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Monday, October 03, 2022

Evolution by chance

Can natural selection occur by chance or accident? No, with qualifications. Can evolution occur by chance or accident? Yes, definitely.

While tidying up my office I came across an anthology of articles by Richard Dawkins. It included a 2009 review of Jerry Coyne's book Why Evolution Is True (2009) and one of Richard's comments caught my eye because it illustrates the difference between the Dawkins' view of evolution and the current mainstream view that was described by Jerry in his book.

I can illustrate this difference by first quoting from Jerry Coyne's book.

This brings up the most widespread misunderstanding about Darwinism: the idea that, in evolution, "everything happens by chance" (also stated as "everything happens by accident"). This common claim is flatly wrong. No evolutionist—and certainly not Darwin—ever argued that natural selection is based on chance ....

True, the raw materials for evolution—the variations between individuals—are indeed produced by chance mutations. These mutations occur willy-nilly, regardless of whether they are good or bad for the individual. But it is the filitering of that variation by natural selection that produces natural selection, and natural selection is manifestly not random. (p. 119)

It's extremely important to notice that Coyne is referring to NATURAL SELECTION (or Dawinism) in this passage. Natural selection is not random or accidental, according to Coyne. This passage is followed just a few pages later by a section titled "Evolution Without Selection."

Let's take a brief digression here, because it's important to appreciate that natural selection isn't the only process of evolutionary change. Most biologists define evolution as a change in the proportion of alleles (different forms of a gene) in the population.

[Coyne then describes an example of random genetic drift and continues ...] Both drift and selection produce the genetic change that we recognize as evolution. But there's an important difference. Drift is a random process, while selection is the antithesis of randomness. Genetic drift can change the frequencies of alleles regardless of how useful they are to their carrier. Selection, on the other hand, always gets rid of harmful alleles and raises the frequencies of beneficial ones. (pp. 122-123)

Now let's look at Richard Dawkins' review of Coyne's book as published in the Times Literary Supplement in 2009 and reprinted in Books Do Furnish a Life (2021). I picked out an interesting passage from that review in order to illustrate a point.

Coyne is right to identify the most widespread misunderstanding about Darwinism as 'the idea that, in evolution, 'everything happens by chance' ... This common claim is flatly wrong.' Not only is it flatly wrong, it is obviously wrong, transparently wrong, even to the meanest intelligence (a phrase that has me actively restraining myself). If evolution worked by chance, it obviously couldn't work at all. (p. 427)

That last sentence is jarring to many scientists, including me. I think that the Dawkins' statement is 'obviously wrong' and 'transparently wrong' because, as Coyne pointed out, evolution by random genetic drift can occur by chance. [Let's not quibble about the meanings of 'random' and 'chance." That's a red herring in this context.] Clearly, evolution can work by chance so why does Dawkins say it can't?

It's not because Dawkins is unaware of random genetic drift and Neutral Theory. The explanation (I think) is that Dawkins restricts his definition of evolution to evolution by natural selection. From his perspective, the fixation of alleles by random genetic drift doesn't count as real evolution because it doesn't produce adaptations. That's the view that he described in The Extended Phenotype back in 1982 and the view that he has implicitly supported over the past few decades [Richard Dawkins' View of Random Genetic Drift].

This is one of the reasons why we refer to Dawkins as an adaptationist and it's one of the reasons why so many of today's evolutionary biologists—especially those who study evolution at the molecular level—reject the Dawkins' view of evolution in favor of a more pluralistic approach.

Note: I wrote an earlier version of this post in 2009 [Dawkins on Chance] and I wrote a long essay on Evolution by Accident where I describe many other examples of evolution by chance.


Mikkel Rumraket Rasmussen said...

It's certainly true evolution can still "work" without producing adaptations all the time, depending on exactly what one means by work.

But the role of natural selection in maintaining fitness against accumulation of deleterious mutations shouldn't be ignored when the average mutation is deleterious. Of course if no adaptation ever happened either, that would eventually be a problem too.

A lot of stuff happens by drift, and there is such a thing as constructive neutral evolution that can build up (usually gratuitous)complexity, but without any natural selection to at least maintain fitness and purge deleterious alleles, all organisms would go extinct.

When I see this selectionist vs neutralist debate I always feel like I'm watching two sides talking past each other. There are different senses or perspectives from which to view evolution. Fish didn't genetically drift unto land those 380 million years ago. They adapted. The fact that most mutations that occurred had fitness effects so small their fixation or loss owed mostly to drift in species with small effective population sizes seems in that sense rather besides the point. What is the right sense in which to look at evolution? Count total mutations? `Large-scale ecological and morphological change? Depends who you ask.

Joe Felsenstein said...

A similar issue arises when people ask what process causes most "evolution". If they mean most adaptation, then natural selection is the answer. If they mean most gene frequency change, including changes that reverse each other, then for sure the answer is genetic drift. The danger is that emphasizing that genetic drift causes most changes of gene frequency will lead people to think that the elephant's trunk evolved by genetic drift, without any natural selection.

John Harshman said...

I seem to recall that to Ernst Mayr, neutral evolution didn't count as evolution.

Larry Moran said...


It's not that the two sides are "talking past each other" but you are correct to note that the adaptationists and the pluralists have different perspectives.

The pluralists recognize that evolution is complex. They do not ignore the importance of adaptation and natural selection but they (including me) often seem to be downplaying the role of selection because we are trying hard to get people to realize that there's more to evolution than that. It often seems like we are pushing random genetic drift and Neutral Theory too hard and losing perspective.

But that's often what you have to do when you are trying to change minds.

I would be happy if the adapationist side simply toned down THEIR rhetoric and admitting that evolution is complicated and there's a role for chance and accident. After all, it's not as if there's no evidence for drift and nearly-neutral alleles. Why ignore it?

Larry Moran said...


Mayr lived for one hundred years and wrote lots of books, essays, and scientific papers. He had lots of ideas but, unfortunately, consistency was not one of his goals. You can find support for almost any idea in evolution if you comb through everything he wrote.

However, there were several themes that he kept coming back to time and time again. He did not like mathematics or genetics ("beanbag genetics"). He thought that the only important thing in evolution was the selection of individual phenotypes. From his perspective, the individual was the unit of selection and genes were a distraction. He also refused to acknowledge any role of drift in affecting phenotypic change; to him, that was all meaningless neutral evolution.

"The neutralists are reductionists, and for them the gene - more precisely the base pair - is the target of selection. Hence, any fixation of a 'neutral' base pair is a case of neutral evolution. For the Darwinian evolutionists, the individual as a whole is the target of selection, and evolution takes place only if the properties of the individual change. A replacement of neutral genes is considered merely evolutionary noise and irrelevant for phenotypic evolution ....

The Darwinian wonders to what extent it is legitimate to describe as evolution the changes in gene frequencies caused by nonselected random fixation."

Ernst Mayr (1991) "One Long Argument" pp. 152-153

Anonymous said...

in evolution, 'everything happens by chance' ... This common claim is flatly wrong.'

The wording there is pretty clear: it is talking about the claim that everything happens by chance, with no selection at all. I think everyone agrees that can't be right.

Dawkins would have been clearer if the next sentence, the one you most objected to, had the word 'solely' added:

If evolution worked solely by chance, it obviously couldn't work at all.

then it is restating the same idea, and is meant to emphasize that chance alone is not sufficient for evolution.

Jonathan Badger said...

But I don't think that's a serious danger (outside of Creationists who doubt all kinds of evolution). Instead the general public thinks natural selection is so powerful that they even think humans were inevitable by it. Think of all the science fiction that thinks that it is reasonable that creatures identical to humans would evolve on other planets. As opposed to the idea that if could "rewind the tape" it is unlikely that humans would evolve again even on Earth

Robert Byers said...

Robert Byers.
For the creationist here the point is both sides can exist because both sides only are using a line of reasoning, There is no scientific evidence for either on this point or both would be persuaded. This is a problem in evolution everywhere. A creationist can nmot debunk the science for either idea because there is no science. One is debunking a line of reasoning. Aha. Caught then.
Anyways. Darwin best explained, to me, that its simply the idea that selection is pouring over every nuance in biology and bringing change in p[oulations based on a advantage for a new nuche or new reality in the envirorment and food supply etc etc.

Evolution is based on random chances of outcomes in those reproduced. it is random how the kids are different at birth. so mutations etc are random. Selection is the engine for change Darwin said. I say this is wrong Mutation is the demand for evolution. the mutations have got to be notable to bring the complexity of evolution.
Evolution just doesn't work and is not true. If you think about very carefully. if i'm right the near future will keep getting worse for Darwin.

John Harshman said...

Can't really get much worse than dead.

Robert Byers said...

Funny but actually in Christian doctrine, unfortunately, it can be worse then just not being alive.

John Harshman said...

Just goes to show that Christian doctrine is evil, and God, if he exists and he's anything like what that doctrine describes, is evil too.

Doug said...

Hi Dr. Moran, you and others have posted that you wrote a nice essay about macroevolution some time ago but the link is no longer valid:

Will you post the essay on your blog so those of us interested can read it?


Larry Moran said...


Ask and you shall receive. :-)


Doug said...

Thank you!