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Faith  Suspended Member (Idle past 1471 days) Posts: 35298 From: Nevada, USA Joined: |
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Author | Topic: Evolution Requires Reduction in Genetic Diversity | |||||||||||||||||||||||||||||||||||||||||||||||
PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
Sickle-cell anaemia is a case where selection maintains diversity.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
I don't think so. The thing about sickle-cell anaemia is that it is the heterozygous condition that is beneficial (in malarial regions). I haven't heard that said about lactase persistence.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
I think you miss the point. Because the heterozygous condition for sickle-cell is advantageous and the homozygous state is disadvantageous, selection acts to keep a balance. Is there a similar disadvantage to lactase persistence alleles ?
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
quote: Still missing the point. Iin the case of the sickle-cell allele, selection will be positive when it is rare and negative when it is common. Thus selection acts to prevent it from becoming either fixed or lost, hence maintaining diversity. Unless you can show something similar with lactase persistence it is not the same.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3
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quote: To correct Faith, I was not even trying to "come up with" examples of mutation, or even challenging jar to come up with more. I was simply pointing out an interesting an relevant fact about suckle-cell. And the other mutation that Faith conveniently forgets - lactase persistence - seems to be unambiguously useful...
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
quote: A little more thought is needed here: Migration will not typically subtract alleles from A. To add alleles to B only requires at least one of the migrants has an allele not found in population B. And adding an allele to B that was previously found in A but not B will decrease the difference between the populations. To subtract alleles from A every individual with that allele must be among the migrants. That's not impossible but I wouldn't expect it to be common. And this would NOT decrease the difference between the populations - the allele would still only be found in one population. So, the typical situation - the one envisioned in the table - is that migration will add alleles to B previously only found in A, but that A will still include these alleles. This increases the diversity within B but decreases the difference between the two populations.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3
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For drift, I will just point out that in reality drift will affect A.
quote: Selection can be positive or negative. Alleles that add to fitness will become more common, those that subtract from it will become less common. In either case it will tend to eliminate alleles. That's how selection reduces diversity within a population. As I pointed out in the discussion with jar, sickle-cell is an unusual case where selection maintains diversity. This is an example of "heterozygote advantage" where - according the the chart - selection increases diversity. Having one sickle-cell allele is better (in the presence of malaria) than having none or (worse) two. As a consequence selection is positive when the allele is rare and negative when it is common. (It requires some thought but it does work out) The difference between the populations is also listed as increase/decrease, but that is easier to understand. If alleles present in A and remaining in A are removed from B this increases the difference. If alleles not in A are eliminated from B (e.g. deleterious mutations) then it will decrease the difference. Edited by PaulK, : No reason given.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
Because drift is random it has more extreme effects in small populations.
quote: Divergence is almost inevitable. To avoid it the same alleles would have to be lost to drift in both populations. While that is possible in a random process it's hardly likely.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3
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quote: He already has Message 931. My post has a simpler version of the explanation. I don't know why you didn't respond to that.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
Perhaps I have misunderstood the terminology, but it seems to me that heterozygote advantage must describe a case where the heterozygote has an advantage over both homozygotes, which is exactly the situation with sickle-cell in malarial regions. If that is not correct I think the term needs to be carefully explained when introduced because it is not obvious. As I have said, an advantage to the heterozygote over both homozygotes will have the consequence of what you called frequency-dependent selection although I would think that a more general term.
I have thought a little more on the matter and it does seem that in this case diversity measured by the proportion of heterozygotes would increase, although diversity measured by the number of different alleles would not. Although this could also be said for selection in general, it is more marked in cases like this. Edited by PaulK, : No reason given.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
quote: For cases like sickle-cell where selection maintains a balance between alleles it must be the case. Maybe there are cases where it isn't true, and it may depend on relative fitnesses but I'm pretty sure that it is for sickle-cell.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3
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Faith:
quote: The first error is that population genetics does not describe phenotypic changes. It describes how alleles change in frequency. The second error is that population genetics DOES work incrementally over generations. (Indeed, a generation would be a 'round of population genetics') The third error is - as usual - to exclude the role of mutation Apparently prayer is no substitute for making the effort to understand what you are talking about.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3
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quote: I'm glad that you accept that you misunderstood what population genetics is actually about.
quote: Unfortunately you've gone right back to repeating your first error immediately after accepting that it was an error. Population genetics does not describe the phenotypic changes. It describes changes in allele frequency and those changes DO take place incrementally over the generations. 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. Declaring that it didn't happen without the records that would let you know if it had is just an unsupported opinion.
quote: That's a typical Faith excuse. You can't make a relevant factor irrelevant just by refusing to talk about it.
quote: Or to put it another way they are really only interested in pure-bred dogs, the problems faced by breeders of those dogs and what to do about them. It's all short-term stuff, related to small, isolated and heavily inbred populations which are rarely found in nature. With artificial selection tending to make the problems worse.
quote: It isn't. It's almost completely irrelevant. You'd have to be nuts to think that a heavily artificial, short-term situation (which would be expected to lead to extinction) has much to do with long term evolutionary change in nature. Edited by PaulK, : Minor clarification
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3
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I'm referring to cases where a sequence of phenotypic changes build on top of each other. It wouldn't be that surprising to find a few short sequences. It doesn't even require mutations. If multiple genes affect a feature select for one until an allele is fixed in the breed. Sometime later select on another and so on.
Maybe comparing a breed derived from another with the "parent" breed might show something of the sort, although obviously it couldn't demonstrate a sequence of more than two steps without getting into the genetics. But I would be very, very surprised if Faith has even made a decent attempt at making comparisons of that sort.
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PaulK Member Posts: 17827 Joined: Member Rating: 2.3 |
quote: Change in allele frequency goes on all the time. It's just faster in small populations.
quote: NO. This is another of your misunderstandings. Selection and drift cause changes in allele frequencies (although selection can also maintain them). Population splits can increase the rate of drift (because the sub-populations are smaller than the whole). But the real reason they are important is that they eliminate gene flow between the sub-populations allowing them to change relative to each other.
quote: Only if you believe that 2 - 1 + 1 = 1. Faith's new math.
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