Evolution Requires Reduction in Genetic Diversity

I realize part of the problem is we are working with two different paradigms. That does make it difficult and I try to be sensitive to that. Sure. The metapopulation would indicate that both populations are the same species and can interbreed. Yes, 'A' and 'B' are separated by some type of geographical barrier. In order to talk about migration, we should not think of this barrier as complete, but will allow some level of migration between populations. The important thing is that these are independent populations.What is the relationship between them? I am not sure how it matters. The diagram is rather abstract and simply represents a population that has been split by some geographical barrier. Maybe population 'B' got blown by a storm to an island; or a road was put in that separated a population of snails; or they are just in fields far enough away from each other that they rarely come in contact, ... any imaginable scenario.A better question may be that if we sampled these populations how would we tell which was the parent and which was the daughter population? Could we even tell which was which? You hypothesis is that the population with the least amount of genetic diversity would always be the daughter population. No. I want you to say you understand what the chart is telling us about how these factors affect genetic diversity with and between populations (same applies with the other 3 factors). If you understand and agree, great. If you understand and disagree, no problem - but you need to then explain why you don't agree and justify it with observational data. If you still don't understand what the chart is saying, then we continue discussing it.All your questions seem to be very similar in what they are asking, so I will address them all in a general way before discussing the specifics. Within populations: Population 'A' has a certain amount of genetic diversity. What the chart indicates is how that particular factor affects the genetic diversity within the population. A mutation in population 'A' would add an allele to the pool and so would INCREASE diversity. Between populations: Population 'A' and 'B' both have a certain amount of genetic diversity. The question the chart addresses is how does the factor in question affect this diversity between these populations. Over time the populations will become increasingly different; that is, diversity will increase. Or maybe they will become more similar and so diversity will decrease. So "between populations" addresses how these factors will affect the differences, or the diversity, between the two populations. A mutation in either population (assuming the exact same mutation does not occur in both populations) will INCREASE the diversity between the populations. Affect all loci addresses how the factor affects the loci. Does it affect them all equally or does it only affect some loci and not others? Mutation will only affect the loci that mutates, not other loci. It means migration between populations 'A' and 'B'. Remember that to properly consider migration, we need to think of it as gene flow, not just individuals moving around. So, migration would be a member of population 'A' mating and having offspring with a member of population 'B' (or visa versa). So what happens to the diversity of population 'B' when a member of population 'A' migrates to population 'B'? It will tend to INCREASE diversity within population 'B' because it would introduce alleles that exist in population 'A' that do not exist in population 'B'. It will tend to DECREASE diversity between populations 'A' and 'B' because alleles that exist in population 'A' that don't exist in 'B' will be moved into 'B' and will reduce the differences between populations. Migration affects all loci because individuals are moving and introducing their whole genome into the other population. Genetic drift results when individuals are removed from the population regardless of their relative fitness. It is the result of serendipitous events and can eliminate even the most fit individuals. So genetic drift refers to fluctuations in allele frequency solely by chance as a result of sampling errors. (Note: The frequency of an allele is also the probability that the allele will be selected at random).The most significant source of genetic drift comes from the fact that only a small proportion of all available gametes are used to create the next generation. Some gametes fail to fuse with another gamete (for example, think of how many sperm are produced in mammals with only 1 or 2 that fertilize the egg) and of those that do form zygotes, only a small proportion may survive. Think about a fish that may lay thousands of eggs but most of them are eaten when they are still fry. This "sampling" is likely to result in a different allele frequency than that of the parent population.The ultimate result of drift is that alleles can be fixed purely by chance. This could allow slightly deleterious alleles to become fixed. This effect can be very pronounced in small populations.So drift tends to DECREASE diversity with a population because it is randomly removing alleles from the population. Drift tends to INCREASE the diversity between the populations since it is a random process it cannot be expected that each population will be "drifting" in the same direction. It will affect all loci because it involves whole individuals and their whole genome, not just specific loci. Increase OR decrease. Selection is kind of a difficult one. It will probably take more explaining that I have time for now, but I will at least explain the chart.Some alleles would tend to be favored and would increase in frequency, some alleles would be less favorable and would tend to decrease in frequency. As selection continues to act on the population the tendency would be that diversity within that population will DECREASE as the population moves towards an optimum fitness. However, there are situations where diversity can INCREASE such as with heterozygote advantage (where the heterozygote is more fit that either homozygote).If different selection pressures are operating on each population we would expect diversity between the two populations to INCREASE as in each population different alleles or combinations are being favored. However, if two populations are already slightly diverse and the same selection pressure begins to act on both populations, then they will become more similar, or diversity will DECREASE.It does not affect all loci because, in general, only loci that are being selected for or against will be affected.Like I said, selection is not quite as straight forward as the others and is a bit more difficult to grasp. But depending on the situation, either a decrease or an increase can be expected (not both at the same time).Also keep in mind that these are all TENDENCIES. This is what these factors TEND to do. The opposite effect may occur for short periods of time or under special circumstances.Hope that helps clear it up. I realize selection will need to be dealt with more vigorously, but hopefully the others make sense.HBD

I think it might be more clear if the diversity within a population were referred to simply as diversity, while the diversity between populations were referred to as diversity difference. Diversity within a population would be measured by factors related to allele frequencies or permutations, etc. Diversity difference between populations would be measured by alleles shared and not shared, allele combinations shared and not shared, etc. I know you're trying to keep it simple, but can't mutations cross loci boundaries? And it might help to be clear that you only mean mutation's direct impact on nucleotide sequences, not the downstream effects, for instance, regulatory genes that turn other genes on and off.

Thank you, I'm glad somebody is at least trying. OK, that's clear now. I meant genetic relationship: cousins, fourth cousins once removed. But I think you were clear enough above, that they are two independent populations of the same species, the genetic relationship not being important. Well, there are exceptions, such as when a population split is pretty much right down the middle. Then each population would get a new set of allele frequencies but roughly the same amount of genetic diversity, or "allelic possibilities." By the way I like "allelic possibilities" as a way to say what I mean by genetic diversity. But if it's confusing I'll forget about it. OK, yes, that one I agree with. This is still hard to grasp.What if I rewrite your sentence, "The question the chart addresses is how does the factor in question affect this diversity between these populations of the two populations combined?To which I would then answer a mutation in either population would increase it. But then it gets more confusing: We're talking about one mutation in one of them, right? Are you saying that over time that one mutation will bring about increasing diversity? Why? Isn't diversity basically the number of alleles or "allelic possibilities" in a population? Wouldn't one mutation between the two increase the diversity only by that one mutation? Similar how? Again, isn't genetic diversity the number of alleles or "allelic possibilities?" Over time low-frequency alleles may drop out of a population altogether and that could cause a decrease in genetic diversity in one and therefore in the combination of the two, but I think you'd have to specify that this is happening, it can't just be assumed. That part is clear but the rest isn't clear at all. Yes, including what Percy points out, anything beyond a particular locus that may also be changed by the particular mutation, though wouldn't just one locus most often be all that's changed?I think I'll stop here and let you respond to this much because I'm still having problems with the "between" factor that should be dealt with before moving on. Edited by Faith, : No reason given.

That's fine with me. However, the track record is readily available for anyone to peruse. I'm quite comfortable with mine. Absolutely. Almost everything I have to say is well covered by other posters anyway.

I only have a few minutes because I actually have work to do today This is why I suggested that we use "genetic differences" to discuss this chart. I realize it doesn't quite get across what you are trying to say in your argument, but it is what we are actually interested in when we study population genetics - how the genetic diversity of populations change over time. These two statements I made are general, overall observations. Imagine two populations that are identical in genetic make up. If those populations are isolated from one another they will become increasingly different in their genetic makeup. This is pretty much a biological fact. (And by this, I am not referring to micro or macro evolution. Genetic divergence can lead to micro or macro evolution, but we can discuss change without inferring those processes) Population genetics is concerned with how and why those populations become different. Imagine you have two populations that are completely homogenous (that is, every individual has the same genetic makeup). And let's say that the alleles are ordered like this:Pop 1: A B C D E F G H I JPop 2: A B C D E F G H I JBoth populations are identical, there is no genetic differences between themD₁=10; D₂=10;D_1-2=0 (D₁ is #alleles in pop 1; D₂ is # alleles in pop 2; D_1-2 is difference in alleles between 1 and 2)Now let's say there is a mutation in pop 2 in gene 'D' so that it becomes allele 'd'. Now the genes look like this.Pop 1: A B C D E F G H I JPop 2: A B C D d E F G H I JD₁=10; D₂=11;D_1-2=1The diversity IN pop 2 has increased from 10 to 11; the difference BETWEEN pop 1 and pop 2 has increased from 0 to 1.Yes, this is very hypothetical and yes, I realize it probably doesn't fit what you are trying to argue. I am simply trying to illustrate what this chart is saying about how these factors affect genetic diversity. Right now this is rather theoretical, I realize that. However, this chart has been developed by making observations of natural and experimental populations. But before we go into the evidence for it, we should understand the principals first. It's OK. That is one way to think of genetic diversity, but not the only way. As I said, genetic diversity is kind of an abstract concept and how we measure it or think about it depends on what question(s) we are asking about the populations being studied.I better stop here. Hope that provides some clarity.HBD

I proposed such terminology, but Faith didn't like it. Not really sure why but I tried to stick with a term she is comfrotable with since I didn't feel it was that much of a issue. Yes, trying to keep it simple, but I did use the plural "loci" which would indicate that it could affect more than one locus. However, the sense of "mutates" was not consistent, so that might have caused some confusion. My sentence should have read:"Mutation will only affect the loci that mutate, not other loci." Yes, especially to keep things simple. Understanding how mutation, selection, ect. affect genetic networks can quickly become extremely complicated.HBD

I thought you were equating genetic diversity with "difference" or "difference in diversity" or something like that. I can't see how they are the same.But now it seems the chart is asking for the difference between the genetic diversity in each of the two populations. Is this right? That's a lot easier to understand, but it also implies needing to know what the genetic diversity of each was before the mutation or other event on the chart took place. Yes or no?

I just answered your other post with the answer that belongs here I think. So the question on the chart is what is the difference between the genetic diversities in each of the populations as a result of each category, beginning with mutation? And again, just as you lay out here what the difference would be if the populations had identical genetic diversity, wouldn't whatever the actual diversity is need to be known in order to answer the question on the chart? And yet there is no indication of that on the chart.The table again, for reference:

	Diversity within pop B	Diversity between A and B	Affect all loci
Mutation	increase	increase	No
Migration	increase	decrease	Yes
Drift	decrease	increase	Yes
Selection	increase/decrease	increase/decrease	No

Edited by Faith, : No reason given.Edited by Faith, : No reason given.

Hi Faith,You don't need to know the value of x to know that x+1 is greater than x.

Oh OK, that simplifies things.

Of course Vice versa? So this is two-way migration? Affecting both populations? Now it sounds one-way. But the phrase "between populations A and B" continues to confuse. Why not say, just as diversity within population B is increased, diversity within population A is decreased? But OK I get that for whatever reason the emphasis is on the "differences between." Yes.Edited by Faith, : No reason given.Edited by Faith, : No reason given.

The point is that we just cannot measure and report total genetic diversity in any kind of absolute way. We need to make comparisons between populations, so that is exactly what we consider - the differences. Genetic diversity and genetic difference are not the same exact thing, but for the purposes of this discussion we can consider differences to be an indication of diversity. Yes. More precisely the chart is talking about changes in genetic diversity. We don't need the absolute value to know what the change will be. The change in diversity is what we are concerned about.HBD

Not sure what the eyeroll is for. When considering the effect of migration you need to consider that the effect could go both directions. In nature it doesn't have to be the case - migration could be in one direction only. I used migration in one direction only to illustrate the principle. I could have said that individuals from population A were migrating to B and individuals from B were migrating to A; but that makes the situation a bit more confusing, I think. The reason is that we are trying to understand what happens to the diversity of populations when these different factors are at work - which should ultimately give us a clue as to how two populations become genetically distinct.For example, let's say we have a subpopulation that has broken off the main population and is stranded on an island for 100 years. It has only a subset of the alleles that the original population has. This population begins to look distinctly different than the parent population.Now, after 100 years the water that kept the island separate dries up and individuals are able to migrate back and forth from the mainland. What will happen to the island population as alleles from the mainland get introduced into that population? It will begin to look more and more like the mainland population. The difference in diversity between the populations will decrease. Right? It is possible that as individuals move out of a population that they take some rare alleles with them and that could eliminate those alleles in that population, but that kind of situation would be rare (drift would be more likely to eliminate rare alleles). The TREND will be that migration out of a population would not be a factor, it is bringing alleles INTO a population that does not already have them that is the chief concern. Of course, if large portions of a population move out of a population, as in the scenario you have been describing there could be enough change in alleles that it could reduce the diversity of the original population. But that would be a special case rather than the trend.That is something to keep in mind here, that deviations from the trend are often what gives us clues as to what happened in the history of a population.HBD

OK, you're doing a good job making this somewhat coherent. However, I'm probably not going to consistently keep in mind why it's a decrease "between populations" for migration because it doesn't really make sense to me that the decrease isn't IN the population that's losing the diversity.And I also have a feeling Drift may not make a lot of sense, but onward nevertheless. How does this show up in actuality. That is, can you recognize drift just by looking at a population, of, say, a herd animal? And what would you see? I've had the general picture in mind of a subpopulation forming within a population but apparently this isn't correct? The concept of "sampling error" has never made any sense to me although I've encountered it many times in reading about drift. I don't know what to picture. What is another source? How much occurs at the gamete level? And yet this is normal, it's how the reproductive system works, so why is there this apparently to-be-expected "error?" I can accept the assessment of decrease within and increase between from the description you give even without really understanding what it is. And all loci.

I'm mostly just trying to get through this now. I get the general gist of the chart much better but it's probably still going to give me problems, especially with the "between" concepts. I also can't see how it relates to anything I've been arguing but I guess you'll get to that.On to Selection jjust to try to get the whole thing covered,. it's actually a lot easier to grasp than Drift. You may have to deal more with selection, not sure, but as I said, I think I get it better than I get Drift.At least I can keep in mind what each category is and why there is an increase or decrease indicated on the chart, without completely understanding it.