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Faith  Suspended Member (Idle past 1764 days) Posts: 35298 From: Nevada, USA Joined: |
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Author | Topic: Evolution Requires Reduction in Genetic Diversity | |||||||||||||||||||||||||||||||||||||||||||
Admin Director Posts: 13140 From: EvC Forum Joined:
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Hi Faith,
You don't need to know the value of x to know that x+1 is greater than x.
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Faith writes: MIGRATION:Adds to one by subtracting from the other = INCREASE WITHIN B, (decrease within A for some reason not mentioned) It's not impossible for there to be a decrease in the diversity of A, but it's very unlikely. Consider just one loci for populations A and B. Population A has alleles X, Y and Z for this loci, while population B has only Y and Z. Now an individual with allele X migrates from A to B, which adds allele X to population B and increases its diversity. But the migration of this single individual from population A to B does not remove the X allele from population A. There are still plenty of individuals in population A with allele X. In order for A to lose diversity, all individuals with allele X would have to migrate away. That's very unlikely.
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Admin Director Posts: 13140 From: EvC Forum Joined: |
As you say, it may be a wording issue. I think that when HBD says "Selection replaces one trait with another..." that he means changing things like beak size or fur color or melanic coloration by drawing upon variation already present in the genome.
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Dr Adequate writes: Oh Faith, not again, not again. Please be more specific.
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Admin Director Posts: 13140 From: EvC Forum Joined:
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PaulK writes: Although to be quite honest I would not be surprised to find some examples of incremental change in dogs if we had a decent record. Not sure what you mean here. Are you referring to incremental changes in what we today consider stable dog breeds? It makes sense in that context, but your comments might be misinterpreted as being aimed at the development of a new breed of dog, where incremental changes are of course apparent, otherwise breeders wouldn't be able to select mates for the next generation. Or maybe you mean some possibility I missed?
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Faith writes: This is really YOUR misunderstanding. Selection and drift both act like population splits, which I've argued before. Yes they both change allele frequencies. When I first read this (and the rest of the paragraph, which I address below) I couldn't make sense of it, so I made a note to myself to come back to it and inquire what you meant. I then read forward to PaulK's response, and he couldn't make sense of it either. One can imagine a population with a large geographic range being subjected to different selection pressures in different geographic regions, and in such cases selection and drift could cause splits, but these genetics discussions have so far been operating under the simplifying assumption of uniform selection pressures across a population. Any shift away from this assumption should be explicitly noted. Since you didn't do that we have to assume you're not talking about populations with large geographic ranges and varying selection pressures. So perhaps you meant to say something else? Maybe that selection and drift act to differentiate isolated subpopulations? Whether I've guessed right or not, please let the discussion know what you meant.
No, selection cannot possibly maintain them... I think you misunderstood PaulK's meaning. He was only saying that selection pressures can maintain allele frequencies in a population. This is what happens to populations that are already well adapted to their environments. Individuals who by the luck of the genetic lottery receive an allele mix that deviates too greatly from the population's will be selected against and produce fewer or no offspring, so these different allele mixes tend to be removed from the population. Those who receive an allele mix that matches the population's pretty well will be selected for and produce more offspring, contributing more individuals with comparable allele mixes to the population. Or maybe I misunderstood what you're trying to say, in which case please clarify for the discussion.
...because, just as population splits and genetic drift do, it reproductively isolates some individuals from the rest of the population, which is THE way gene frequencies change, and THE way genetic diversity is reduced. You again appear to be saying that selection within a population reproductively isolates individual members of the population from one another. This makes no sense. I would guess that you're instead referring to isolated subpopulations, and that you're saying that the different selection pressures will encourage changes in the direction of reproductive isolation, but you're reference to population splits seems to rule out this interpretation. So I'm unable to figure out a meaning that makes sense. Could you please clarify for the discussion?
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Faith writes: One thing I used to do was identify population splits as a FORM of selection, random selection, because individuals are in a sense "selected" to be the founders of a new population when they are geographically (and therefore reproductively) isolated from the parent population. So you could say the Pod Mrcaru lizards were "selected" in that sense. This isn't natural selection because there is no selection pressure, it's random, but my point has always been that the genetic effect is the same because the same mechanisms are involved, only with different causes. Genetic drift too can be described as a form of random selection. You're drifting toward creating your own definitions again, which only creates confusion. Yes, population splits and genetic drifts can be viewed as performing a form of selection, but not natural selection, which is what is meant in this discussion by the shortened "selection." Natural selection operates on the phenotype and influences the degree to which individuals pass their genes on to the next generation. Population splits and genetic drifts are not operating on the phenotype.
Both selection and genetic drift occur within the population, but they nevertheless become reproductively isolated from that population and possess their own different gene/allele frequencies. Individuals that are selected for reproductive advantage, or just the random isolation of individuals in genetic drift both become isolated and that's THE mechanism that brings out new phenotypes and ultimately creates a new variety or race. Negative selection, selection against a group of individuals, would have the same effect, whichever group is reproductively favored creating a new subpopulation within the main population. (Reproductive isolation in any of these cases may not be absolute, even in population splits, but for the purposes of the discussion it helps to think in terms of absolute isolation.) You're envisioning a scenario where new breeds emerge as a subpopulation within a population without some form physical separation or isolation. While some kind of non-physical impediment or isolation is not impossible, it must be by far the less common case. Is there some reason you want to focus on this uncommon circumstance? If it's not central to your point then it seems best avoided.
If the isolation continues for enough generations a distinctive new population within the parent population can form,... If the new population is "within the parent population," could you make clear for the discussion what you are imagining is the barrier that creates isolation?
In other words the only real difference between these three populations is the reason for the isolation. All the different ways these subpopulations form follow the same pattern: changed gene frequencies, new phenotypes, reduced genetic diversity from the original population (which, incidentally, could also undergo a similar sequence unless it is a very large population). By "three populations" that have become isolated you evidently mean a population created via split, another created via selection, and another created via drift. Isolation through physical splits is easy to understand, but could you please make clear for the discussion how natural selection and drift can cause a subpopulation to break off from a parent population while still within that parent population?
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Faith writes: I know that and my point was that it makes no difference to the basic pattern. The mechanisms and results are the same no matter what the cause of the reproductive isolation. This is just self-evidently wrong. You're declaring that the mechanisms of physical population splits, natural selection and genetic drift are the same, despite that they're completely different mechanisms. And you're also declaring the results the same, despite that the results of these mechanisms differ, particularly natural selection which produces adaptation, something the other mechanisms do not. I was hoping that by noting those portions of your messages that seemed hard to interpret that I might bring greater clarity to the discussion, but your clarifications seem to be marching off in directions of greater error. I don't want the thread to begin spending all its time trying to figure out what you mean, or trying to convince you that what you're saying makes no sense. You don't have to agree with anything anyone else says. You can have a different opinion on everything. But there *are* a few rules. You can't change existing terminology. And you have to say things that most other people can understand. I'll comment on a few more things.
Natural selection in operating on the phenotype reproductively isolates that phenotype from the other phenotypes in the population,... You need to make clear to the discussion how isolation occurs while still in the parent population, i.e., explain what prevents a creature (one that is only very modestly changed in phenotype) from breeding with any other member of the population that it still resides within. As PaulK notes, a favorable allele will spread through a population, not create an isolated subpopulation co-resident within the parent population.
...the highest frequency alleles in this case being those for those traits that are selected for. I just want to note that traits can also emerge or be emphasized by diminished allele frequency, or by new allele combinations. Allele frequency is just a measurement across a population. It doesn't tell you the allele makeup of individuals.
Genetic drift for whatever reason is the random reproductive favoring of some individuals over others and their traits come to dominate the subpopulation simply randomly. The isolation is brought about simply by the random favoring of those individuals, random "selection." As PaulK notes, if natural selection is unlikely to produce isolation within a population, then genetic drift is even more unlikely.
Natural selection isolates the individuals it selects by selecting them,... Except that it doesn't isolate the selected individuals. A creature with a new and advantageous allele can only breed with other members of its population. Its offspring can also only breed with other members of its population. The process you're describing is how favorable alleles spread through a population, not how isolated subpopulations form right in the middle of their parent populations. These claims that natural selection and genetic drift can cause isolation within a parent population seem like a digression from the main topic. I think it would help the discussion if you could either drop these claims, or make clear how they bear on the main topic. Please help the discussion along by doing one or the other.
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Faith writes: Admin writes: These claims that natural selection and genetic drift can cause isolation within a parent population seem like a digression from the main topic. I think it would help the discussion if you could either drop these claims, or make clear how they bear on the main topic. Please help the discussion along by doing one or the other. PaulK brought up the subject of genetic drift and natural selection and I gave my view of it in response, I have no other reason to continue it. I haven't changed my view, I've had it for years so I'm not going to drop it beyond not pursuing it after this post unless somebody else keeps it alive. What you're talking about actually has a name: sympatric speciation. The link is to the Widipedia article. If you read it you'll find that plausible mechanisms remain elusive, and that it is not a common consequence of genetic drift and natural selection (you've been arguing the opposite and are clearly wrong). According to Wikipedia:
quote: If you believe this form of speciation is necessary to arguing your position then please explain how. Otherwise please drop it.
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Admin Director Posts: 13140 From: EvC Forum Joined: |
Faith writes: It's not speciation until it leads to speciation... You said it would lead to a subpopulation that was reproductively isolated from the parent population though co-located with it. Inability of a subpopulation to interbreed with the parent population is part of the definition of speciation for sexual species, so you're still trying to redefine speciation. I've already ruled against inventing new definitions for existing terminology. Responding to your next message:
Faith in Message 1025 writes: The effects, as I clearly described them, are the same in all three cases. This is a bald declaration without any support, while the rebuttals clearly explained why you were wrong.
I think it's about time to leave. I asked you to stop threatening to leave, yet you have kept doing it, though veiling it somewhat. The reason you shouldn't do this is because it brings discussion to a halt, and then you don't leave, causing a fitful and disconnected restart of the discussion. I will shortly be suspending your right to post in this thread.
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