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Author Topic:   What exactly is natural selection and precisely where does it occur?
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 151 of 303 (390330)
03-19-2007 8:54 PM
Reply to: Message 148 by Chiroptera
03-19-2007 5:21 PM


Re: More nonsense?
Chiroptera wrote:
When some individuals leave more surviving offspring behind than others due to these heritable differences, we call that "natural selection"...In fact, I suspect that Modulous will basically agree with this definition (although he might, and should, have some reservations about details). Neither he, nor anyone else, has objected to this since I have proposed it twice, so I am assuming that most of us are in broad agreement that it is a useable definition...As far as I know, this definition is or is close to the actual definition actually used by biologists.
Just curious: Do you think that 'individuals selected to reproduce' is different from 'replicators selected to replicate'?
I do agree that Darwinian evolution by "natural selection" is Malthusian in principle, and
I think I understand it: A population may grow faster than its supply of food and resources, possibly causing differential reproduction success amongst it members. And I do understand that individuals succeeding in reproduction may be seen as “being selected”” in the sense of “selected to reproduce.” But I still don’t see this as Darwinian "natural selection."
I agree that natural selection can be correctly defined as differential reproductive success of individuals across a population. But you confuse me when you imply in Message 143 that natural selection occurs even in populations whose members enjoy effectively equal reproductive success:
3. The allele frequencies of population C over time t remain unchanged despite the presence of selective pressure.
Selective pressure with no change in allele frequecies? How could that be? If there is no change in alllele frequencies how else would you measure selection pressure? And just because an organism successfully reproduces doesn’t mean it has been “naturally selected." (I think natural selection operates on the relative favorability of genetic traits inherited generationally”I'll call them "allele frequencies"”not on the ephemeral individuals who bear them temporarily.)
”HM

This message is a reply to:
 Message 148 by Chiroptera, posted 03-19-2007 5:21 PM Chiroptera has replied

Replies to this message:
 Message 152 by Chiroptera, posted 03-19-2007 9:15 PM Fosdick has replied

Chiroptera
Inactive Member


Message 152 of 303 (390334)
03-19-2007 9:15 PM
Reply to: Message 151 by Fosdick
03-19-2007 8:54 PM


Re: More nonsense?
quote:
But you confuse me....
Sorry for the confusion. Let me try to explain the situations where these things might occur; these are theoretical situations -- whether such a situation can or does occur in reality is a different question.
[qs]4. The allele frequencies of population D over time t change despite the absence of selective pressure and/or random genetic drift.[/quote]
This is a situation where there is little or not selective pressures, but mutations continue to occur. Since mutations produce new alleles, the allele frequencies are changing. Some alleles are increasing from 0 to something, and other alleles are decreasing in frequency as new mutations make up more of the genome.
3. The allele frequencies of population C over time t remain unchanged despite the presence of selective pressure.
Now this is a situation where a mutation produces a deleterious allele. Natural selection, then, will try to eliminate it. However, if the mutation occurred once, it can occur again; in a large enough population, this same mutation will repeated occur. The production rate is steady as long as the population is steady; however, the elimination rate will depend on the number of individuals with this allele. A basic differential equation problem. At some point, production and elimination will balance each other, and an equilibrium will be reached. It will be eliminated exactly as fast as it is produced, meaning that the allele frequency will hold steady.
As I said, a classic differential equations problem. I wonder if some alleles like that producing cystic fibrosis can be explained in this manner?
-
quote:
Do you think that 'individuals selected to reproduce' is different from 'replicators selected to replicate'?
I actually don't like the language "individuals selected to reproduce" or "replicators selected to replicate". I don't think it makes sense to speak of "natural selection" occurring to an individual; you can only speak of natural selection when you compare the number of progeny produced by individual A with that of individual B. In fact, accidents do happen, and even very "fit" individuals may just happen to be standing beneath falling branches. What you really need to check is whether, over time, all of the individuals with allele A produce more progeny on average than the individuals with allele B.

Actually, if their god makes better pancakes, I'm totally switching sides. -- Charley the Australopithecine

This message is a reply to:
 Message 151 by Fosdick, posted 03-19-2007 8:54 PM Fosdick has replied

Replies to this message:
 Message 153 by Percy, posted 03-19-2007 9:40 PM Chiroptera has not replied
 Message 154 by Fosdick, posted 03-20-2007 2:21 PM Chiroptera has replied

Percy
Member
Posts: 22393
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 153 of 303 (390340)
03-19-2007 9:40 PM
Reply to: Message 152 by Chiroptera
03-19-2007 9:15 PM


Re: More nonsense?
Chiroptera writes:
I don't think it makes sense to speak of "natural selection" occurring to an individual; you can only speak of natural selection when you compare the number of progeny produced by individual A with that of individual B.
This sounds more like the differential reproductive success that results from natural selection. Wikipedia: "Natural selection acts on individuals..."
I write the above with strong reservations as I've been trying to avoid getting into quibbles with others while keeping my focus on explaining natural selection to Hoot Mon, but I can only see further and more profound confusion resulting from telling Hoot Mon that natural selection doesn't operate on individuals. He's going to be quoting that back to us for pages and pages.
--Percy

This message is a reply to:
 Message 152 by Chiroptera, posted 03-19-2007 9:15 PM Chiroptera has not replied

Replies to this message:
 Message 159 by JustinC, posted 03-20-2007 9:31 PM Percy has replied

Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 154 of 303 (390462)
03-20-2007 2:21 PM
Reply to: Message 152 by Chiroptera
03-19-2007 9:15 PM


Re: More nonsense?
But, Chiro, you haven't yet explained how natural selection occurs in a population with unchanging allele frequencies, owing to effectively equal reproductive success amongst its individuals. You're just saying that any individual that reproduces successfuly is naturally selected. I'm still confused. I thought Darwinian natural selection happens, or can happen, when the individuals of a populations have differential reproductive success.
”HM

This message is a reply to:
 Message 152 by Chiroptera, posted 03-19-2007 9:15 PM Chiroptera has replied

Replies to this message:
 Message 156 by Chiroptera, posted 03-20-2007 2:45 PM Fosdick has replied

Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 155 of 303 (390464)
03-20-2007 2:38 PM
Reply to: Message 144 by crashfrog
03-19-2007 4:10 PM


Re: More nonsense?
1. The allele frequencies of population A over time t remain unchanged, owing to the absence of selective pressure and/or random genetic drift.
Even the maximally simple population, the Hardy-Weinberg population, moves towards equilibrium, doesn't it? And wouldn't mutation also cause changes in allele frequencies?
crashfrog, just what do you mean by "the Hardy-Weinberg population [that] moves towards equilibrium"? And why would mutations cause changes in allele frequencies? Wouldn't a population have to assimilate those changes homologically? And wouldn't a population have to experience selection pressure of random genetic drift to alter those allele frequencies?
”HM

This message is a reply to:
 Message 144 by crashfrog, posted 03-19-2007 4:10 PM crashfrog has replied

Replies to this message:
 Message 157 by crashfrog, posted 03-20-2007 8:00 PM Fosdick has not replied

Chiroptera
Inactive Member


Message 156 of 303 (390469)
03-20-2007 2:45 PM
Reply to: Message 154 by Fosdick
03-20-2007 2:21 PM


Re: More nonsense?
I've been told that I am being confusing, so I am going to bow out of this conversation.
Have fun, though.

Actually, if their god makes better pancakes, I'm totally switching sides. -- Charley the Australopithecine

This message is a reply to:
 Message 154 by Fosdick, posted 03-20-2007 2:21 PM Fosdick has replied

Replies to this message:
 Message 160 by Fosdick, posted 03-20-2007 10:02 PM Chiroptera has not replied

crashfrog
Member (Idle past 1467 days)
Posts: 19762
From: Silver Spring, MD
Joined: 03-20-2003


Message 157 of 303 (390508)
03-20-2007 8:00 PM
Reply to: Message 155 by Fosdick
03-20-2007 2:38 PM


Re: More nonsense?
crashfrog, just what do you mean by "the Hardy-Weinberg population [that] moves towards equilibrium"?
By "Hardy-Weinberg population", I meant to refer to a population operating under the strictures assumed in the Hardy-Weinberg equation, to wit:
quote:
The original assumptions for Hardy-Weinberg equilibrium (HWE) were that the organism under consideration:
* Is diploid, and the trait under consideration is not on a chromosome that has different copy numbers for different sexes, such as the X chromosome in humans (i.e., the trait is autosomal)
* Is sexually reproducing, either monoecious or dioecious
* Has discrete generations
In addition, the population under consideration is idealised, that is:
* Random mating within a single population
* Infinite population size (or sufficiently large so as to minimize the effect of genetic drift)
and experiences:
* No selection
* No mutation
* No migration (gene flow)
and by "moves towards equilibrium", I mean that, even under the Hardy-Weinberg assumptions, the allele frequencies, starting from an arbitrary distribution, reach the distribution specified by the Hardy-Weinberg equation (aka HW equilibrium) within a few generations.
And why would mutations cause changes in allele frequencies?
Obviously a mutation that creates a new, mutated allele has just changed allele frequencies. If you imagine a population of 100 where:
25 are homozygous AA;
50 are heterozygous (Aa);
25 are homozygous aa
and, one homozygous individual experiences a germline mutation to a that creates a new allele a', you now have new frequencies:
25 AA;
50 Aa;
24 aa; and
1 a'a.
See? Mutation alone can change allele frequencies. You went from a frequency of 100 A and 100 a to 100 A, 99 a, and 1 a'.
Wouldn't a population have to assimilate those changes homologically?
When you say words like "homologically", I have no idea what you're talking about. Homology is defined in a biological context as:
quote:
any similarity between structures that is due to their shared ancestry. There are examples in different branches of biology. Anatomical structures that perform the same function in different biological species and evolved from the same structure in some ancestor species are homologous. In genetics, homology is measured by comparing protein or DNA sequences, and homologous genes share a high sequence identity or similarity, supporting the hypothesis that they share a common ancestor.
It's not clear from your usage of the term how you intended it to apply to anything that we're talking about. Can you clarify? I can't answer a question that I don't understand.
And wouldn't a population have to experience selection pressure of random genetic drift to alter those allele frequencies?
I assume you meant "or", there, because genetic drift is non-selective. But, no, my point is that mutation alone is another influence on allele frequencies, in addition to selective forces and genetic drift.
Sorry, references:
Hardy—Weinberg principle - Wikipedia and
Homology (biology) - Wikipedia
Edited by crashfrog, : No reason given.

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 Message 155 by Fosdick, posted 03-20-2007 2:38 PM Fosdick has not replied

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 Message 158 by Adminnemooseus, posted 03-20-2007 9:04 PM crashfrog has not replied

Adminnemooseus
Administrator
Posts: 3974
Joined: 09-26-2002


Message 158 of 303 (390518)
03-20-2007 9:04 PM
Reply to: Message 157 by crashfrog
03-20-2007 8:00 PM


You're quoting from some reference w/o citing the reference
quote:
The original assumptions...
and
quote:
any similarity between structures...
I think you should include references/links to your source(s) for the above, and for any other like situation. For all I know, you may be pulling it from Answers in Genesis .
Please take any comments about this moderation message (and I do encourage such) to the "General discussion..." topic, link below. Please link back to this message.
Adminnemooseus

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This message is a reply to:
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JustinC
Member (Idle past 4844 days)
Posts: 624
From: Pittsburgh, PA, USA
Joined: 07-21-2003


Message 159 of 303 (390522)
03-20-2007 9:31 PM
Reply to: Message 153 by Percy
03-19-2007 9:40 PM


Re: More nonsense?
quote:
This sounds more like the differential reproductive success that results from natural selection.
I know you said you are trying to avoid quibbles with others, but can you unpack this statement a little? It doesn't really seem to make sense, from my perspective.
Natural selection is a type of differential reproductive success that results from heritable properties of a particular unit (unit being the focus of this debate).
When you say differential reproductive success results from NS, it sounds like you are implying that NS happens first and this, then, causes differential reproductive sucess. This doesn't seem quite right, though we could be using words differently.
The only way I could see that making sense is if by NS you meant fitness, that is:
This sounds more like the differential reproductive success that results from different fitnesses.

This message is a reply to:
 Message 153 by Percy, posted 03-19-2007 9:40 PM Percy has replied

Replies to this message:
 Message 161 by Percy, posted 03-21-2007 9:27 AM JustinC has replied

Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 160 of 303 (390523)
03-20-2007 10:02 PM
Reply to: Message 156 by Chiroptera
03-20-2007 2:45 PM


Re: More nonsense?
I've been told that I am being confusing, so I am going to bow out of this conversation.
I suppose you can go hang upside down in your cave, Chiroptera, if you want to. Does this mean, then, that you have abandoned your position that natural selection happens even in populations experiencing no differential reproductive success amongst its individuals?
”HM

This message is a reply to:
 Message 156 by Chiroptera, posted 03-20-2007 2:45 PM Chiroptera has not replied

Replies to this message:
 Message 162 by Percy, posted 03-21-2007 9:36 AM Fosdick has not replied

Percy
Member
Posts: 22393
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 161 of 303 (390586)
03-21-2007 9:27 AM
Reply to: Message 159 by JustinC
03-20-2007 9:31 PM


Re: More nonsense?
JustinC writes:
I know you said you are trying to avoid quibbles with others...
Not that the discussion about how best to think of natural selection isn't important and useful, but I see this thread as focused on explaining natural selection to Hoot Mon, who posed this question in the thread's title: "What exactly is natural selection and precisely where does it occur?"
If someone had explained natural selection to Hoot Mon from the selfish gene perspective and Hoot Mon had replied, "Oh, I get it," then no worries from me. But Hoot Mon is saying he accepts the gene level view and not the individual level view, views that are merely different perspectives on the same facts, which tells us he doesn't understand natural selection at all. He still thinks natural selection involves genetic change.
I see the digression onto the relative merits of creature-level versus gene-level perspectives as inherently confusing to someone like Hoot Mon who is just embarking upon trying to understand the concept of natural selection, but I wasn't going to quibble about it until I saw the comment about it not making sense to think of natural selection operating on individuals. With the exception of Hoot Mon I think we all understood perfectly well the very valid point Chiroptera was making, but the comment was made in the context of a digression not related to the main purpose of this thread, and when interpreted in the main context it appeared to directly contradict Darwin's original definition of natural selection, so I felt I had to point that out. I regret that Chiroptera interpreted it as a comment that he was being confusing. He wasn't. It was the presence of the side-discussion that was confusing.
JustinC writes:
quote:
This sounds more like the differential reproductive success that results from natural selection.
  —Percy
...but can you unpack this statement a little? It doesn't really seem to make sense, from my perspective.
Language is infinitely flexible and nuanced. There are countless ways to define the same thing. You'll often hear the comment, "Well, I wouldn't have described it that way myself, but I guess that's correct," or "Yes, I get what you're saying," and what people mean when they say things like this is that they don't think the person has expressed himself accurately or correctly or with sufficient precision, but they grasp his meaning anyway.
It is virtually impossible to come up with definitions that everyone agrees upon, including natural selection. The best we can do is get people to say, "Yeah, close enough." That's why the side-discussion about the best way to think of natural selection was taking place, because people don't even agree on whether the views possess equal validity.
If you've ever been in a meeting whose goal is to develop a clear mission statement or an unambiguous statement of a project's purpose, you know that several hours can be wasted just arguing about accurate and unambiguous expression. The meeting only ends as people gradually realize that they've set themselves an impossible goal. The ways in which language can be interpreted and misinterpreted are probably infinite.
So I'm not going to set myself the impossible goal of coming up with a clear and unambiguous definition of natural selection. It isn't possible. I think all one can reasonable expect is to start with an explanation of natural selection, and then respond to questions as they arise, and hopefully the discussion will gradually improve the understanding of natural selection in the other person's mind.
So if you already understand natural selection but don't like that definition from me that you quoted (probably only one of at least four or five different ways I've described natural selection in this thread), but you get my meaning, then that's fine. Or if you're uncertain of the definition of natural selection, then I think your definition is fine, too, and actually reflects a rather nuanced understanding of the term. We could even combine our two definitions and arrive at, "Natural selection operates on individuals, and their different fitnesses result in differential reproductive success."
--Percy

This message is a reply to:
 Message 159 by JustinC, posted 03-20-2007 9:31 PM JustinC has replied

Replies to this message:
 Message 172 by JustinC, posted 03-21-2007 6:48 PM Percy has not replied

Percy
Member
Posts: 22393
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 162 of 303 (390590)
03-21-2007 9:36 AM
Reply to: Message 160 by Fosdick
03-20-2007 10:02 PM


Re: More nonsense?
Hoot Mon writes:
I suppose you can go hang upside down in your cave, Chiroptera, if you want to. Does this mean, then, that you have abandoned your position that natural selection happens even in populations experiencing no differential reproductive success amongst its individuals?
This is crass and pretty rude. Chiroptera made clear he was ceasing participation because I told him he was being confusing (not what I was trying to say, but that's how he heard it).
Chiroptera is not abandoning any positions. He was attempting to make the process of natural selection clear to you by enumerating a number of possibilities. I think your time would be well spent trying to find an understanding of natural selection within your own mind that is consistent with everything Chiroptera said. Trying to find fault with the individual examples he provided is preventing you from seeing the overall picture. You don't have to accept Chiroptera's viewpoint on natural selection, but unlike most others in this thread, you don't understand it, and that is preventing you from discussing it in any meaningful way.
--Percy

This message is a reply to:
 Message 160 by Fosdick, posted 03-20-2007 10:02 PM Fosdick has not replied

Replies to this message:
 Message 163 by Chiroptera, posted 03-21-2007 9:51 AM Percy has not replied

Chiroptera
Inactive Member


Message 163 of 303 (390598)
03-21-2007 9:51 AM
Reply to: Message 162 by Percy
03-21-2007 9:36 AM


Re: More nonsense?
I don't think that Hoot Mon was trying to be rude. I think he was just making a friendly joke. At least that is how I read it.
I recognize that the side discussions probably aren't helping much. That is why I am bowing of the thread for a while. People gave their different suggestions as to how to explain the concept; now I think the fewer people are talking, the less potential there will be for confusion.

Actually, if their god makes better pancakes, I'm totally switching sides. -- Charley the Australopithecine

This message is a reply to:
 Message 162 by Percy, posted 03-21-2007 9:36 AM Percy has not replied

Replies to this message:
 Message 164 by Fosdick, posted 03-21-2007 11:11 AM Chiroptera has not replied

Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 164 of 303 (390620)
03-21-2007 11:11 AM
Reply to: Message 163 by Chiroptera
03-21-2007 9:51 AM


Re: More nonsense?
I don't think that Hoot Mon was trying to be rude.
No, I wasn't trying to be rude. I was only trying to understand how natural selection happens when there is no natural selection. If natural selection happens even when there is no differential reproductive success, as you claim, then how do you explain it?
”HM

This message is a reply to:
 Message 163 by Chiroptera, posted 03-21-2007 9:51 AM Chiroptera has not replied

Replies to this message:
 Message 165 by Percy, posted 03-21-2007 11:44 AM Fosdick has replied

Percy
Member
Posts: 22393
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 165 of 303 (390635)
03-21-2007 11:44 AM
Reply to: Message 164 by Fosdick
03-21-2007 11:11 AM


Re: More nonsense?
Hoot Mon writes:
If natural selection happens even when there is no differential reproductive success, as you claim, then how do you explain it?
I would explain this differently than Chiroptera, but please realize it is only the approach to explanation that is different, not the underlying concept being explained.
Say you have two first-year brown rabbits that have never reproduced that have wandered a bit too far north and find themselves in a snow covered landscape in the spring. While seeking food they are both eaten by foxes. They have both been deselected by natural selection. The difference in their reproductive success is zero, but natural selection has occurred. Whatever it was in their phenotype that caused them to place themselves in a vulnerable position will not be passed on to the next generation.
--Percy

This message is a reply to:
 Message 164 by Fosdick, posted 03-21-2007 11:11 AM Fosdick has replied

Replies to this message:
 Message 168 by Fosdick, posted 03-21-2007 12:45 PM Percy has replied

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