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Fosdick  Suspended Member (Idle past 5500 days) Posts: 1793 From: Upper Slobovia Joined: |
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Author | Topic: Is there more than one definition of natural selection? | |||||||||||||||||||||||||
Fosdick  Suspended Member (Idle past 5500 days) Posts: 1793 From: Upper Slobovia Joined: |
After 300+ posts following Message 1 we failed to agree on the exact meaning or precise difinition of natural selection. I have been saying that NS is the differential reproductive success amongst invividuals of a population. This agrees with E. O. Wilson's defintion (from Sociobiology, 2000, p. 589):
quote:He does not mention sexual selection, mutation, gene flow, or drift in his definition; he only refers to the differential reproductive success amongst individuals in a population. Is there a cause/effect relationship within the context of NS that is not yet well understood? Do members of this forum think that natural selection is a cause, an effect, or both, as it associates with a microevolutionary event? It certainly does seem causal to me. So why do we still disagree on how to define NS? Does anyone have a better definion of NS than the one I provided? ”HM
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Admin Director Posts: 12998 From: EvC Forum Joined: Member Rating: 2.3 |
Thread moved here from the Proposed New Topics forum.
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Fosdick  Suspended Member (Idle past 5500 days) Posts: 1793 From: Upper Slobovia Joined: |
Here’s a different angle on differential reproductive success”a 'twofer':
Nature Online (March 26, 2007) reports Semi-identical twins Discovered:
quote: The researchers called the twins chimaeras; so rare, they said, that they didn't to expect to ever see another occurrence. Do semi-identical twins ever occur in other species? I suppose they do, but probably rarely. Co-fertilization, if I may call it that, would seem to be another possible cause of differential reproductive success amongst individuals in a population. ”HM Edited by Hoot Mon, : Fixed link
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macaroniandcheese  Suspended Member (Idle past 3928 days) Posts: 4258 Joined: |
your link's dead.
try to find a stable one.
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Fosdick  Suspended Member (Idle past 5500 days) Posts: 1793 From: Upper Slobovia Joined: |
brennakimi, I fixed the link.
”HM
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JustinC Member (Idle past 4844 days) Posts: 624 From: Pittsburgh, PA, USA Joined: |
quote:I'd add that the differential reproductive success would have to be due to some heritible trait or property. If a meteor lands directly on you (i'm not talking about the subsequent change in the environment after a meteor hits. That could be a valid selective pressure), you have differential reproduction but not due to any fault or virtue of your own; there does not exist a phenotype that could survive such an event.
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
Co-fertilization, if I may call it that, would seem to be another possible cause of differential reproductive success amongst individuals in a population. In what way? I thought you weren't looking from the genes eye perspective, which would seem to be the only one from which this statement would make any sense whatsoever. This just suggests that you don't even understand what 'differential reproductive success' means. TTFN, WK
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
Is there a cause/effect relationship within the context of NS that is not yet well understood? There are thousands. Natural selection is a result of the action of the environment on an organism with a particular phenotype, and at the gene level with particular genes and gene complements. Until we understand every interaction between an organism and its environment and the contribution of those factors to the organisms reproductive success then we will not have a full accounting of the cause/effect relationships shaping natural selection in that population and the various genotypes/phenotypes within it.
He does not mention sexual selection, mutation, gene flow, or drift in his definition Well since it is a general definition he would be unlikely to mention more specific forms such as sexual selection and artificial slection. As to mutation this is a significant source of the variation upon which natural selection can act but not a component of natural selection itself.
Do members of this forum think that natural selection is a cause, an effect, or both, as it associates with a microevolutionary event? I'd say it can be both in as much as the constitution of any generation resulting from previous selection will form part of the environment shaping subsequent selection.
So why do we still disagree on how to define NS? I'm not sure that any of the things you have said here actually reflect the disagreements that arose on the previous thread. Maybe you should read some of Philosopher of Biology Roberta Millstein's work on the relationship between Genetic drift and natural selection and their respective roles as causes of evolution. The page I have linked to contains further links to the full text of many of Millstein's writings. I think that her paper “Are Random Drift and Natural Selection Conceptually Distinct?” is particularly relevant. TTFN, WK
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Modulous Member Posts: 7801 From: Manchester, UK Joined: |
Does anyone have a better definion of NS than the one I provided? It's good - but its not all encompassing since it misses out the generation after the next generation. If individuals contribute poorly towards the offspring of the next generation, but contributed excellently towards the contribution of the offspring of the offspring of the next generation, that wouldn't be natural selection according to Wilson's basic formulation. I prefer something along the lines of Variation in a trait + Heritability of the trait + Differential fitness conferred by the trait = natural selection Where fitness is the ability of the trait to assure its inheritance. Creating or helping to create more copies of the trait implies a higher fitness. This definition, such as it is, also allows us to discuss natural selection acting in a non standard biological setting. Natural selection can occur in man made entities or life or whatever.
Do members of this forum think that natural selection is a cause, an effect, or both, as it associates with a microevolutionary event? Natural selection is universally acknowledged as a cause of evolution. Hence why natural selection is one of the mechanisms under the umbrella of the theory of evolution.
It certainly does seem causal to me. So why do we still disagree on how to define NS? There maybe some disagreement on how best to word natural selection, but on its general definition there is no disagreement. The only debate I've seen is with regards to what exactly is being naturally selected.
I have been saying that NS is the differential reproductive success amongst invividuals of a population. The problem with this is that differential reproductive success can exist without any natural selection existing. This occurs when the differential reproductive success is not caused by a heritable trait. Think of 1000 robots:- Each robot has 100 instructions in memory on how to create more robots, these 1,000 robots are all made from this same instruction set. There are 10 robots built with each instruction set. They are ranked in order of how many copies they are able to make. A robot built according to the instruction set 30 will be able to make 30 more robots itself before it dies. When it comes to building a new robot, the parent robot picks a random number between 1 and 100 and builds a robot to the corresponding specification. Clearly, there is a differential reproductive success here, but there is no natural selection - the frequency of each robot-type remains basically fixed at around 1 in 100.
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Fosdick  Suspended Member (Idle past 5500 days) Posts: 1793 From: Upper Slobovia Joined: |
Co-fertilization, if I may call it that, would seem to be another possible cause of differential reproductive success amongst individuals in a population.
In what way? I thought you weren't looking from the genes eye perspective, which would seem to be the only one from which this statement would make any sense whatsoever. This just suggests that you don't even understand what 'differential reproductive success' means. I do prefer the gene’s-eye view of evolution and NS. So, from that stand point, I am interested in the frequency distributions of alleles, and how those distributions might change. Given that, I’ll try to back myself out of a corner: Differential reproductive success amongst individuals in a population would mean that not all individuals contribute the same number of offspring to the next generation. Simple enough: if every individual of a population produces the same number of offspring there is no differential reproductive success (i.e., no natural selection). Now, taking into consideration what this means genetically, every individual’s gametes carry their own meiotically shuffled complements of alleles. So, upon successful fertilization, the zygote gets an even half-and-half allele mixture from one sperm and one egg. In this normal arrangement there are two allele-mixing opportunities”meiosis and fertilization. But, in the case of those semi-identical twins, there were three allele mixing occasions: one in meiosis and two in co-fertilization. This suggests to me that, on such a rare occasion, the allele frequencies can be altered (albeit slightly) by this extra mixing opportunity. As such, it would be an additional cause, possibly explaining the change in a population's allele frequencies. In any that case, though, NS remains only a measure of differential reproductive success amongst individuals. Thanks for being sharp on that. ”HM
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Wounded King Member Posts: 4149 From: Cincinnati, Ohio, USA Joined: |
Your reply ties in with the paper I was suggesting you look at. The question is whether the co-fertilisation or subsequent change in allele frequency were affected by heritable differences or just the result of stochastic noise blind to the phenotype/genotype.
In other words this may lead to evolution in terms of the allele frequencies in the next generation being affected but it is not natural selection as the process is blind to the genetic makeup of what it is acting upon, similar to the effect of genetic drift.
In any that case, though, NS remains only a measure of differential reproductive success amongst individuals. Is it a measure? TTFN, WK
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Percy Member Posts: 22392 From: New Hampshire Joined: Member Rating: 5.3 |
Hi Hoot Mon,
I'm not going to take issue at this time with your definition of natural selection relative to differential reproductive success. I'm only going to note an internal inconsistency in your approach:
Hoot Mon writes: Simple enough: if every individual of a population produces the same number of offspring there is no differential reproductive success (i.e., no natural selection). When the differential reproductive success is 0, that is not the same thing as no differential reproductive success, no natural selection. The individuals happened to be equally fit and well adapted, but selection pressures were present just like they always are, and so the process of natural selection was still in force. It just didn't happen to have a measurable effect on that generation. Consider an analogy to your definition of natural selection. Just as natural selection is a measure of differential reproductive success, an automobile race is a measure of differential speed success. If all cars cross the finish line at the same time, that doesn't mean there was no race. It only means that all cars were equally fit and well adapted for that race. Or consider another analogy, this time to math. If at the end of your solution to a complex math problem you discover that the answer is 0, that doesn't mean that there is no answer. It means the answer is 0. --Percy
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Fosdick  Suspended Member (Idle past 5500 days) Posts: 1793 From: Upper Slobovia Joined: |
Modulous wrote:
There maybe some disagreement on how best to word natural selection, but on its general definition there is no disagreement. The only debate I've seen is with regards to what exactly is being naturally selected.
I do agree.
Variation in a trait + Heritability of the trait + Differential fitness conferred by the trait = natural selection.
Modulous, I would venture to challenge your algorithm this way: 1. What does “variation in a trait” mean? Alleles are variations of genes, of course, and in this way they ARE the traits. The genes themselves are the not the traits, per se; that job goes to the alleles. For example, I don’t call an eye-color gene a “trait.” I call the alleles of the eye-color gene “traits.” So, the first term in your equation actually pertains to different allele frequencies. The Hardy-Weinberg (dis)equilibrium would seem to account for that. 2. What does “heritability of that trait” mean here? Do you mean that certain alleles or their frequencies are more heritable than others? Probably. But all it implies to me is that traits can have variation, even in their heritability, which seems already accounted for by the first factor of your algorithm. 3. “Differential fitness conferred by that trait” could be another way of saying “favorability of that trait.” As such, the favorability part seems close to meaning of “selection.” But, of course, selection here seems to be accounted for in "natural selection," on the other side of the equation.
The problem with this is that differential reproductive success can exist without any natural selection existing. This occurs when the differential reproductive success is not caused by a heritable trait.
Good and subtle point, but confusing. Doesn’t this cast doubt again on the meaning of a "trait"? Isn’t a trait, by way of viewing it generationally, really just a lineage of allele frequencies? I don’t know of any non-heritable “traits.” How would they last long enough to be a trait? Let’s go back to someone’s model of natural selection in an earlier thread, I think it was Quetzal’s. He used a game of marbles cascading through tiers of barriers with holes in them to model NS. The marbles and the holes were of different sizes, so selection, fitness, or favorability was decided on the basis of size. (The author of this model pointed out that its simplicity may not fairly represent the complexities of natural selection and evolution.) So, thinking adventurously, perhaps your algorithm could be revised this way: N = dF x S Where N is natural selection, dF is the disturbance of the HW equilibrium, S is a measure of favorability or selectivity (selective pressure? information?). I multiply the terms of N for arbitrary reasons, mainly because they don’t seem to be just additive to me, and also in this way the model resembles the structure of Ohm’s Law: V = IR, were voltage V equals the current I times the resistance R. So, the resistance metaphor relates to the tiers, and the current is the movement of marbles through the tiers. What results is differential reproductive succcess ("the voltage of natural selection"). Thanks for letting me experiment a little with your model. I'm sure I will learn soon that this was a bad idea. ”HM
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Allopatrik Member (Idle past 6187 days) Posts: 59 Joined: |
At the risk of jumping in here,
quote: Eye-color is a trait, the different colors are variations of that trait.
quote: Heritability refers to that proportion of a variation in a trait which is controlled genetically. Environment may have some effect on the expression of the trait by the organism. Consider beak depth in Darwin's Finches. This trait is approximately 85% heritable, with 15% contributed by the environment. A Natural Selection is not Evolution-- R.A. Fisher
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Modulous Member Posts: 7801 From: Manchester, UK Joined: |
What does “variation in a trait” mean? Alleles are variations of genes, of course, and in this way they ARE the traits. The genes themselves are the not the traits, per se; that job goes to the alleles. For example, I don’t call an eye-color gene a “trait.” I call the alleles of the eye-color gene “traits.” So, the first term in your equation actually pertains to different allele frequencies. The Hardy-Weinberg (dis)equilibrium would seem to account for that. The first term does actually pertain to different allele frequencies, when we are discussing biological evolution.
What does “heritability of that trait” mean here? Do you mean that certain alleles or their frequencies are more heritable than others? Probably. But all it implies to me is that traits can have variation, even in their heritability, which seems already accounted for by the first factor of your algorithm. It isn't a measure. You are confusing my formulation for an algorithm of sorts. It is not, all I am saying is that when the three conditions are met, natural selection results.
“Differential fitness conferred by that trait” could be another way of saying “favorability of that trait.” As such, the favorability part seems close to meaning of “selection.” But, of course, selection here seems to be accounted for in "natural selection," on the other side of the equation. Perhaps a better way of phrasing the last term would be 'The trait variations affect their own replication.' There is probably a better, less clumsy way of phrasing this.
Isn’t a trait, by way of viewing it generationally, really just a lineage of allele frequencies? I don’t know of any non-heritable “traits.” How would they last long enough to be a trait? If I lost my legs in an explosion I'd have a trait of 'absense of legs'. If I keep my legs and exercise heavily I become very muscled, this is a trait. Acquired traits are non-heritable.
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