Talk:Genetic drift/Archive 3

From Talk:Genetic drift

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Not to presume that others aren't bored stiff and gone on a hiatus by now, but since this came to my mind, I'll post it now: In an earlier version of the article, I thought to do some explaining by way of a naturalistic example. I decided it was too long winded, but if people want examples, and if we keep the examples section I suggested, perhaps a piece of text along the following lines would be useful:

"Consider the quasi-realistic example of a hypothetical hoof-related gene of wildebeests, in which a rare allele confers a superior propulsion when treading water. Suppose that each year half of a herd drowns in crossing the roaring river that separates its summer and winter grazing areas. Who survives depends only partly on ability and partly on luck--whether a stampeding animal's neighbors in the water are prone to panic, for example. Nevertheless, carriers of the rare hoof allele do better when given a fighting chance, so they drown less often on average. As the years pass, the allele becomes more common. This is a case of natural selection.

"Consider now the roughly 50,000 other genes in the wildebeest genome for which there exist multiple alleles...."

I don't know how ecologically or "populo-genetically" sound an example this is (I do know from a nature video that wildebeests migrate to breeding grounds annually, that they may cross a big river to do so, and that many die in the river) so maybe someone can propose something better. What I like about the above is that it names a specific animal that I think will engage people's interest, it's explicit and naturalistic, it's oriented toward action happening on the human scale, it's told as a narrative and (to the extent it isn't confused about any of them) it makes the concepts stark 168... 16:18 21 May 2003 (UTC)

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I reverted the article because of two problems which I have been reviewing with 168. In his comments on this page he writes:

It amazes me that anyone could infer from the Nelson & Jurmain quote that lifespan is irrelevant to allele frequency, as you assert

which once again reveals his confusion. No one said that lifespan is irrelevant to allele frequency. Changes in allele frequence is one simple way of defining evolution, and clearly, lifespan is relevant to evolution. The question is not whether lifespan is relevant to evolution, but whether it is relevant to genetic drift. At this point I wonder whether 168 believes that drift is evolution, and not just one factor in evolution. According to the modern synthesis, drift is only one factor (sexual selection and natural selection are two other forces, and there are others). So to answer 168's question, lifespan may be relevant to drift, it may not be -- it depends. Accidental death is relevant to drift, but death as a result of selective forces is not. So the article must make clear that we are talking about accidental death (hence my revert).Slrubenstein

SLR, do you read the responses I make to your accusations? Let me quote myself: "If a disproportionate number of individuals of a certain genotype die earlier than the average, fewer than the expected number of such individuals will be present at the census from which allele frequency is calculated. Lifespan also may be the cause for why an individual produces more or less offspring than the average for its genotype." I am not supposing any selection as the cause of deviations from average lifespan this scenario. Do you deny that lifespan has a variance, that it fits something like a bell curve? Do you believe selection needs to be invoked to explain this? I have made a simple argument for why mortality matters to drift (and also referenced the assertion that appears in one teacher's one-sentence definition of drift on the Web). You provide no counter argument and you provide a text which does not even speak to the issue.168... 18:00 21 May 2003 (UTC)

It occurs to me that you are counting any death that isn't from "old age" as "selection," but when a piano falls silently on a sleeping gerbil, that is not selection. No alleles in the gerbil gene pool could have saved that gerbil. In common speach we talk of chance having determined the day that the piano movers would come. Likewise with all kinds of things that bring death to individuals. The bell curve I mentioned above is not one for individuals that die of old age, but for all individuals no matter the cause. For many species, old age is rarely the cause in the wild. 168... 18:21 21 May 2003 (UTC)

Sorry, but wrong -- remember, I included the example of accidental death in a car accident (which you deleted) tomake precisely this point. Please read what I write -- I never mentioned old age, but I did mention accidental death many times as a factor in drift. What I als did was explain how death may not be drift if it is a form of natural selection (the driver had poor vision) or if the population is very large. This is a very clear example that makes a number of important distinctions in any discussion of drift. Slrubenstein

I believe the right way to think about alleles that are subject to selection (and as I tried to explain the issue in my proposed version of the article) is to think about them making an individual's odds different than the odds of individuals with different genotypes. Let's talk about the odds of survival, and consider my wildebeest example above. The river crossing in a sense is "selecting" some individuals by drowning others. But not all possesors of the good-for-swimming hoof trait survive, and not all of those who don't possess it drown. So not all drownings represent "deaths by selection," and even under circumstances that select, there will be outcomes that deviate from the average, providing fuel for drift. It's hard for me to see how "death by selection" could be a sensible category of thing. This seemingly selective cause of death also leads to deviations from the average lifespan for individuals with no traits that confer unusual fitness or unfitness with respect to the potential cause of death. It seems to me that it is as much "death by drift" as "death by selection." 168... 19:18 21 May 2003 (UTC)

168 also continues to question the importance of HW, and where it should come in the article. Clearly, as Dobzhansky and Mayr make clear, HW is a baseline for any discussion of drift. In a way, drift is any deviation from a HW equilibrium. So before one discusses drift, one must explain the HW equilibrium (hence my revert). Slrubenstein

What I question is you placing HWE high and in the center of introductory and one-would-hope general remarks about drift. Dob. and Mayr are old guys and they are explaining drift in a way that mirrors the history of ideas in their field--a tradition in text book explanations. That tradition represents one heuristic device, but not always the best. But it is merely a tradition, and definitely not the best choice in this instance. I left HWE in my last posted version, but I had it lower down in the article. You might notice that in my edited version, I reversed your order, describing how the gamete issue can contributes to drift before remarking when it does, it constitutes a deviation from HWE. This shows that one need not explain HWE before explaining drift. SInce you base your latest reversion on the above points, and since I believe I have refuted them, I will undo your reversion, childish as this game is. 168... 18:00 21 May 2003 (UTC)

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Here we go again. An excerpt from the above discussion:

It amazes me that anyone could infer from the Nelson & Jurmain quote that lifespan is irrelevant to allele frequency, as you assert, 168...

No, I said it is not necessarily relevant to drift. This article is about drift, not evolution in general (i.e. changes in allele frequency)Slrubenstein

It was an awkward sentence and you misread it. Read it like this: "It amazes me that anyone could infer from the Nelson & Jurmain quote that lifespan is irrelevant to allele frequency unless it influences numbers of offspring, as you assert." It's largely my fault, but you misunderstand my point. 168... 18:05 21 May 2003 (UTC)

Although I appreciate 168's efforts at clarification, it does not bear on my point. I did not assert that lifespan is irrelevant unless if influences the number of offspring. (Let's leave aside the fact that offspring are relevant, since evolution in any form involves changes in gene frequencies from one generation to the next; offspring is how alleles are carried into the next generation, and the number of offspring is one factor in fitness.) What I asserted is that lifespan is irrelevant if it is determined by fitness/natural selection, rather than chance. If someone dies at any age because of natural selection, the consequent change in the allele frequencies in the next generation are not an example of drift. If someone dies at any age because of an accident, and if the population is relatively small, the consequent changes in allele frequencies in the next generation are an example of drift. Slrubenstein

My issue is that your concept of "accidental death" is faulty. That's why I overlooked to exclude it from the category of deaths you call irrelevant. It's all the same to me: If "accidental" to you means inducing drift, then all deaths are accidental, b/c they all are occasions for deviation from the average life span. I explained that in my 2nd discussion of the wildebeests above. I just get tired of repeating myself. 168... 02:15 23 May 2003 (UTC)

Another example from above:

My point was that the HWE didn't belong high up in the article. It's not _general_ to drift, whereas the the chance factors I emphasized were.

HW is not a "chance factor," it is the baseline against which chance factors may be compared and is thus generalizable to drift, which is precisely why other discussions of drift by scientists begin with HW. Slrubenstein

"it can't be right to describe it as the essense or the be-all and end all of drift" I wrote. That is my point.168... 22:57 20 May 2003 (UTC)

Hmmm. Your point is that "it can't be right to describe it as the essence or the be-all...?" That is funny, because I never said it was the be-all and end-all, what I did say is that it is a baseline that needs to be up front. The reason I said it needs to be up front was that I was responding to your point, "HWE didn't belong high up in the article." You see, that is what I thought your point was. I thought that was your point because you said "My point was..." Can you see how I get confused when I try to discuss these matters with you? You make a point -- you write "My point is that HWE didn't belong high up..." and I explain why I do think it belongs high up and you say tell me I am not responding to your point! But HWE is the baseline, and belongs up front for good reason. I take very seriously your desire to communicate these issues clearly to a lay audience. So let us strive to explain them as clearly as possible. This may take more concrete examples, or more explanation -- fine. But let us not distort the science. Slrubenstein

I addressed this in my remarks preceeding "This shows that one need not explain HWE before explaining drift" above.168... 19:39 21 May 2003 (UTC)

In my version, HWE does not come in the very beginning. It is not in the introduction, it is only in the section explicitly labeled "Drift in sexually reproduction", because it is essential -- yes, essential -- in understanding drift in sexually reproducing species.Slrubenstein