Jerry Fodor and Massimo Piattelli-Palmarini have just published a book called What Darwin Got Wrong. I haven't read the book but from the reviews I've seen, it's not something that I'm looking forward to. However, their main thesis is that natural selection has been oversold as an explanation for evolution and I have a great deal of sympathy for that point of view. Furthermore, I think that adaptationism—the assumption that adaption is the default explanation for everything that evolves—is a scientifically bankrupt position. I'm a pluralist.Michale Ruse has reviewed the book for boston.com and I'd like to analyze his review in order to reveal where he goes wrong.
Like I said, I haven't read the book so I can't really comment on the specifics. But I can comment on what Micahel Ruse says about the book.Origin of the specious
This new critique intends to rebut Darwin’s ideas but seems largely to misunderstand evolutionary theory.
“What Darwin Got Wrong’’ is an intensely irritating book. Jerry Fodor, a well-known philosopher, with coauthor Massimo Piattelli-Palmarini, a cognitive scientist, has written a whole book trashing Darwinian evolutionary theory - the theory that makes natural selection the main force of change in organisms through the ages.
He begins by claiming that the authors misunderstand evolutionary theory. That may be true but it will become painfully obvious that Michael Ruse is not enough of an authority to make such a claim.
Let's begin by seeing how Ruse describes evolution. He says that "Darwinian evolutionary theory" is the theory proposing that natural selection is the main force of evolution. Strictly speaking, that's correct. What we're interested in debating is whether "Darwinian evolutionary theory" is correct as defined.
The answer is clearly "no." Random genetic drift is the most common mechanism of evolution as long as you define evolution properly. Thus, as a explanation of evolution, Darwinism is not as good as a pluralistic evolutionary theory. Although Ruse isn't clear on this, it's well known from his previous writings that he thinks of Darwinism as the preferred explanation of evolution and not just of adaptation. In fact, he rarely distinguishes between the two.
I conclude, right from the beginning of the review, that Michael Ruse has a poor understanding of evolutionary theory.
You would think that somewhere in the pages there would be one - just one - discussion of the work that evolutionists are doing today to give a sense of how the field itself has evolved. Peter and Rosemary Grant on Darwin’s finches for example; Edward O. Wilson and Bert Hölldobler on ant social structures perhaps; David Reznick on Trinidadian guppies perchance? But no such luck. A whole book putting in the boot and absolutely no serious understanding of where the boot is aimed.Aren't those interesting examples? Just what you'd expect from a myopic adaptationist. What about studies of molecular evolution which are almost entirely based on neutral changes and random genetic drift. You'd think that someone who claims to be on top of modern evolutionary theory would recognize the growing evidence of non-adaptive change, wouldn't you?
Why write such a book? The authors would respond in two ways. First, in a section that would be better described as “What Darwin Didn’t Know,” rather than “What Darwin Got Wrong,” they tell us that today’s cutting-edge biology has all sorts of explanations of organic origins that make Darwinism otiose. We learn that life is constrained by the laws of physics and chemistry, and that something like natural selection, which supposedly molds organic life into sophisticated bundles of adaptations, simply cannot get off the ground. To the contrary, evolution is all a matter of molecular development, guided by the self-organizing laws of the physical sciences.In their various published articles Fodor and Piattelli-Palmarini have over-emphasized "constraints" and they come off sounding like some new-agers who have just discovered molecular biology.
To which Darwinians can only respond, wearily again, that they have known about constraints since “The Origin of Species.’’ Because body weight cubes as length increases, you cannot build a cat the size of an elephant. The elegant feline legs needed for jumping must be replaced by tree trunks able to carry many pounds. And examples of plausible self organization have been fitted into the Darwinian picture for many years. A favorite example is the way that many flowers and fruits (like pine cones) exhibit patterns following the Fibonacci series, made famous by “The Da Vinci Code.’’ Chauncey Wright, a 19th century pragmatist, discussed these patterns in detail, showing how formal rules of mathematics can nevertheless yield organisms that are highly adapted and that natural selection is the vital causal element. The rules give the skeleton, and then selection fills in the details. The order of a plant’s leaves may be fixed, but how those leaves stand up or lie down is selection-driven all of the way.
But the fundamental point about constraints is interesting and it's true that adaptationists have been forced to recognize it every since Gould and Lewontin published the Spandrels paper back in 1977. Most adaptationist still don't get it and Ruse is no exception, although in this case he probably gets it better than Fodor and Piattelli-Palmarini. Ruse admits that there are certain physical constraints on the way plant leaves evolve, for example, but he then goes on to say that everything else is an adaptation. How does he know this? How does he know for sure that the differences in the leaves of red maples, silver maples, and sugar maples are all due to natural selection?
The second half of the book is a frontal attack on natural selection itself. The main argument is very odd. It is allowed that there is differential reproduction. Some organisms have many offspring, and some have just a few. It is even allowed that the reason why some succeed and others don’t might have to do with the superior features possessed by the winners and not the losers. At which point you might think: Darwinism wins, because what else is there to natural selection?I take it as "gospel" that random genetic drift is an important mechanism of evolutionary change. Why do I get the impression that Michael Ruse has doubts about this? Why does he use the word "gospel" to refer to the ideas of Gould and Lewontin but not Dawkins and E.O. Wilson? Isn't that strange?
Not so fast, however. Our authors take as gospel the argument of the late Stephen Jay Gould and the geneticist Richard Lewontin that although some features may be adaptive others may not. This argument is then used to say that if an organism succeeds in life’s struggles, you can never conclude that a particular feature was essential for this success, because there may be other features linked to it. Perhaps it was the latter features that were essential. Natural selection fails therefore as a mechanism of change.
Hundreds of evolutionary biologists have written about random genetic drift and other possible mechanisms of evolution (e.g., molecular drive, species sorting). They do not claim, as Ruse implies, that non-adaptive traits become fixed because they are "linked" to adaptive ones. Is this how Ruse dismisses random genetic drift—by treating it as a by-product of natural selection?
In fairness, Fodor and Piattelli-Palmarini do go on about linkage in their published articles so Ruse is right to mock those silly claims. However, I wish he didn't make things worse by implying that hitchhiking is the explanation for drift.
The existence of random genetic drift does not mean that natural selection "fails. " It just means that natural selection by itself is not a sufficient explanation for evolutionary change. Perhaps Fodor and Piattelli-Palmarini are confused about this—other reviews suggest that this is the case—but Michael Ruse seems to be trashing the very idea that something other than natural selection could be at play.
I read all of this stuff a couple of times. I am just not used to people giving the opposition everything for which they have asked and then plowing on regardless. But, even if you ignore the apparently shared belief that selection is at work - we may not know which features were crucial, but that hardly stops us saying that there was selection at work - the other points hardly crush the Darwinian. It has long been known that features get linked. And in any case, we can ferret out which features are most useful and which are just along for the ride. Suppose eyes, which are surely necessary, are linked to tufts of hair, which may not be. Well, experiment and see how the organisms get along without eyes and then without hair.Non-adaptive features can arise even if they are completely unlinked to adaptive feature. Ruse doesn't seem to understand this basic concept of population genetics. And Ruse needs to take his own advice. Rather that just assume that a feature is an adaptation, you need to do the experiments. This applies to the leaves on a tree and the beaks of the finch.
Fodor and Piattelli-Palmarini will not allow this, because apparently we are now ascribing conscious intentionality to the nonconscious world. We are saying the eyes were designed for seeing in a way that the tufts were not. And they stress that the whole point of a naturalistic explanation, to which the Darwinian is supposedly committed, is that the world was not designed.Darwinist are always saying that the world has the appearance of design. Of course it's a metaphor but it's a metaphor based on the idea that natural selection, and not God, is the designer.
In response, one can only say that this is a misunderstanding of the nature of science. The Darwinian does not want to say that the world is designed. That is what the Intelligent Design crew argues. The Darwinian is using a metaphor to understand the material nonthinking world. We treat that world as if it were an object of design, because doing so is tremendously valuable heuristically. And the use of metaphor is a commonplace in science.
Ruse and his fellow adaptationists treat the world as if it were an object of design because they are psychologically committed to the idea that natural selection is responsible for almost everything. They cannot adjust to the fact that much of what we see in living things could be due to accident, or even the fixation of deleterious mutations. That's one of the reasons they have so much trouble with junk DNA and it's why they can't account for so much diversity in populations.
Here's a clue. Life doesn't actually look terribly designed. Get over it. Abandon the metaphor—it just feeds into a false notion of evolution and, incidentally, lends support to the IDiots.
Why then do we have these arguments? The clue is given at the end, when the authors start to quote - as examples of dreadful Darwinism - claims that human nature might have been fashioned by natural selection. At the beginning of their book, they proudly claim to be atheists. Perhaps so. But my suspicion is that, like those scorned Christians, Fodor and Piattelli-Palmarini just cannot stomach the idea that humans might just be organisms, no better than the rest of the living world. We have to be special, superior to other denizens of Planet Earth. Christians are open in their beliefs that humans are special and explaining them lies beyond the scope of science. I just wish that our authors were a little more open that this is their view too.This is despicable. Evolutionary psychology is a broken discipline. And it's not because there is no genetic components to behavior—of course there is. It's because the field is dominated by adaptationist explanations and crazy "just-so" stories that would make Rudyard Kipling proud.
If you accept, as I do, that humans do all kinds of silly things just because of their culture and superstitions, and not necessarily because they are adaptive, then that makes us more like the other animals and not more special. If you accept that we are products of evolution by accident and not "design" (metaphorically) then that makes us farther removed from a potential designer and not closer to God as Ruse would have you believe.
Ruse and the adaptationists are the ones who skate close to the edge when it comes to supporting Christian concepts of life. They do this by conceding that we look designed when that's simply not the full story.
[Image Credit: The photo is from Paul Nelson on the Intelligent Design website. It refers to Ruse's idea that evolution is a form of religion. There's something to be said for this idea, especially when it's applied to confirmed Darwinists.]
31 comments :
Ruse has always irritated me, spending so much time making nice with the creationists that he now carries many of their fleas. One such flea seems to be their inability to understand modern evolutionary theory as being far more than RM&NS.
"Darwinist are always saying that the world has the appearance of design. Of course it's a metaphor but it's a metaphor based on the idea that natural selection based on the idea that natural selection, and not God, is the designer"
Appearance of design is not just a metaphor based on a previous idea; it's a reality: living beings are -in certain important aspects- strikingly similar to designed objects. You can think of this appearance as a proof of existence of a designer, or you can be a scientists and think of it as a fascinating scientific problem that can be solved. And it has been solved.
I hope being a "pluralist" doesn't imply being a denialist of this fascinating aspect of Biology.
"you can think of this appearance as a proof of existence of a designer, or you can be a scientists and think of it as a fascinating scientific problem that can be solved. And it has been solved"
Some have never understood that scientifically accepting a notion as weird as "APPEREANCE of design" falls nicely on the ground set by rev paley. The most pitiful side- effect of this is that, because darwin used natural selection to counter paley's argument, some are hard-set on this being true, such that to deny natural selection as the explanation is the equivalent of giving in to inteligent design creationism.
True evolutionary scientists (yes, paper-making) know evolution is true and don't give a shit what creationists think, so they can calmly examine the question on the origin of adaptation without assuming a prioiri that natural selection will be the answer.
We can say that selection definitely affects the direction the evolution of an adaptation can take, and that it is a necessary factor to understand the origin of adaptations. Be we can also confidently say that natural selection is being overhyped when considered a sufficient explanation of tne evolutionary origin of adaptation. Constraint, phenotypic plasticicty, exapatation, drift, are some notions different from selection that are often needed to understand the evolution of specific biological adaptations.
The fact that the truth is more complex, and that we must use many notions other than natural selection to understand how adaptations evolve, is no reason to panic: it does not question in any way the fact of evolution itself. I think it is sad but true, that adaptaionist philsophers like Ruse and Dennet, adn well, Dawkins himself have all failed utterly to see this point. Anticreationism is the true source of their adaptationism, in a quite transparent case of cultural contamination of science.
You say: "What about studies of molecular evolution which are almost entirely based on neutral changes and random genetic drift." Part of the problem may be that such studies never figure in popular accounts of evolution. I would be interested in counter-examples, i.e. treatment of neutral change and drift in popular science books, and would be grateful if you could share any reference.
I think F&PP have a good point (already developed, e.g., by Lewontin), that you can have selection (a change of a given shape in statistical distributions) without necessarily having adaptation (driven by improvements in function). The problem is that from this starting point (which IMHO is correct, but still eludes not a few philosophers) they proceed to deny any role to natural selection in evolution. In doing so they are dumping every existing populational model of evolution, including those which account for neutral changes and drift.
ElPaleoFreak says,
Appearance of design is not just a metaphor based on a previous idea; it's a reality: living beings are -in certain important aspects- strikingly similar to designed objects. You can think of this appearance as a proof of existence of a designer, or you can be a scientists and think of it as a fascinating scientific problem that can be solved. And it has been solved.
Do you think humans look as if they have been designed to walk upright? Do they have the right backbone for upright walking? Do they have the right kind of abdominal muscles? The right kind of feet?
On behalf of all those people with sore backs, hernias, and aching feet I'd like to challenge your assumption of design.
Wanna talk about wisdom teeth?
Any competent engineer could do a much better job of designing living things. You must have a strange concept of design if you think that living things look designed. Perhaps Rube Goldberg is your favorite designer? :-)
Dr. Moran, I think it would be very helpful to have a post listing with short explanations, all the mechanisms known to caused evolutionary change.
This is because as a lay reader interested in the theory of evolution and not just in the Evolution/Creation/ID debates i find the intermediate level of information lacking (pitched at like a scientific American level article) and so have to be scouring blog post like this one to hear about mechanisms like Molecular Drive and Species Sorting.
your blog and genomicron are the only ones i know that target a general audience with explanations in evolution beyond the beginner level but not quite yet at the journal article stage and i would like to encourage that.
Larry says,
"Do you think humans look as if they have been designed to walk upright?"
and
"Any competent engineer could do a much better job of designing living things."
As a competent engineer, I can tell you that we deal with constraints all the time (many of them historical in nature). Some of those constraints were "adaptive" for customers at one point and too costly to redesign from scratch, some are more like Spandrels (which the marketing folks like to sell as features :), some may even be a loose metaphor for drift - supplier changes or lack of knowledge of engineering or physics. Most of us can still infer many of the differences between the "designed" elements and the constraints as well as how the constraints impacted the "designed" elements, even without a complete documentation of the history.
I think the claim of PaleoFreak was that "in certain important aspects" living beings are similar to designed objects. That doesn't sound like any arch-adaptionist claim that adaption by natural selection explains everything and is not inconsistent with sore feet. Blame us engineers for bad shoe designs!!
I think it would be very helpful to have a post listing with short explanations, all the mechanisms known to caused evolutionary change.
If you define evolution as a change in allele frequencies at the population level (which is often used), the list of evolutionary mechanisms becomes:
-natural selection
-random genetic drift
-mutation
-migration
There are alternative definitions of evolution, so this is by no means an exhaustive list. Good for starters though :-)
"Do you think humans look as if they have been designed to walk upright? Do they have the right backbone for upright walking?" (etc.)
No, Larry, but:
First, I said there is appearance of design in *certain* important aspects of the living beings. Not in every aspect or detail of life.
Second, -and more important- thanks to the work of biologists, we know lots of "bad designed" structures and functions. That's part of the solution of the problem. Bad designed structures prove that *there is not design*. You have *appearance* of design plus evidence against design. You can have both before start thinking about natural selection, lamarckian forces or other possible mechanisms. You don't need to be a "Darwinist" to acknowledge the -I repeat- fascinating scientific problem of appearance of design in Biology.
"The fact that the truth is more complex, and that we must use many notions other than natural selection to understand how adaptations evolve, is no reason to panic"
Right, Vargas: the truth is more complex, and I am trying to introduce some complexity here. Natural selection haters and denialists won't be harmed, so don't panic :o)
ElPaleoFreak says,
Second, -and more important- thanks to the work of biologists, we know lots of "bad designed" structures and functions. That's part of the solution of the problem. Bad designed structures prove that *there is not design*. You have *appearance* of design plus evidence against design. You can have both before start thinking about natural selection, lamarckian forces or other possible mechanisms. You don't need to be a "Darwinist" to acknowledge the -I repeat- fascinating scientific problem of appearance of design in Biology.
So, here's what a pluralist might say ...
Some aspects of living organisms appear to be adaptations consequently they look superficially like they were designed. Other features look haphazard and still others appear to be maladaptive. It is not appropriate to say that "life" looks designed because much of it doesn't.
Here's what an adaptationst would say, ...
Like has the appearance of design.
Which statement do you think is more scientifically accurate?
Here's a clue. Life doesn't actually look terribly designed.
Are you missing a word here? Did you mean
Here's a clue. Life doesn't actually look terribly well designed?
If not, I don't understand your point.
"So, here's what a pluralist might say ..."
I'm not discussing what things a pluralist, darwinist, adaptationist, or whateverist might say. I tried to say that appearance of design is not just a metaphor linked to the concepts of a) God, b) natural selection, but a real scientific problem that deserved research and explanation.
I do think life has the *appearance* of design, but now we know it is not designed. And the Earth has the *appearance* of being flat and fixed, but we know it isn't.
So, blame expressions like "appearance of design" (or "appearance of flat Earth") for its scientific inaccuracy? It makes no sense.
There is a much better, biological term for 'appereance of design". It's called adaptation. I simply refuse to adopt the terms of a XIXth century debate against creationism.
The case in hand is not "how much" do organisms look designed or how much of their structure is adaptive. The problem with the overhyping of selection is when it is considered a sufficient notion for understanding the evolution of adaptation. That simply hides the truth, that many other notions, like those I metioned above, are needed to understand the evolution of adaptation.
A. Vargas, nobody pretends that "appearance of design" is a biological term that can be used instead of "adaptation". So, again, don't panic!
If you don't want to use the expression "appearance of design", whatever personal reason you have, it's up to you. Of course I don't care.
Isn't random genetic drift simply the 'material' that is culled by natural selection? Are the traits we see in animals today explainable by random genetic drift without the influence of natural selection? It seems to me that, in place of natural selection, random genetic drift is being overemphasized here. I've always understood (or misunderstood) RGD as being part of a continuum with NS.
Anonymous asks,
Isn't random genetic drift simply the 'material' that is culled by natural selection?
No, random genetic drift is a mechanism of evolution. Alleles can be fixed by random genetic drift and advantageous alleles can be eliminated by random genetic drift.
See Random Genetic Drift
Are the traits we see in animals today explainable by random genetic drift without the influence of natural selection?
Yes, some of them are but the most important point is that you should not just assume that a trait is an adaptation since random genetic drift can also fix alleles that are not selected.
Random genetic drift should be the default assumption when considering how a trait came fo be present in a population.
It seems to me that, in place of natural selection, random genetic drift is being overemphasized here. I've always understood (or misunderstood) RGD as being part of a continuum with NS.
That's not my understanding but it's a common belief among adaptationists.
Anonymous: It seems to me that, in place of natural selection, random genetic drift is being overemphasized here. I've always understood (or misunderstood) RGD as being part of a continuum with NS.
Larry Moran: That's not my understanding but it's a common belief among adaptationists.
Let us see what Kimura said: The word "neutral" is not used in a strict, literal sense. The emphasis is not on neutrality per se, but on mutation and random drift as the main explanatory factors. The mutant genes that are important in molecular evolution and polymorphism are assumed to be nearly enough neutral for chance to play the major role. This seems to me to be close to Larry's view.
However, Kimura also wrote Although much progress has been made in biology since Darwin's time, his theory of natural selection still remains as the only scientifically acceptable theory to explain why organisms are so well adapted to their environments. This is important to keep in mind when reading creationist claims that the neutralist theory has made natural selection obsolete.
I think the reason why most thoughtful biochemists are much closer than other sorts of biologists to being neutralists is that in biochemistry a great deal of what we see (trivial differences between enzymes catalysing the same reaction in different organisms, for example) makes little sense in terms of adaptation, but a great deal of sense in terms of random drift, whereas other biologists tend to be interested in variations that do seem to make adaptational sense.
"I think the reason why most thoughtful biochemists are much closer than other sorts of biologists to being neutralists..."
It's interesting how different disciplines that interact with or use evolutionary theory see things from very different perspectives - I'm thinking of paleontologists and punctuated equilibrium. I'm just a layman trying to learn about evolution, so I don't really have a dog in the fight over adaptationism (although what I've learned, such as it is, has probably come from the adaptationists for the most part). Is there a good, neutral source that might parse these issues out in a clear manner?
Some get a little "personal" if you point out that natural selection in fact is insufficient to understand the evolution of adaptations. After all, Paley's argument for design was 'solved" by Darwin's natural selection. So goes the legend. Supposedly, darwin made his case of evolution soooo convincing because he was able to provide an 'universal" evolutionary mechanism (I think this is BS, darwin made a pretty good job demonstrating common descent for itself).
In fact, selection explains only a small part of how adaptations evolve. It is NEVER exclusively responsible for the origin of an adaptation. The legend is, in fact, a myth. Selection has been unnecessarily overhyped to counter silly ole creationism. Biology is quite beyond that.
To the engineer above I would ask: how much do you think selection actually explains? Pointing to an eye and saying "this evolved through selection for seeing" does not give much information about mechanism. Organisms. like machines, have an internal structure and organization that explains how they work. We need to understand the evolution of adaptations in these terms as well, beyond the mere identification of selective conditions.
Question: has the coelacanth evolved over all the millions of years it has existed as a "living fossil"?
The adaptionists would seem to say no.
The pluralists would seem to say yes.
For a layperson looking in on this fight is this what the argument between to two camps is basically about?
I get a sense that there is a sort talking at cross purposes over genotypes and phenotypes in these sort of arguments, but I realize as a lay person I'm just not qualified to comment on whether Ruse, Dawkins or whoever really "understand evolutionary theory".
I realize academic tiffs often get very personal and I assume Ruse wouldn't appreciate Larry's choice of words; some wag once explained that academic tiffs get so involved due to them being of such little consequence!
As an outsider trying to understand the issues I find it very hard to disentangle what either of the two sides are really saying - apart from telling the other side they don't understand the subject.
I have no idea if I'm an adoptionist or a pluralist. I'm a layperson looking to be educated - my understanding is that genomes alter overtime often due to random changes. These random changes can sometimes mean that some good traits can get weeded out and some bad traits spread. But this can only happen if the good and bad traits are within a certain threashold - if they are outside this threashold selection will drive the bad traits to extinction and fix good traits.
From this you can say the genome is partially made up of the random circumstance of life and partially from those that have a phenotopical effect which is large enough to be selected for.
My guess is that alot of the genome which is down to random drift does not have any impact on the phenotype, but the fascinating area is where it does, but where the phenotypical response is not enough for it to become altered by selection.
So I suppose you could say the genome has at least 3 parts - 1) phenotypically useless, 2) phenotypically selected and 3) phenotypically unselected/neutral. Adaptionists are interested in 2) and pluralists 3).
Larry's beef is with adaptionists coming up with just so stories to explain aspects of life, when he is conscious of the fact that these traits could be non-selected and exist just from blind chance - ie coming from area 3).
Is any of this right? And does my interpretation make me an adaptionist or a pluralist?
Re-reading what I've written I see an error - I shouldn't have said area 3) is phenotypically unselected/neutral. It doesn't have to be neutral at all - it can contain maladaptive traits, and also adaptive ones as well as neutral ones - the point is the fact that the selective pressure they manefest isn't enough for selection to drive there fixation.
Is that right?
A.V.,
I guess I am the engineer above. I am not an expert on evolutionary biology so the question about selection and what it explains is out of my depth. I will try to answer your question from a non-biologist perspective. If I saw a device that could capture the dominant wavelengths of electromagnetic radiation from the sun and process them in a way that lead to outputs that associated with the inputs in a more than trivial manner, (like it reflected a certain color), I would have high confidence that it was "designed". If I wanted to understand the mechanism, as a first step I would break down the hardware and/or software to determine the causitive elements based on my background knowledge and training. I think I could tell many of the causitve elements from the constraints - for example the exisistence of commercially available analog electrical components could be treated as a constraint, but their arrangement in a way that generated an output to visible light would be a design element for a specific purpose. Not sure if that answered your question - maybe it is really more of a philosophical question between ultimate and proximate causes? For ultimate causes (not in the cartoonist theological sense) but in the sense of "what is this for", the design (adaptation) view has a lot of value IMHO.
"The adaptionists would seem to say no"
The "coelacanth" has changed morphologically enough to become lots of different genera and families. Also, it's well known that a lot of evolution (including adaptive evolution) is invissible, it doesn't fossilize but it happens. So if the adaptationist knows something about the diversity and morphology of Coelacanthiformes, and if he is aware of several very basic things about evolution, he would say yes: the coelacanth has evolved over all those millions of years.
But perhaps you prefer talking about the stupid and ignorant, caricaturized strawman adaptationist.
Anonymous asks,
Isn't random genetic drift simply the 'material' that is culled by natural selection?
And Larry answers:
No, random genetic drift is a mechanism of evolution. Alleles can be fixed by random genetic drift and advantageous alleles can be eliminated by random genetic drift.
IANABiologist, but it seems to me that even despite the ability of genetic drift to be the sole actuator behind evolutionary changes on the level of a single or a few alleles, Anonymous is still partially correct in that genetic drift is impotent to facilitate larger scale changes without the assistance of natural selection.
(I'm trying very hard to avoid the Creationist-favored terminology of microevolution and macroevolution, but it almost might be helpful here)
In thinking about the evolution of dinosaurs into birds... we could point out individual traits and say that a major determining factor for many characteristics of bird morphology was genetic drift, but to eventually have all of these characteristics manifested together in a viable class of animal is obviously impossible without natural selection.
Where I'm going with all of this is that the accuracy of the statement "natural selection is the main mechanism of evolutionary change" is highly dependent on how you define the words. If by "main mechanism" you mean that it accounts for the majority of fixed mutations, then okay, probably genetic drift is more responsible. But if by "main mechanism" you mean that it facilitates the development of not just diversity, but of classes of organisms that are radically different from one another, then I think that statement is defensible.
James Sweet says,
But if by "main mechanism" you mean that it facilitates the development of not just diversity, but of classes of organisms that are radically different from one another, then I think that statement is defensible.
How does natural selection account for diversity?
Take the diversity among human populations, for example. Do you think that most of the obvious differences between Japanese and Nigerians are due to natural selection?
Species Diversity
Why Five Fingers?
The Cause of Variation in a Population
Visible Mutations and Evolution by Natural Selection
The most obvious phenotypic difference between Nigerian and Japanese people is perhaps skin color.
Much work has been done about natural selection (including sexual selection) and skin color, in humans and other species. Not "just so stories". Scientific work. And very interesting.
I'm aware of hypotheses concerning skin color and why Japanese might have a different skin color than Nigerians.
However, there are a lot more differences between the populations. The question is whether most of these differences are adaptations or accidents.
The pluralist argues that we don't know until we have more data but it certainly seems possible that a lot of the diversity we see in human populations could be due to random genetic drift of nearly neutral mutations.
The adaptationist assumes they are adapatations unless proven otherwise. In these discussions, adaptationists frequently point to those few traits where there is some evidence for adaptation (like skin color) and not those that seem to be non-adaptive, like blood type or whether you can role your tongue.
"The question is whether most of these differences are adaptations or accidents"
IMHO, thats "a question", not "the question". You seem very interested in that question :o)
According to my particular experience, adaptationists don't assume most of the differences are adaptations. They focus on few conspicuous or salient characters that (they think) are good candidates for adaptations. They simply are not very interested in *all* the characters or differences and they don't deny genetic drift as a cause.
People who think most or all characters are adaptative should be called *panadaptationists*, Panadaptationists often are non biologists, interested in science but lacking basic knowledge of evolutionary theory.
At the 'digital', single-organism level, an allele either does or does not take part in its own survival/reproduction in a given life. If it never does, it can only be subject to genetic drift (you'll be aware how hot-under-the-collar those mathematicians get over that word "random"). If it sometimes does, then in all those 'other' lives genetic drift is still a significant motor of change. Selected lives generate extra "rolls" in the Allele's Progress over chance. Only if every life involves the positive contribution of the allele can we eliminate the "random" element.
I think the 'pluralist" approach implies a false dichotomy - or rather, a dichotomy where there is actually a continuum. Anything better than purely neutral can be seen as an adaptation – a change in allele frequency caused by the allele’s effect on fitness, by it being a better "answer" to a challenge than the alternatives.
I really don’t see anything 'bankrupt' about assuming an adaptive explanation as your default, rather than your oft-advanced insistence on assuming none - unless there is clearly no phenotypic effect. Neutral drift probably drives more allele-level change than does selection, but change of the grosser features presented is much less clearly in the grip of drift alone, given the lack of linear mapping of allele to phenotypic feature.
Allan Miller says,
I think the 'pluralist" approach implies a false dichotomy - or rather, a dichotomy where there is actually a continuum.
The pluralist approach recognizes that there are many different mechanisms of evolution. It's neither a dichotomy nor a simple continuum since other possible mechanisms include things like species selection and molecular drive—and even fixation of acquired characteristics (although there are no known examples).
I'm also very sympathetic to mutationism, which doesn't fit on your continuum either.
Have you heard of hierarchical theory? That's another pluralist approach that doesn't fall on your imagined continuum.
You are stuck in the adaptationist mindset where everything is seen as a variation on adaptation.
Anything better than purely neutral can be seen as an adaptation – a change in allele frequency caused by the allele’s effect on fitness, by it being a better "answer" to a challenge than the alternatives.
Don't make the mistake of confusing the selective value of a mutation with the mechanism of fixation. Beneficial alleles can be lost by random genetic drift and deleterious alleles can be fixed by random genetic drift.
Neutral theory requires random genetic drift but drift doesn't only affect neutral alleles. Its good to keep in mind that an allele with a 10% selective advantage (s=0.1) will be lost 90% of the time.
While it's true that if an allele has become fixed in the population AND it's selective advantage is known to be positive, then natural selection was responsible (in the vast majority of cases).
Thus your statement is correct. However, I'm not sure you realize that a beneficial allele can be replaced by a deleterious allele. What that means is that non-neutral alleles can also be fixed by random genetic drift.
Nearly Neutral Theory and Random Genetic Drift are not synonyms.
The pluralist approach recognizes that there are many different mechanisms of evolution. It's neither a dichotomy nor a simple continuum since other possible mechanisms include things like species selection and molecular drive—and even fixation of acquired characteristics (although there are no known examples).
OK, I misunderstood the point you were making. My point was that compartmentalisation of the fitness-related rationale of allele spread - even into the trinity of neutral, nearly neutral and adaptive - obscures the fact that there is an element of drift throughout. Each shades into the other, and drift is a significant component that does indeed have the power to fix detriment and remove benefit. That doesn't, however, lead me to the conclusion that drift is the best default position. That would depend upon the known prevalence of non-adaptive vs adaptive alleles. If most are one or the other, then that would be a solid basis for what seems at the moment like a matter of taste.
I'm not sure you realize that a beneficial allele can be replaced by a deleterious allele. What that means is that non-neutral alleles can also be fixed by random genetic drift.
I didn't realise I'd given the impression I thought otherwise. The involvement of probabilities does indeed blunt the deterministic edge of "raw" expectations on fitness alone. The random sampling element is consistent - lives in which a specific allele's advantage was not 'tested' are chalked up to random drift - ie s<1 is always the resultant of both 'actual' selection and drift.
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