A number of misconceptions about evolution are described on the website Understanding Evolution for Teachers. One of them is Misconception: “Most biologists have rejected ‘Darwinism’ (i.e., no longer really agree with the ideas put forth by Darwin and Wallace).
Response: Darwin’s idea that evolution generally proceeds at a slow, deliberate pace has been modified to include the idea that evolution can proceed at a relatively rapid pace under some circumstances. In this sense, “Darwinism” is continually being modified. Modification of theories to make them more representative of how things work is the role of scientists and of science itself.
Thus far, however, there have been no credible challenges to the basic Darwinian principles that evolution proceeds primarily by the mechanism of natural selection acting upon variation in populations and that different species share common ancestors. Scientists have not rejected Darwin’s natural selection, but have improved and expanded it as more information has become available. For example, we now know (although Darwin did not) that genetic mutations are the source of variation acted on by natural selection, but we haven’t rejected Darwin’s idea of natural selection—we’ve just added to it.
Here's how I would re-word the first sentence of the second paragraph.We now know that natural selection is just one of several mechanisms of evolution. One of the others is random genetic drift where variants can become established in a species purely by chance. There is clear evidence that variants that are neither harmful or beneficial can contribute to evolution and, in fact, the evidence suggests strongly that this is the most common form of evolution. However, there have been no credible challenges to the basic Darwinian principles that adaptive evolution proceeds exclusively by the mechanism of natural selection acting upon variation in populations and that different species share common ancestors.I realize that my version is more complicated but it is also more accurate. This is a case where over-simplification comes at the expense of accuracy and I don't think the
Ironically, the Understanding Evolution website has a good description of Genetic Drift, which they describe as "one of the basic mechanisms of evolution." I don't know why don't mention these other mechanisms on the page where they discuss Darwinism.
Now, look at the first paragraph of the website version. I think they're referring to punctuated equilibria. In this case, the extra complication isn't worthwhile because it doesn't really represent a significant change from Darwin's idea of evolution by natural selection. As a matter of fact, the description is downright misleading. The key concept in punctuated equilibria is that evolutionary change is associated with speciation by cladogenesis and the idea that evolution can occur rapidly isn't all that significant.
17 comments :
Larry - haven't you heard the news: your fellow country person, Denyse "PLEASE, Please, Buy-my-book" O'Leary has declared Darwinism dead and buried:
http://post-darwinist.blogspot.com/2008/08/darwin-now-that-its-all-in-ruins-theyre.html#links
According to her, Darwin's theory is a state of collapse, and beside Darwin was not a very pleasant person. So there!
You must be SO proud that she is a fellow Canadian...who knew you had your very own Ann Coulter of ID.
I don't think they were trying to oversimplify so much as misrepresenting the mechanisms involved. Like you said, they should have been referring to genetic drift when instead they were trying to mention punctuated equilibrium.
Maybe an "E" for effort, on their part I suppose.
"adaptive evolution proceeds exclusively by the mechanism of natural selection acting upon variation in populations"
Ehem... NEVER a process of adaptation is a matter of selection exclusively, even if it is evidently involved.
You may also want to look up the origin of adaptations by axaptation, Mr "Gould fan"
Larry: "... random genetic drift ..., the evidence suggests strongly that this is the most common form of evolution."
This depends critically on how you define "evolution" and how you quantify it. It is more likely to be true (but in a trivial way) if you define evolution as any change in the DNA sequence, and weighting them them equally to arrive at a measure of evolution (e.g., changes in junk dna, synonymous substitution, and phenotypically trivial AA changes that leave the protein function equivalent are all quantified as equal to mutations that have phenotypically significant effects.)
Do you have a good reference that summarizes the "strong evidence" for this claim? I would be very interested to understand the method used to quantify evolution, and to see how, quantitatively, it compares to the quantity of evolution that is driven by natural selection.
It seems to be a common issue that you raise (e.g., your most recent post today), so it would be helpful to me to see the evidence that supports it.
divalent says,
This depends critically on how you define "evolution" and how you quantify it. It is more likely to be true (but in a trivial way) if you define evolution as any change in the DNA sequence, and weighting them them equally to arrive at a measure of evolution (e.g., changes in junk dna, synonymous substitution, and phenotypically trivial AA changes that leave the protein function equivalent are all quantified as equal to mutations that have phenotypically significant effects.)
Evolution is any change in the hereditary characteristics of a population over time. When we examine these changes at the molecular level we can't always tell if an allele has been fixed by natural selection or by random genetic drift but we can safely assume that most changes are neutral since the big picture of molecular evolution indicates a molecular clock. Furthermore, when we examine individual changes most of them look neutral—especially if they're in junk DNA.
There's no particular reason to "quantify" evolution by giving more weight to changes that are presumed to be adaptive. Thus, the vast majority of fixed alleles are almost certainly neutral or nearly neutral. That means random genetic drift is a more common mechanism of evolution than natural selection.
Why is this a problem for you?
[BTW, I hope you are reading this on Sunday (or later) after a nice night out for you and your wife’s 40th.]
If I’m parsing your response correctly, I think you are saying is that “evolution” is: 1) any change in sequence (even in junk) that eventually gets fixed, and 2) that any such change is considered quantitatively equivalent (in terms of the amount of “evolution” it produces) to any other change in sequence (even those that have phonotypic effects that enhance survival).
The problem I have with this is that it is a very broad, and mostly academic, measure of evolution. So it may be true that, by this definition, most evolution is due to drift, but what does that tell us about what the type of evolution that virtually the whole world has in mind when they use the term outside of a molecular dating lab? The type of evolution that Darwin and his contemporaries, and those who followed for at least the next 90 years, endeavored to explain?
“Given that they arose from a common ancestor, what was the process that gave rise to the differences between [insert any two species: dog-cat, lion-gazelle, shark-lizard, human-chimpanzee, tree-worm, etc]?” No one who asks this question is thinking about predominantly neutral changes (e.g., sequence-variations in junk DNA, or synonymous substitutions, or even functionally equivalent AA substitutions). Both questioner and questionee understand that it is the differences in the characteristics of the phenotype that define those organisms that make the question important and interesting.
And that in turn brings me to the issue about claims of the relative importance of drift vs selection. A recurring (obsessive?) theme of your postings is the claim that drift is as important as (or, as you claim in this post, there is strong evidence that it is *more* important than) selection in the evolution of life.
The problem is that drift takes you nowhere in particular; it’s change without effect (or change with little effect). Drift won’t give rise to limbs, immune systems, brains, livers and kidneys, eyes and ears, and so on and so on. Drift won't give rise to the Krebs cycle, or Calvin cycle, enzymes. Drift won’t endow that newly duplicated gene with a new function. It won’t lead to increased complexity, only trivially different complexity. None of the things we marvel at when we study, say, a rainforest ecosystem, with its various species of animals and plants (and fungi and bacteria, etc etc) occupying specific roles in an integrated web of life, is the product of drift. NS is the process that led to the diversity of life as we observe it. NS allows species to adapt (behaviorally, morphologically, metabolically) to diverse environments by bringing about those characteristics that we now used to define those species, and which distinguish them from their ancestors. It is natural selection that does ALL the heavy lifting. (Yeah, drift might alter the Km of this reaction, or the Vmax of that, but only after NS created the enzyme in the first place; big deal).
To the extent that the term Evolution has any meaning to society, it is my definition, not yours. The school board in Cobb County Georgia did not put a disclaimer that “evolution is only a theory” on their biology textbooks because they feared the implications of neutral mutations in junk DNA. (In fact, the Discovery Institute is actually quite accepting of the types of changes that drift can produce: it’s a creationist-friendly form of “microevolution”.)
The phenomenon of drift is important in evolution (e.g., helping to preserve some variability under weak selective pressure, helping homozygous recessive beneficial mutations penetrate to the point where NS can act on them, and in accounting for variations in allele frequency in different populations), but to put it on par with natural selection in bringing about the last 3 billion years or so of the development of life from simple beginnings is (in my mind) just absurd.
Anyway, I hope I was not addressing a strawman version of your position. (I did try to get a citation out of you).
Regards.
divalent says,
If I’m parsing your response correctly, I think you are saying is that “evolution” is: 1) any change in sequence (even in junk) that eventually gets fixed, and 2) that any such change is considered quantitatively equivalent (in terms of the amount of “evolution” it produces) to any other change in sequence (even those that have phonotypic effects that enhance survival).
I'm saying that the minimal definition of evolution is ...
Evolution is a process that results in heritable changes in a population spread over many generations. [What Is Evolution?]
I'm happy to entertain any other reasonable definitions, do you have one?
I'm also saying that we have no reason to put a value judgment on the kinds of evolutionary change that occur. Yes, it's true that some people are more interested in the kind of evolution that produces large visible changes in animals; but it also true that some of us are interested in all changes at the molecular level inducing those that gave rise to junk DNA.
The problem I have with this is that it is a very broad, and mostly academic, measure of evolution. So it may be true that, by this definition, most evolution is due to drift, but what does that tell us about what the type of evolution that virtually the whole world has in mind when they use the term outside of a molecular dating lab?
It tells us that "virtually the whole world" needs to be better educated about evolution. I'm trying to help but I find that I have to educate my fellow scientist first. That's disappointing.
The type of evolution that Darwin and his contemporaries, and those who followed for at least the next 90 years, endeavored to explain?
It is a mistake to assume that all visible evolutionary changes are due to natural selection. I was hoping you wouldn't make that common adaptationist error, but, of course, you are.
“Given that they arose from a common ancestor, what was the process that gave rise to the differences between [insert any two species: dog-cat, lion-gazelle, shark-lizard, human-chimpanzee, tree-worm, etc]?” No one who asks this question is thinking about predominantly neutral changes (e.g., sequence-variations in junk DNA, or synonymous substitutions, or even functionally equivalent AA substitutions).
And that's a real shame because many of the differences between these species are neutral changes that have been fixed by random genetic drift. The fact that most people don't realize this, is a problem in science education—a problem that isn't made any easier by people like Richard Dawkins (and you?) who refuse to admit the possibility.
Both questioner and questionee understand that it is the differences in the characteristics of the phenotype that define those organisms that make the question important and interesting.
I agree that it's an interesting question. We'd like to know whether these characteristics are mostly due to random genetic drift or adaptations. You seem to be making the assumption that all the interesting differences are due to natural selection. Do you have any evidence to back this up or are you just reflecting your bias?
And that in turn brings me to the issue about claims of the relative importance of drift vs selection. A recurring (obsessive?) theme of your postings is the claim that drift is as important as (or, as you claim in this post, there is strong evidence that it is *more* important than) selection in the evolution of life.
That's correct. In terms of the number of heritable changes that have become fixed in a population over time the evidence strongly suggests that random genetic drift is a more common mechanism of fixation than natural selection.
The problem is that drift takes you nowhere in particular; it’s change without effect (or change with little effect). Drift won’t give rise to limbs, immune systems, brains, livers and kidneys, eyes and ears, and so on and so on. Drift won't give rise to the Krebs cycle, or Calvin cycle, enzymes. Drift won’t endow that newly duplicated gene with a new function. It won’t lead to increased complexity, only trivially different complexity.
What you say above is mostly true but not entirely. You are mostly expressing a blind faith in the idea that all visible changes (including enzymes) must be adaptations. That is not a scientifically defensible position, is it?
None of the things we marvel at when we study, say, a rainforest ecosystem, with its various species of animals and plants (and fungi and bacteria, etc etc) occupying specific roles in an integrated web of life, is the product of drift.
Now you're "drifting" into dangerous territory. If you claim that speciations are all due to natural selection then you are definitely wrong. Anyone interested in the diversity of the rain forest has to understand random genetic drift and the role it plays in speciation.
Do you honestly believe that every single species of beetle is the result of adaptation to a particular clump of trees? Are you absolutely convinced that differences in the shape of the leaves or the texture of the bark on closely related trees are adaptations? What is the evidence for such a sweeping generization—a generalization that allows you to completely dismiss random genetic drift as a possible explanation? Do you know something I don't?
NS is the process that led to the diversity of life as we observe it.
That is not correct. Random genetic drift plays an important role in generating diversity. Only a biased adapationist would make the statement you just made.
NS allows species to adapt (behaviorally, morphologically, metabolically) to diverse environments by bringing about those characteristics that we now used to define those species, and which distinguish them from their ancestors.
Natural selection is the only mechanism responsible for adaptations. What you are doing here is making an incorrect assumption. You are assuming, without proof, that every difference between species is an adaptation.
What is your evidence that all the differences between say, the African and Indian rhinoceros, are adaptations? [Visible Mutations and Evolution by Natural Selection].
It is natural selection that does ALL the heavy lifting. (Yeah, drift might alter the Km of this reaction, or the Vmax of that, but only after NS created the enzyme in the first place; big deal).
You are making the classic adaptationist error. First, you are assuming that the only interesting thing about evolution is what you, personally, are interested in; namely, adaptation. Second, you are assuming, without evidence, that all the important differences between species are adaptations.
The important question is, "Is it true that all the "heavy lifting" is due to natural selection?" Don't you see that by avoiding this question you are not doing proper science?
To the extent that the term Evolution has any meaning to society, it is my definition, not yours.
The fact that your personal bias conforms to that of the general public is not an argument in your favor.
The school board in Cobb County Georgia did not put a disclaimer that “evolution is only a theory” on their biology textbooks because they feared the implications of neutral mutations in junk DNA. (In fact, the Discovery Institute is actually quite accepting of the types of changes that drift can produce: it’s a creationist-friendly form of “microevolution”.)
The fact that your personal biases are similar to those of creationists is not a scientific argument concerning the relative importance of random genetic drift and natural selection. The fact that you think it is, is very troubling to me.
The phenomenon of drift is important in evolution (e.g., helping to preserve some variability under weak selective pressure, helping homozygous recessive beneficial mutations penetrate to the point where NS can act on them, and in accounting for variations in allele frequency in different populations), but to put it on par with natural selection in bringing about the last 3 billion years or so of the development of life from simple beginnings is (in my mind) just absurd.
Nobody is saying that natural selection isn't important and nobody is saying that adaptations aren't a large part of the history of life. But that's no reason to completely dismiss random genetic drift by relegating it to a trivial "helper" status for natural selection. That's just wrong.
Anyway, I hope I was not addressing a strawman version of your position. (I did try to get a citation out of you).
No problem. I think you understand the gist of my position. The most important deficit in your understanding of evolution is your inability to admit that very visible differences between species can be due to random genetic drift. But that's almost the definition of an adaptationist so I'm not surprised.
"The problem is that drift takes you nowhere in particular; it’s change without effect (or change with little effect)"
Absolutely false; an event of, say, founding effect means the new population is but an expansion of that small sample. Neutral traits become FIXED this way, without selection.
"Drift won’t give rise to limbs, immune systems, brains, livers and kidneys, eyes and ears, and so on and so on. Drift won't give rise to the Krebs cycle, or Calvin cycle, enzymes. Drift won’t endow that newly duplicated gene with a new function"
I am convinced that none of these would have evolved without non-adaptive neutral changes. Non-adaptive traits, fixed by drift (some of which are "spandrels") allow exaptation and new function.
The role of drift in facilitating or impeding adaptive pathways was discussed by Sewall Wright. Nowadays it's a favorite topic of population genetics theorists: how drift interplays with evolvability.
"selection is the exclusive mechanism for the origin of adaptations"
This is simply false. The fact that this mantra is repeated like holy gospel by some is a good sign that it's not a statement based on facts, but on ideology or simply cultural bias. An oldie beloved, but still, false and shoud be headed to the wastepaper basket.
The role of drift is contingent to the origin of adaptations.
And as I've repeated to you so many times, exaptation is not natural selection. If you think about that, Larry, it's enough to falsify your assertion. I thought you were good at logic, but it's juts tossed out of the window when it comes to some "gospel truths" of yours.
This is the last time I'm making the argument as I have lost hope in saving Larry from repeating that little bit of hugely dated and unscientific "folk wisdom". "exclusive", my ass.
and that's without even mentioning how phenotypic plasticity has been thrust upon the frontline discussion of adaptation
Well, I certainly could have written a bunch more, but I think I made my points in my prior post and I won’t get into a pissing match over them. I will say that in many places your response uncharitably (and patronizingly) distorted what I said.
Just a couple of items:
1. Regarding the definition of evolution: there is nothing wrong with defining it they way you did, and I’m sure it is a useful definition for your research purposes. But when you say “It tells us that "virtually the whole world" needs to be better educated about evolution.” I take that as an admission that you understand that your definition (the “correct” one, IYO) is not recognized as the default meaning by a large segment of the world. Consequently, you should understand that when you claim that “the evidence suggests strongly that [drift] is the most common form of evolution” that people will be misled by what you say, because they will have a different concept of the term in their mind. Your definition does not match the one used in common parlance, nor the one used by anybody prior to Watson and Crick.
However, I don’t so much disagree with your definition, as I do with how you then quantify the relative importance of (in this case) drift vs selection. Your claim leaves the impression that there is strong evidence that drift is even more important than NS. My point is that, even if it is true that most genetic changes are due to drift, that this claim is misleading because the *important* changes were most likely due to NS. It’s a little like defining “weight gain” as any process that adds weight to a persons body, and declaring that the accumulation of billions of particles of atmospheric dust on a person is more important than their habit of overeating as the cause of their obesity.
2. African vs Indian rhinos: I’m floored by your use of this example. Are you really advancing the argument from Lewontin’s personal incredulity to support drift? And your flavor of it sounds eerily like Behe! (“What is your evidence that *all* the differences between say, the African and Indian rhinoceros, are adaptations?”) I don’t study rhinos, so I don’t know, but I do know that drift is not established as the cause if I or anyone else can’t answer the question. Drift only wins if someone shows that drift did it.
Regarding Lewontin’s horn issue: what do the fossils say? Did the African gain one, or did the Indian lose one? Was it all at once, or gradual? What else accompanied the gain (or loss) of the horn? And when did this all happen? What were the climate and ecosystems like for each species during the millions of years they have been separated? What is the developmental sequence that produces the horns in the two different species and what is the fate of the segments that do it in the African one and don’t in the Indian? I need to know this stuff and so much more to even begin to intelligently approach the problem.
But on the face of it, I can imagine MANY plausible explanations, which include developmental changes in the face and head structure driven my natural and sexual selection.
But again, Lewontin’s lack of imagination is no evidence one way or the other, and our current inability to establish that all features that differ between two species of rhino is no evidence that “drift did it”. Is there evidence that drifts did it? Indeed, is there evidence that drift did anything like this? In my view, the burden is on those who claim that something is drift to establish that fact.
divalent asks,
But again, Lewontin’s lack of imagination is no evidence one way or the other, and our current inability to establish that all features that differ between two species of rhino is no evidence that “drift did it”. Is there evidence that drifts did it?
There's as much evidence that "drift did it" as there is that natural selection did it.
I think you missed the point of Lewontin's example. It's not his lack of imagination that's at issue, it's yours. The adaptationist fallacy is that adaptationists are unwilling, or unable, to consider alternative explanations for their observations. They assume that all the diversity of life is due to natural selection.
I'm glad to see that you are beginning to think about other possibilities. You need to start using your imagination to consider the possible role of random genetic drift instead of just dismissing it as something that is uninteresting and unimportant.
Let me remind you of what you said a few days ago ...
None of the things we marvel at when we study, say, a rainforest ecosystem, with its various species of animals and plants (and fungi and bacteria, etc etc) occupying specific roles in an integrated web of life, is the product of drift. NS is the process that led to the diversity of life as we observe it.
Do you still maintain that position?
Larry: “I think you missed the point of Lewontin's example.”
Possibly. I interpreted it as an affirmative assertion that it’s a prima facia example of a situation that will defy an adaptationist explanation. (And that that was why you brought it up, to suggest that perhaps drift is involved). Is it not? However, to avoid arguing at cross purposes, could you please state Lewontin’s point (and your purpose) *plainly* for me?
While I await clarification, I’ll just note that in the absence of knowledge about how the two different horns came about, one is left to guess at what might plausibly have happened. If a second horn arose (or disappeared) gradually over time, then I don’t easily see how drift could account for it. But I can easily imagine a large number of plausible explanations based on selection. A gradual change would imply the accumulation of many changes to the genome, producing a directional shift in size, and I just don’t see drift as likely in that circumstance. OTOH, if one arose (or disappeared) abruptly, then drift becomes more plausible, but there will also remain plausible explanations based on selection.
Larry: “The adaptationist fallacy is that adaptationists are unwilling, or unable, to consider alternative explanations for their observations.”
Strawman. Just because they initially focus on the most plausible explanations, and/or address the ones that best generate testable hypotheses, doesn’t mean that they are oblivious to other processes. The problem with testing for genetic drift in phenotypically visible traits is that it is very very hard to establish (it has to “look” like drift, and you have to rule out any selection pressure, and much of the evidence needed to do this has been lost to the past).
Larry: “There's as much evidence that "drift did it" as there is that natural selection did it.”
I’m sorry, but this is irrelevant. Just because we don’t know, doesn’t mean that one of many possibilities has a 50/50 chance of being correct. Adaptive changes to major head features has been documented numerous times in other species (e.g., whale nostrils), but AFAIK, no one has yet convincingly shown how drift could solely account for anything that approaches the scale of a large horn on the face of a large mammal. (If you have a citation, I’ll take it). We could add “God did it” to the mix of possibilities (and, of course, the creationists do), but at some point you have to winnow down the possibilities to those that are plausible, based on our understanding of what those processes are capable of, and that are likely to lead to productive lines of inquiry.
If you study point mutations in junk DNA, then drift would be perfect starting hyposisis, but selection is a better starting position for a major phenotypically visible characteristic.
I have no major problem with you defining (and quantifying) evolution as any change in the genome, and will concede that under that definition it is not unreasonable to claim drift is a major mechanism. I do have a problem when you use your definition (which you concede is not the one used by most other people) to make claims about the relative importance of drift and selection in “evolution” when you don’t also make clear the definition you are using. When you tell high school biology teachers (the target audience of the web site text you propose changing) and (by proxy) high school biology students (and parents, and school board members, and voters, and politicians) that there is strong evidence that drift is the most common mechanism of “evolution”, you are misleading them, and that is not a good thing to do. Because you know that their conception of "evolution" is not what you are refering to. (And their concept of "evolution" is not unreasonable for a high school level biology class.)
Update: a bit of hunting clarified Lewontins example. According to how Gould characterises it, it is an example of multiple adaptive peaks. (Although its not really relevant to our discussion, it turns out that he guessed wrong in assuming that the two forms arose independently: the two types did not evolve separately, but rather the one-horned species derived from a two-horned ancestor).
So he wasn't making an argument from person incredulity. (my appologies to him).
Based on this, it's not clear why this is an appropriate example, as he didn't propose it to support genetic drift, and it doesn't illustrate the result of genetic drift. Indeed, he felt they were both adaptations: he was just illustrating the point that the final form is contingent on local factors and random chance that determines what direction you starting heading towards from a starting position.
Please give us your adaptationist just-so story that explains the evolution of the one-horned species from the one with two horns.
While you're at it, you might try explaining the prevalence of male pattern baldness, the ability to roll your tongue, and the shape of eyelids on east Asians. According to you, they must all be adaptations, right?
Aw, man. Now you're just trollin' me!
I'll pass on the first, because I'm sure you would be able to think of that same ones I thought of (you're stubborn, but not stupid).
I'd pass on the other three, but I think they are interesting, so I'll let you jerk me along just a bit longer :)
Pattern baldness: tough one, because the onset is well past sexual maturity, so sexual selection is probably unlikely (or if it is, it would be negative). I don't know the mechanism, so I really can't say what the gene (if it is due to just one gene) is involved in, so its just speculation, but drift would be a reasonable candidate.
Roll your tongue: a really good candidate for drift (assuming the gene that controls it is limited to just this function), although my guess would be that the *inability* to roll your tongue is really the "new thing".
Eye shape in Asians (and many other facial features often noted as characteristic of the "classic" race groupings): probably sexual selection. (Jared Diamond had an interesting essay on this phenomenon in, I think, the Third Chimpanzee).
There are lots of others that are good candidates for drift, such as the inability to smell certain things (like asparagus in pee).
Ok, I'm done with the topic. I've said my piece.
divalent,
Ok, I'm done with the topic. I've said my piece.
Me too. I'm glad we're ending on a note of agreement. We both agree that much of the diversity within and between species could be due to random genetic drift. This was the surprise of the '60s when Lewontin and others first uncovered the huge amount of diversity within a species.
gawd you're a butt! Like I said, your wife must be a saint! :)
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