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Thursday, October 22, 2009

Richard Dawkins' View of Random Genetic Drift

The Greatest Show on Earth is Richard Dawkins' latest book. It's his eighth book on evolution: the others are The Selfish Gene (1976), The Extended Phenotype (1982), The Blind Watchmaker (1986), River Out of Eden (1995), Climbing Mount Improbable (1996), Unweaving the Rainbow (1998) and The Ancestors Tale (2004).

I'm interested in the evolution of Richard Dawkins' ideas about evolution; in particular, his ideas about random genetic drift and mechanisms of evolution other than natural selection.

In Chapter 1 Dawkins says, "All reputable biologists go on to agree that natural selection is one of its most important driving forces, although—as some biologists insist more than others—not the only one."

This looks promising. Dawkins is saying— in chapter 1—that there are two mechanisms (driving forces) of evolution. He implies that he accepts random genetic drift as a "driving force" of evolution. (Assuming that random genetic drift is what he has in mind.) It's clear that "some biologists" have influenced him, although it's not clear from the sentence whether those biologists are "reputable"!

Since this is a book about the evidence for evolution, I eagerly anticipated his explanation of random genetic drift. Would it be as good as Jerry Coyne's?1 In fact, I was so eager that I couldn't wait. I jumped to the index to look under "random."

Nothing. Not to worry. The other important mechanism must be here somewhere. Is it indexed under "genetic"? No. What about "drift"? No, not there either.

What gives? How can you write a book about evolution in the 21st century without mentioning random genetic drift as an important mechanism of evolution? Even the other adaptationist, Jerry Coyne, has it in the index to Why Evolution Is True.

Maybe Dawkins uses another term for the second mechanism of evolution. I recalled that he often gets mixed up about the difference between neutral theory and random genetic drift. Let's see if "Neutral Theory" is in the index. Nope.

What about "Kimura"? Success at last! Check out page 332.

Page 332 is in the middle of a section on The Molecular Clock in Chapter 10. It seems a bit late to begin discussing the second mechanism of evolution, but, as I said before, it's promising that Dawkins even concedes that there is one.

Dawkins explains that the reason why there's a molecular clock is because the majority of changes at the genetic level are neutral and these changes are fixed in a regular, clock-like, albeit stochastic, process. He then goes on to say...
When the neutral theory of molecular evolution was first proposed by, among others, the great Japanese geneticist Motoo Kimura, it was controversial. Some version of it is now widely accepted and, without going into the detailed evidence here, I am going to accept it in this book. Since I have a reputation as an arch-"adaptationist" (allegedly obsessed with natural selection as the major or even only driving force of evolution) you can have some confidence that if even I support the neutral theory it is unlikely that many other biologists will oppose it!
I can't think of any serious biologists who would deny that neutral mutations exist. The essence of Neutral Theory, or Nearly Neutral Theory as it is currently called, is undoubtedly correct. The fact that Richard Dawkins accepts it in this book is not remarkable. What's remarkable is that he has to tell us that he accepts it, especially in a book about the evidence for evolution.

Meanwhile, we are still waiting for the explanation of the "other" mechanism of evolution. The one that was mentioned in Chapter 1 when he said that natural selection does not account for all of evolution. He can't have been thinking about "Neutral Theory" since that's not a mechanism of evolution. And he can't just have been thinking about a mechanism for fixing neutral mutations since he surely knows that the "other" mechanism can result in the loss of beneficial alleles and the fixation of detrimental ones.

Still waiting. What we see in Chapter 10 is an explanation of neutral mutations but no mention of random genetic drift—the mechanism responsible for fixing neutral mutations in a population. He does briefly mention on page 335 that neutral mutations can "go to fixation by chance." I get the impression that he goes out of his way to not name the other mechanism of evolution. You know what I'm referring to, it's the mechanism that gets a whole chapter to itself in all the evolutionary biology textbooks [Evolution: Table of Contents].

Dawkins concedes that the vast majority of the human genome consists of sequences that aren't genes. Here's how he puts it ...
It is a remarkable fact that the greater part (95% in the case of humans) of the genome might as well not be there, for all the difference it makes. The neutral theory applies even to many of the genes in the remaining 5%—the genes that are read and used. It applies even to genes that are totally vital for survival. I must be clear here. We are not saying that a gene to which the neutral theory applies has no effect on the body. What we are saying is that a mutant version of the gene has exactly the same effect as the unmutated version.
In other words, the vast majority of the DNA in our genome is junk. Mutations that occur in junk DNA will become fixed in spite of the fact that they are not seen by natural selection. This is what he means when he says that most mutations are neutral and it's equivalent to saying that the dominant mechanism of evolution, in terms of overall frequency, is random genetic drift and not natural selection. I just wish he'd come right out and say it.

It's a shame that Dawkins does not actually mention the mechanism by which those neutral mutations become fixed but instead continuously refers to neutral theory as the alternate mode of evolution. The general public needs to hear about random genetic drift and Dawkins is—like it or not—the most prominent evolutionist on the planet.

Dawkins has not changed his mind about the existence of these neutral mutations and he has not changed his mind about their importance. While they may exist, they are not important as far as evolution is concerned. He makes this very clear—once again—in this book.
As it happens, it is probably true to say that most mutations are neutral. They are undetectable by natural selection, but detectable by molecular geneticists; and that is an ideal combination for an evolutionary clock.

None of this is to downgrade the all-important tip of the iceberg—the minority of mutations that are not neutral. It is they that are selected, positively or negatively, in the evolution of improvements. They are the ones whose effects we actually see—and natural selection "sees" too. They are the ones whose selection gives living things their breathtaking illusion of design. But it is the rest of the iceberg—the neutral mutations which are in the majority—that concerns us when we are talking about the molecular clock.

As geological time goes by, the genome is subjected to a rain of attrition in the form of mutations. In that small portion of the genome where the mutations really matter for survival, natural selection soon gets rid of the bad ones and favors the good ones. The neutral mutations, on the other hand, simply pile up, unpunished and unnoticed—except by molecular geneticists.
This is the way the adaptationist dismisses non-adaptive evolution. It's not really of interest to real biologists. It's only interesting to molecular geneticists. And we all know that those people are not real evolutionary biologists!

Now we come to one of the most interesting sentences in the entire book; at least as far as I'm concerned. As most Sandwalk readers know, we have long debated whether or not visible mutations can be neutral. Once you have an observed phenotype, can you ever attribute it to neutrality? Many adaptationists argue that you can't.

Here's what Richard Dawkins says in his latest book.
It is also possible (although "ultra-Darwinists" like me incline against the idea) that some mutations really do change the body, but in such a way as to have no effect on survival, one way or the other.
This is progress. Back when he wrote The Extended Phenotype, in 1982, Richard Dawkins said.
The adaptationism controversy is quite different. It is concerned with whether, given that we're dealing with a phenotypic effect big enough to see and ask questions about, we should assume that it is the product of natural selection. The biochemist's "neutral mutations" are more than neutral. As far as those of us who look at gross morphology, physiology and behavior are concerned, they are not mutations at all. It was in this spirit that Maynard Smith (1976) wrote: "I interpret 'rate of evolution' as a rate of adaptive change. In this sense, the substitution of a neutral allele would not constitute evolution ..." If a whole organism biologist sees a genetically determined differences among phenotypes, he already knows he cannot be dealing with neutrality in the sense of the modern controversy among biochemical geneticists.
Finally, in 2009, Richard Dawkins admits that it is "possible" that visible mutations could be neutral. Hallelujah!

I'm looking forward to book #9.

1. Jerry Coyne's View of Random Genetic Drift


Anonymous said...

Who cares about mutations in junk DNA?

Anonymous said...

Larry: Do you recommend Futuyma's book on evolution, or is there a better one?

Psi Wavefunction said...

"The neutral mutations, on the other hand, simply pile up, unpunished and unnoticed—except by molecular geneticists."

Condescending much? So I guess a REAL biologist is someone who lounges about in the African savannah counting how many giselles a lion tribe has devoured that week. Right. What do we labrats know about life anyway?

@anon: "Who cares about mutations in junk DNA?"

Brian said...

It's funny, I used to think Darwinist was a snide American term for someone who accepted evolution. Like it's a bad thing to accept a fact of nature. It turns out that it is this, from Creationists, but it also means Evolution by (and only by) natural selection. I'm a bit confused however as I thought Darwin thought there were other mechanisms of evolution such as sexual selection. Not that I'm making an argument from authority. I'd just never noticed that the Dawk was exclusively a Darwinist as I'd assumed that the so called Darwin's pitbull would not be an a so called adaptionist (I thought that name was reserved for Alvin Plantinga, or at least his misunderstanding of evolutionary theory). Shows what I know.

Still, I have a lot of respect for the man. Unfortunately, even the greatest amonst us are human, and subject to human foibles and prejudices, no less than mere plebs. like me. At least that's what I tell myself. :)

Very interesting Larry. Thanks.

Veronica Abbass said...

"He implies that he excepts random genetic drift as a "driving force" of evolution."

Is the use of "excepts" a Freudian slip or did you mean to imply that Dawkins accepts and excepts random genetic drift as a "driving force" of evolution?

Rich Lawler said...

There's a pretty large literature on "neutral models of morphological evolution." Dawkins is aware of these but for purposes of his general views, one need not focus on these traits. Work by Mike Lynch, Russ Lande, and a lot of work in biological anthropology, which uses Fst models and applies them to skull morphology, provide examples of neutral phenotypic evolution.

Dawkins, to be fair, does understand a lot about constraints on adaptation and how other evolutionary processes influence phenotypes apart from adaptation (Chapter 3 in The Extended Phenotype outlines his views on this topic).

Kerry Cunningham said...

i just read Prof Dawkins book, and I came away thinking random genetic drift was a pretty important force.

How I, a lay-person, see it:
This book was aimed at people who don't know much about evolution.
Dawkins is very, very famous.
If he says an idea was controversial, or not widely accepted anyway, but now it is, that adds a great deal to the credibility of that idea.
The above phenomenon may not hold for people who's scientific qualifications are similar to his.

dave said...

I'm just reading Dick Dawkins' book, and finding it a very entertaining primer with a fund of good well written illustrations. Amusingly topical right now, as it includes the Darwinius debacle and Richard Lenski's response to what PZ called "the goons and fools at Conservap├Ždia" in a post linked from Panda's Thumb on October 16, 2009, when discussing Joe Thornton.

One slight annoyance, Dawkins keeps trying to get readers to buy Why Evolution Is True by Jerry Coyne for the bits Dawkins thinks are better covered there. Think he's right?

Jeffrey Shallit said...

What's a neutral "vacation"?

Zarquon said...

A vacation in Switzerland.

anthrosciguy said...

On the Dawkins forum last February, Richard Dawkins essentially said we just shouldn't talk about drift because it's going to confuse creationists and give them ammo:

To refute creationists, we have to explain the evolution of adaptation, and that means we have to concentrate on natural selection as the important driving force of evolution. You and I know that, if you actually measure gene frequencies over generations, the majority of changes are random: fixations of adaptively neutral mutations. But adaptively neutral mutations are totally IRRELEVANT to the problem of design. They might as well not be mutations at all. You will only CONFUSE creationists, and give them spurious aid and comfort (something your hero Steve Gould spent much of his life doing), if you bang on about evolution being random.

Richard Dawkins Feb 16, 2009

I can understand his not wanting to give creationists something they can misrepresent, but since they mispresent evrything to do with evolution there's no way to stop that happening. I can't see that ignoring a part of evolution is a smart move.

Psi Wavefunction said...

"But adaptively neutral mutations are totally IRRELEVANT to the problem of design."

Dawkins must HATE the constructive neutral evol people then... ^_^ Heh!

(see Lynch 2007 PNAS, Stoltzfus 1999 JME; he probably has never read those because he seems to have a thing against molecular biologists...)

Anonymous said...

Dr. Moran, i have just read the 1979 spandrel paper by gould and lewontin and found it very informative.
what i would like to see is a update/crtique of section "5: a partial typology of alternatives to the adaptationist programme."
i think it would be wonderful to see the other types of explanations that are considered important by pluralist.

Baking Soda said...

Chemically speaking how can a protein mutation ever be strictly "neutral" if it causes a change in the local chemical environment in a protein?

LK said...

Great post. Thanks Larry.

The Other Jim said...

Baking Soda said...

Chemically speaking how can a protein mutation ever be strictly "neutral" if it causes a change in the local chemical environment in a protein?

Even if there is a change in the peptide's properties / function, if it has no effect on organismal fitness, it is neutral.