, five have characteristic B. Neither A nor B provide any selective advantage in the current environment. The traits confer no difference in reproductive rate, survival, etc. In other words, all the marbles are the same size, but some are red and some are blue. A chance event occurs - lightning strikes the tree in which they all live. Simply by the luck of the draw, 3 A's and 4 B's are killed. Is this selection? I would argue "no". The proportion of A's and B's has changed - and assuming this breeds true - but it wasn't natural selection that changed them, simply because the differences between them had no bearing on either survival or reproduction. There may have arguably been selection of A's, since more of them survived, but there was no selection for A - which is the definition of NS.2. Natural Selection: Assume the same population. In this case, however, there is a climatic fluctuation - an ENSO event, for instance - where the characteristic B provides a slight survival advantage. More B's live to reproduce in subsequent generations. Slowly (depending on the severity of the pressures) B's will come to represent a larger proportion of the population over time. Both A's and B's are still reproducing "after their kind" , but B's are surviving better - and hence are providing a greater contribution to subsequent generations. If the pressure is maintained over sufficiently long time frames, it is possible that A's may become extinct, and the population contains only B's. This is natural selection, and at the end there it constituted evolution. If things go back to "normal" before this occurs, however, thenThis latter case is essentially what the Grants found in the Galapagos. You probably know the story. There was a severe, multi-year drought brought on by an extended ENSO. Long-beaked finches began to die off because they were able to forage less and less on the fewer, hard-shelled seeds. Short-beaked finches, better able to deal with harder seeds, had a slightly better chance of survival. Both types were hard-pressed, and died in droves (the population collapsed). It finally came down to a 4mm difference determining the potential for life or death. Natural selection for different beak sizes based on available food supply. However, unlike what someone mentioned on another thread, this wasn't evolution. Short- and long-beaked finches kept being produced in the normal proportions. The long-beaked birds simply died of starvation before reaching adulthood. This particular selection pressure simply didn't last long enough for the overall change in alleles to become fixed. When the weather went back to normal, so did the proportion of different sized beaks.Chance may promote one individual over another in a population, but it is random, and thus can not really be considered natural selection. Selection, on the other hand, promotes an individual based on its posession of a particular characteristic.

WK wrote: Should I also consider altruistic anti-selection? Not if you care to use the agree-upon definition of natural selectrion. Why do you need to complexify it? Don't anwser; I think I understand. It's that nagging issue of cause v. effect”process v. outcome”isn't it?Maybe so. In this respect, the paper by Millstein you asked me to read”“Are Random Drift and Natural Selection Conceptually Distinct?””is worth the reading. If I get the major thrust of it he is saying that they may be indistinguishable, depending upon whether you view them as processes or outcomes. I think he's right, mostly; cause and effect are not at all the same things. And this casts certain doubt on our efforts to differentiate the roles of NS and drift. (There's more to say about this paper, but I'll postpone it for now.)So we're back to worrying over cause-and-effect relationships. Indeed it may be productive for us to differentiate process from outcome, because many of us here don't bother to make that distinction.”HM