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Define Evolution
- Thread starter Warrior FC
- Start date
DNAunion: Lets take the clauses one at a time, in reverse order.
The development of new species is not a requirement of evolution (and therefore should not be in the terms definition). Populations in the wild are always undergoing evolution, even when no new species are being produced.
Evolution is not the result of natural selection only other factors, such as immigration, emigration, non-random mating, genetic drift, etc., also produce evolution (any population that is not in Hardy-Weinberg equilibrium is evolving). So the part about natural selection needs to be taken out (or the other factors need to be added in, turning it into more of a paragraph than a definition).
The first clause is pretty good: [Evolution is a] change in the genetic composition of a population during successive generations.
But even then it is only one of the definitions of evolution for example, it has a problem in that is does not apply to the best evidence of "evolution" (the fossil record) since we dont have genetic material from trilobites or other organisms from around that time period. So how can we determine what alleles were actually present in a given population, what their actual frequencies were in that population, and how those allelic frequencies changed from one generation to the next? Also, keep in mind that it has been said that evolution is in fact falsifiable because finding a fossilized human in pre-Cambrian strata would destroy evolution (at least as it is now formulated)*. But how would such a finding falsify the observation that the genetic composition of populations changes over time? It wouldn't. So obviously people making the claim of "grossly out of place fossils" are using a different definition of evolution than the population-genetics one. Something like, "all the changes in biological forms over vast spans of time as evidenced by the fossil record".
*Here's the quote:
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Just because we don't have direct access to the DNA doesn't mean we can't observe changes in gene frequencies over time. Gregor Mendel, the father of modern genetics, was able to learn a great deal about genetics simply by noting changes in the visible characteristics of his peas over time. He was able to do this because the gross morphology of the pea is a proxy for its genetic composition. And what Mendel did with peas, paleontologists can do with trilobites.
Yes it would. Again, knowing that gross morphology is a proxy for genetic composition, it would provide strong evidence that at least one organism had not undergone significant genetic change in billions of years and was not linked to earlier, more "primitive" forms.
No, they're just using morphology as a proxy for genetic composition.
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DNAunion: Uhm, if we cant observe the alleles, we cant observe changes in the allele frequencies obviously. What we can do is INFER things about changes in alleles based on observed changes in morphology, but thats different than observing the changes in allele frequencies themselves. And, since we could not experiment directly with the organisms (since they died hundreds of millions of years ago), some problems arise: there might not necessariy be a direct correlation between the genetic composition and morphology. Here's two reasons why.
1) It is possible to have a change in morphology/phenotype without it's being accompanied by a change in allelic frequencies simply by having the ratio of homozygotes to heterozygotes change while holding the frequencies of the alleles constant. Suppose for demonstrative purposes that population P, consisting of 100 individuals, has two distinct phenotypes for trait T, and that it follows complete dominance. Let the alleles be S (for long) and s (for short), with S dominant over s.
At generation G there are 10 heterozygotes, 45 SS homozygotes, and 45 ss homozygotes. That gives 100 copies of S and 100 copies of s, and the phenotypes are 55 long vs. 45 short.
At generation G + x there are 60 heterozygotes, 20 SS homozygotes, and 20 ss homozygotes. That still gives 100 copies of each of the two alleles, but now the phenotypes are 80 long vs. 20 short.
Thus, while the allelic frequencies remained constant, the long (vs. short) phenotype went from 55% to 80%. The above just presented the general idea the magnitude of change in the phenotype could potentially be greater.
2) For many/most organisms that lived hundreds of millions of years ago, we dont have a complete populations remains, let alone complete representation of multiple successive generations of a given population (as mentioned in the definition above). Random bias in preservation could yield flawed inferences as to the actual phenotypic frequencies that were present for a given trait.
DNAunion: No they cant. Mendel learned what he did mostly by observing how certain traits acted across multiple generations, while specifying the specific cross-breedings that would occur. Mendel breeded pea plants: paleontologists cant breed trilobites.
Also, Mendel worked directly with the individuals he was stuyding, and so his work didnt suffer from the two problems I listed earlier in my reply (he was using traits that definitely followed complete dominance and which he could examine/follow by specifying the crosses and checking the results, and he could observe all individuals of the population of interest).
DNAunion: No it wouldnt. How can something that we find from more than half a billion years ago (or even a hundred years ago) falsify something else we have directly observed in the present? It doesnt matter if we found fossilized human remains in 3.5 GYA strata, or fossilized bird remains on Mars that date back that far nothing like that can falsify the fact that contemporary scientists have observed changes in allelic frequencies in populations.
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That's what I meant.
I agree with your reasons. They don't change the fact that morphology is correlated at some level with genetic composition.
Again, I suppose you are technically correct, but Mendel and paleontologists operate on different scales of time and detail. Adjusting for scale, the inferences each is able to make are similar.
Again, this would only be a concern if paleontologists tried to analyze the same level of genetic detail as Mendel. They don't, however.
Because it could show whether or not the mechanisms we observe in the present are consistent on a larger time scale. Clearly, a human fossil in the Cambrian would show that the mechanisms are NOT consistent.
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DNAunion: But the mechanisms are not part of the definition being discussed. Remember, we both quoted the following before making our follow-up comments.
DNAunion: That was the part of the original definition that remained after I pointed out problems with the other two clauses. So mechanism is not involved in the defintion under consideration.
That puts us back to where we started. Finding a fossilized human in pre-Cambrian strata would not falsify evolution when defined as change in the genetic composition of a population during successive generations because it is a fact that contemporary scientists have observed changes in allelic frequencies in populations.
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If, for a while, the ruse of desire is calculable for the uses of discipline soon the repetition of guilt, justification, pseudo-scientific theories, superstition, spurious authorities, and classifications can be seen as the desperate effort to "normalize" formally the disturbance of a discourse of splitting that violates the rational, enlightened claims of its enunciatory modality.
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lucaspa
Legend
Appendix and Frequently Asked Questions Science and Creationism, A View from the NAS, the section "What is Evolution?" says:
"Evolution in its broadest sense explains that what we see today is different from what existed in the past. Galaxies, stars, the solar system, and Earth have changed through time, and so has life on Earth.
"Biological evolution concerns changes in living things during the history of life on Earth. It explains that living things share common ancestors. Over time, biological processes such as natural selection give rise to new species. Darwin called this process "descent with modification," which remains a good definition of biological evolution today." pg 27
So, descent with modification is the shortes definition of evolution. Not necessarily the most accurate.
LFOD and all the rest quoting the "change in allele frequencies over time" definition, I'm sorry, but I can't find any evolutionary biologist who endorses that definition. It is simply too limiting. I find several evolutionary biologists who vehemently disagree that this should be the definition. One of the most vehement is Ernst Mayr. I really think you should abandon that definition.
" 'Evolution' implies change with continuity, usually with a directional component. Biological evolution is best defined as change in the diversity and adaptation of populations of organisms." pg. 47
"No Darwinian I know questions the fact that the processes of organic evolution are consistent with the laws of the physical sciences, but it makes no sense to say that biological evolution has been "reduced" to physical laws. Biological evolution is the result of specific processes that impinge on specific systems, the explanation of which is meaningful only at the level of complexity of those processes and those systems. And the classical theory of evolution has not been reduced to a "molecular theory of evolution," an assertion based on such reductionist definitions of evolution as "a change in gene frequencies in natural populations." This reductionist definition omits the crucial aspects of evolution: changes in diversity and adaptation. (Once I gave a lump of sugar to a racoon in a zoo. He ran with it to his water basin and washed it vigorously until there was nothing left of it. No complex system should be taken apart to the extent that nothing of significance is left.)" Ernst Mayr, Evolution, Scientific American 239: 47-55, Sept. 1978.
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lucaspa
Legend
DNAUnion, you spent a lot of energy simply trying to say that the reductionist definition of evolution is insufficient.
However, your argument about allele frequency not relating to morphology is also reductionist. Are you saying that morphological characteristics are not heritable or a result of alleles or genes? That we don't know the exact alleles in trilobites or other fossil organisms is irrelevant to the point that we know the alleles must have changed because the morphology changed over generations. That morphological change could not have happened without changes in the alleles.
Yes, the reductionist definition is in error. Several evolutionary biologists have said so and presented compelling arguments. However, your argument above is not one of the compelling ones.
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Finding a fossil human in the pre-Cambrian would provide strong evidence that evolution as defined is a grossly incomplete model for the origin of species. First, it would indicate that there exists some sort of limit to the amount of genetic change over long periods of time. Second, it would suggest that there is some other method of spontaneous origin of genetic change.
It would not falsify the observation of genetic change, that is true. But it would falsify that definition of evolution as a useful model for the origin of species.
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lucaspa
Legend
Originally posted by DNAunion 1) It is possible to have a change in morphology/phenotype without it's being accompanied by a change in allelic frequencies simply by having the ratio of homozygotes to heterozygotes change while holding the frequencies of the alleles constant. Suppose for demonstrative purposes that population P, consisting of 100 individuals, has two distinct phenotypes for trait T, and that it follows complete dominance. Let the alleles be S (for long) and s (for short), with S dominant over s.
At generation G there are 10 heterozygotes, 45 SS homozygotes, and 45 ss homozygotes. That gives 100 copies of S and 100 copies of s, and the phenotypes are 55 long vs. 45 short.
At generation G + x there are 60 heterozygotes, 20 SS homozygotes, and 20 ss homozygotes. That still gives 100 copies of each of the two alleles, but now the phenotypes are 80 long vs. 20 short.
But you can't have this scenario. If there is no selection, then the distribution of alleles remains the same -- Hardy-Weinberg Law.
If selection is working, OTOH, then your shift is impossible unless s has some selective value, but your example says it doesn't. Any mating and selection for long that you can construct is going to have a decrease in "s" in the population.
Random bias in preservation could yield flawed inferences as to the actual phenotypic frequencies that were present for a given trait.
You can't have "random bias". Non sequitor. Random means no bias. If you have "random preservation" you will not have a bias in phenotypic frequencies. To get a bias you have to have an external cause, such as one phenotype puts the organism in an environment where preservation of fossils is not likely to occur.
DNAunion: No it wouldnt. How can something that we find from more than half a billion years ago (or even a hundred years ago) falsify something else we have directly observed in the present?
Apples and oranges. Human fossils in the Pre-Cambrian falsifies evolution. What it won't falsify is the erroneous definition that you are arguing against. I'm afraid that you lost track of the original claim and your claim.
However, thinking about it some more, human fossils in the Pre-Cambrian would even falsify the reductionist claim, because the alleles represented by the human fossils are not in any of the other populations of organisms present at the time or for hundreds of millions of years later, as evidenced by the absence of correlating morphology. So, even if evolution were "change in allele frequencies over time", you can't have alleles (as represented by the morphology) before they have appeared in time.
At generation G there are 10 heterozygotes, 45 SS homozygotes, and 45 ss homozygotes. That gives 100 copies of S and 100 copies of s, and the phenotypes are 55 long vs. 45 short.
At generation G + x there are 60 heterozygotes, 20 SS homozygotes, and 20 ss homozygotes. That still gives 100 copies of each of the two alleles, but now the phenotypes are 80 long vs. 20 short.
But you can't have this scenario. If there is no selection, then the distribution of alleles remains the same -- Hardy-Weinberg Law.
If selection is working, OTOH, then your shift is impossible unless s has some selective value, but your example says it doesn't. Any mating and selection for long that you can construct is going to have a decrease in "s" in the population.
Random bias in preservation could yield flawed inferences as to the actual phenotypic frequencies that were present for a given trait.
You can't have "random bias". Non sequitor. Random means no bias. If you have "random preservation" you will not have a bias in phenotypic frequencies. To get a bias you have to have an external cause, such as one phenotype puts the organism in an environment where preservation of fossils is not likely to occur.
DNAunion: No it wouldnt. How can something that we find from more than half a billion years ago (or even a hundred years ago) falsify something else we have directly observed in the present?
Apples and oranges. Human fossils in the Pre-Cambrian falsifies evolution. What it won't falsify is the erroneous definition that you are arguing against. I'm afraid that you lost track of the original claim and your claim.
However, thinking about it some more, human fossils in the Pre-Cambrian would even falsify the reductionist claim, because the alleles represented by the human fossils are not in any of the other populations of organisms present at the time or for hundreds of millions of years later, as evidenced by the absence of correlating morphology. So, even if evolution were "change in allele frequencies over time", you can't have alleles (as represented by the morphology) before they have appeared in time.
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DNAunion: 1) It is possible to have a change in morphology/phenotype without it's being accompanied by a change in allelic frequencies simply by having the ratio of homozygotes to heterozygotes change while holding the frequencies of the alleles constant. Suppose for demonstrative purposes that population P, consisting of 100 individuals, has two distinct phenotypes for trait T, and that it follows complete dominance. Let the alleles be S (for long) and s (for short), with S dominant over s.
At generation G there are 10 heterozygotes, 45 SS homozygotes, and 45 ss homozygotes. That gives 100 copies of S and 100 copies of s, and the phenotypes are 55 long vs. 45 short.
At generation G + x there are 60 heterozygotes, 20 SS homozygotes, and 20 ss homozygotes. That still gives 100 copies of each of the two alleles, but now the phenotypes are 80 long vs. 20 short.
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Lucaspa: But you can't have this scenario. If there is no selection, then the distribution of alleles remains the same -- Hardy-Weinberg Law.
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DNAunion: You are forgetting about the several other factors (which I listed earlier, such as non-random mating, genetic drift, immigration, and emigration) than can cause a population to not be in Hardy-Weinberg equilibrium. Genotypes can shift without natural selection, and without changes in allelic frequencies. For example, suppose 8 Ss members of a different population immigrate into the population of interest and 4 SS and 4 ss emigrate out of it. Natural selection wouldnt be involved and there would be no change in allelic frequencies in the population yet the genotype frequencies would have shifted.
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Lucaspa: If selection is working, OTOH, then your shift is impossible unless s has some selective value, but your example says it doesn't.
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DNAunion: I dont see anywhere in the statements of mine you selected (nor do I recall off the top of my head) my having stated that both the s and S alleles are selectively neutral. Could you point it out?
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Lucaspa: Any mating and selection for long that you can construct is going to have a decrease in "s" in the population.
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DNAunion: Not necessarily. What about a case where the heterozygous state is fitter than either of the homozygous states? In that case, we would expect heterozygotes to eventually compose the largest portion of the population, due to natural selection (which I still dont recall having ruled out of play).
DNAunion: 1) It is possible to have a change in morphology/phenotype without it's being accompanied by a change in allelic frequencies simply by having the ratio of homozygotes to heterozygotes change while holding the frequencies of the alleles constant. Suppose for demonstrative purposes that population P, consisting of 100 individuals, has two distinct phenotypes for trait T, and that it follows complete dominance. Let the alleles be S (for long) and s (for short), with S dominant over s.
At generation G there are 10 heterozygotes, 45 SS homozygotes, and 45 ss homozygotes. That gives 100 copies of S and 100 copies of s, and the phenotypes are 55 long vs. 45 short.
At generation G + x there are 60 heterozygotes, 20 SS homozygotes, and 20 ss homozygotes. That still gives 100 copies of each of the two alleles, but now the phenotypes are 80 long vs. 20 short.
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Lucaspa: But you can't have this scenario. If there is no selection, then the distribution of alleles remains the same -- Hardy-Weinberg Law.
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DNAunion: You are forgetting about the several other factors (which I listed earlier, such as non-random mating, genetic drift, immigration, and emigration) than can cause a population to not be in Hardy-Weinberg equilibrium. Genotypes can shift without natural selection, and without changes in allelic frequencies. For example, suppose 8 Ss members of a different population immigrate into the population of interest and 4 SS and 4 ss emigrate out of it. Natural selection wouldnt be involved and there would be no change in allelic frequencies in the population yet the genotype frequencies would have shifted.
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Lucaspa: If selection is working, OTOH, then your shift is impossible unless s has some selective value, but your example says it doesn't.
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DNAunion: I dont see anywhere in the statements of mine you selected (nor do I recall off the top of my head) my having stated that both the s and S alleles are selectively neutral. Could you point it out?
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Lucaspa: Any mating and selection for long that you can construct is going to have a decrease in "s" in the population.
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DNAunion: Not necessarily. What about a case where the heterozygous state is fitter than either of the homozygous states? In that case, we would expect heterozygotes to eventually compose the largest portion of the population, due to natural selection (which I still dont recall having ruled out of play).
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DNAunion: 2) For many/most organisms that lived hundreds of millions of years ago, we dont have a complete populations remains, let alone complete representation of multiple successive generations of a given population (as mentioned in the definition above). Random bias in preservation could yield flawed inferences as to the actual phenotypic frequencies that were present for a given trait.
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Lucaspa: You can't have "random bias". Non sequitor. Random means no bias. If you have "random preservation" you will not have a bias in phenotypic frequencies. To get a bias you have to have an external cause, such as one phenotype puts the organism in an environment where preservation of fossils is not likely to occur.
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DNAunion: Poor choice of one word on my part. However, the point I was making still stands.
It is quite possible for the determining factor - for which individual(s) of a population became fossilized - to have been more a matter of shear luck (which individual just happened to be in just the right place at just the right time) than to be due to a simple phenotypic difference (such as a short tail vs. a long tail). Therefore, when examining fossilized remains, we can end up with a small, random sampling of a population. Such a small, random sample may not be representative of the population at large, and that could lead to flawed inferences as to the actual phenotypic frequencies that were present in that population for a given trait.
DNAunion: 2) For many/most organisms that lived hundreds of millions of years ago, we dont have a complete populations remains, let alone complete representation of multiple successive generations of a given population (as mentioned in the definition above). Random bias in preservation could yield flawed inferences as to the actual phenotypic frequencies that were present for a given trait.
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Lucaspa: You can't have "random bias". Non sequitor. Random means no bias. If you have "random preservation" you will not have a bias in phenotypic frequencies. To get a bias you have to have an external cause, such as one phenotype puts the organism in an environment where preservation of fossils is not likely to occur.
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DNAunion: Poor choice of one word on my part. However, the point I was making still stands.
It is quite possible for the determining factor - for which individual(s) of a population became fossilized - to have been more a matter of shear luck (which individual just happened to be in just the right place at just the right time) than to be due to a simple phenotypic difference (such as a short tail vs. a long tail). Therefore, when examining fossilized remains, we can end up with a small, random sampling of a population. Such a small, random sample may not be representative of the population at large, and that could lead to flawed inferences as to the actual phenotypic frequencies that were present in that population for a given trait.
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