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Friday, April 23, 2010

What Is Evolutionary Theory? Futuyma vs Coyne

I've been under the impression that the distinction between the fact of evolution and evolutionary theory is not controversial—at least among evolutionary biologists. Ever since Gould, the point has been that the facts of evolution include things like common descent and the history of life on Earth. Evolutionary theory attempts to provide a mechanism that accounts for those fact and observations.

Richard Dawkins makes this clear in his book The Greatest Show on Earth (p. 17).
Biologists often make a distinction between the fact of evolution (all living things are cousins) and the theory of what drives it (they usually mean natural selection, and they may contrast it with rival theories such as Lamarck's theory of 'use and disuse' and 'the inheritance of acquired characteristics'). ... Nowadays it is no longer possible to dispute the fact of evolution itself—it has graduated to become a theorum or obviously supported fact—but it could still bedoubted (just) that natural selection is its major driving force.
The distinction is important. Things like common descent and the history of life are the facts that demonstrate evolution. Evolutionary theory offers a solid, widely-accepted, explanation of how evolution happens.

Douglas Futuyma has written one of the most respected textbooks on evolution. He agrees with this distinction—as do all other textbook authors that I know of. Here's what Futuyma says in Evolution 2nd ed. p. 4.
The explanation of how modification occurs and how ancestors gave rise to diverse descendants constitutes the theory of evolution. We now know that Darwin's hypothesis of natural selection on hereditary variation was correct, but we also know that there are more causes of evolution than Darwin realized, and that natural selection and hereditary variation themselves are more complex than he imagined. A body of ideas about the causes of evolution, including mutation, recombination, gene flow, isolation, random genetic drift, the many forms of natural selection, and other factors, constitute our current theory of evolution or "evolutionary theory." Like all theories in science, it is a work in progress, for we do not yet know the causes of all of evolution, or all the biological phenomena that evolutionary biology will have to explain. Indeed, some details may turn out to be wrong. But the main tenets of the theory, as far as it goes, are so well supported that most biologists confidently accept evolutionary theory as the foundation of the science of life.
No doubt you're puzzled about the purpose of this posting. You are probably saying to yourself. "So what? We all know that, already."

Apparently, not all of us agree. In an otherwise excellent review of Richard Dawkin's book, Jerry Coyne says the following [see: The Improbability Pump].
Demonstrating the truth of natural selection is just one of Dawkins's aims, for the theory of evolution is composed of several more or less independent parts, which I like to describe in one longish sentence: "Life on earth evolved gradually, beginning with one primitive species; it then branched out over time, throwing off many new and diverse species--and the process producing the illusion of design in organisms is natural selection." This sentence constitutes a scientific theory, which is not just a guess but an informed statement about the general principles that explain many observations about nature.
I think that's very wrong. First, it's wrong because it states that the history of life is a theory. Second, it's wrong because it states that the "illusion of design" is part of modern evolutionary theory (it isn't). Third, it's wrong because it only mentions natural selection and modern evolutionary theory is much more than that.

I hope this was just an attempt to (over-)simplify evolution for the readers of The Nation. In that case it might be (just) excusable. But I can't wait until the creationists get a hold of this review. They'll be delighted to learn that, according to Jerry Coyne, the gradual descent and diversification of life is only a theory.

They'll also be happy to learn from a prominent evolutionary biologist that design is part of modern evolutionary theory.


Harriet said...

I posted this to my facebook account. I understand what is going on, but alas, many of the creationist/ID pinheads won't. They'll exclaim: "see, I told you that evolution was just a theory and not a fact"....

This is the yin/yang of the internet age. Still, I love having this window of access to a field other than my own (mathematics).

Alexander said...

I'm not sure this distinction carries a lot of weight. Unless you exclude historical hypotheses from your definition of theories, I don't see how common descent isn't a theory. Common descent is a set of propositions about the history of life that accounts for certain facts about genetic homologies, geographical distribution, etc etc. It's a theory because the truth of common descent isn't directly observable but is inferred because it makes sense out of a wide range of superficially disparate facts.

But if you for some reason want to exclude well-confirmed historical hypotheses from being theories and consider them facts, then I guess you're right. But I think the analogy between historical theories and scientific theories is close enough to warrant using the same terminology.

Eamon Knight said...

Have you read Coyne's book? He commits the same sin in few places, and at greater length. (However, I have to make clear that on the whole WEIT is excellent).

gillt said...

Based on examples such as this, it's a wonder science journalists get anything correct isn't it.

Marcello Pucciarelli said...

I think you could give a more charitable interpretation of Coyne's statement (though he could have put it better than that): 1. gradual descent from an ancestral species, and further branching, are the patterns of evolution predicted by the theory, not at the same level of "fact" as evolution itself; 2. replace "illusion of design" with "adaptation" and you get a valid statement describing a theory (though I guess you would disagree with such a theory, which you can because it's not a fact).

Re-read the beginning of Coyne's article. He says of the germ theory of infectious diseases that "although it's called germ theory, the idea that infections are spread by small creatures is also a fact, supported by mountains of evidence." Instead of relying on the dichotomy of fact and theory, Coyne shows that some theories are so much supported by evidence to make some of their statements count as facts. Being a fact in this sense depends on the degree of support from evidence.

The bottom line is: you cannot say that evolution is a fact unless you can show that the theory of evolution (broadly conceived, descent with modification and branching) is true, supported by evidence beyond any reasonable doubt.

Anonymous said...

Coyne also said this almost a the beginning of his article:

"But that's fiction, right? Well, not entirely, for it applies precisely to another "theory" that is also a fact: the theory of evolution. "

A. Vargas said...

prominent? maybe among adaptationists

El PaleoFreak said...

In my opinion:
Common descent is fact.
Branching diversification of life is a fact.
Speciation is a fact.
Adaptation is fact.
All evolution being adaptive is of course not a fact.
Gradual evolution is a fact ("gradual" meaning accumulative, "by steps").
All evolution being gradual is not a fact.
Appearance of design is not a natural fact itself, but it's a real scientific "problem" (that has been solved).
Natural selection is a fact.
That natural selection explains a lot of evolution, including adaptation, is not a fact. It's part of the Evolutionary Theory.
Evolutionary Theory is not a fact. It's a theory that explains facts (Gould's classic distinction).

I think in his book Coyne fails at telling apart fact of evolution and evolutionary theory.

In the Spanish translation of the book, its title is "Por qué la teoría de la evolución es verdadera" (Why evolution theory is true). Strange.

paul01 said...

Is evolution a *fact*?

Or would it be better to say it is a *finding of fact*, as in a court of law.

In this sense evolution is one of the findings of modern science and hence a fact, but not a fact directly.

Larry Fafarman said...

The relationships between living things are facts. Evolution is an attempt to explain the origins of those relationships. That's a theory, not a fact.

El PaleoFreak said...

Of course evolution is a fact. Evolution is change in the genetic composition of populations, something that occurs in the real world. Not "an attempt to explain" something.

Anonymous said...

Evolution is something that happens all the time in the world of life as we know it.

That is a fact. That is the sense in which "evolution is a fact".

There are theories which explain how and why evolution happens. They tell us that "X explains evolution". That is one sense in which "evolution is (part of) a theory".

And there are theories which explain features of the world of life by invoking evolution in the explanation. They tell us that "evolution explains Y". That is another sense in which "evolution is a theory".


Mike from Ottawa said...

"Of course evolution is a fact. Evolution is change in the genetic composition of populations, something that occurs in the real world. Not "an attempt to explain" something."

It's also a completely uncontroversial fact, since, in those terms, even the young Earth creationists accept it.

Mike from Ottawa said...

"They'll also be happy to learn from a prominent evolutionary biologist that design is part of modern evolutionary theory."

Only because they won't pay attention to the difference between "the illusion of design" and "design".