Thursday, August 18, 2011

Evolution Is a Fact and a Theory

This year is the 30th anniversary of the publication of Stephen Jay Gould's famous essay, Evolution as Fact and Theory in Discover magazine (May 1981).1

Back in 1993, I wrote a essay for talk.origins promoting the basic concepts that Gould, and others, advocated [Evolution is a Fact and a Theory (1993)]. This essay has been modified and updated several times since then—the latest version was on my website [Evolution Is a Fact and a Theory (2007)].

You would think that the simple concept described in all those articles would be widely understood by creationists but that's not the case. Even today, there are creationists who struggle to understand the difference between fact and theory. That's why I'm posting the 2011 version of my essay.

Evolution Is a Fact and a Theory

When non-biologists talk about biological evolution they often get confused over several different meanings of the word "evolution." On the one hand, there's the question of whether or not modern organisms have evolved from older ancestral organisms or whether modern species are continuing to change over time. On the other hand, there are questions about the mechanism of the observed changes... how did evolution occur? Biologists consider the existence of biological evolution to be a fact. It can be demonstrated today and the historical evidence for its occurrence in the past is overwhelming. However, biologists readily admit that they are less certain of the exact mechanism of evolution; there are several theories of the mechanisms of evolution. Stephen J. Gould has put this as well as anyone else:

In the American vernacular, "theory" often means "imperfect fact"--part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is "only" a theory and intense debate now rages about many aspects of the theory. If evolution is worse than a fact, and scientists can't even make up their minds about the theory, then what confidence can we have in it? Indeed, President Reagan echoed this argument before an evangelical group in Dallas when he said (in what I devoutly hope was campaign rhetoric): "Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science—that is, not believed in the scientific community to be as infallible as it once was."

Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered.



Moreover, "fact" doesn't mean "absolute certainty"; there ain't no such animal in an exciting and complex world. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are not about the empirical world. Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us falsely for a style of argument that they themselves favor). In science "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional consent." I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.

Evolutionists have been very clear about this distinction of fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory—natural selection—to explain the mechanism of evolution.


Stephen J. Gould, "Evolution as Fact and Theory"; Discover, May 1981
Gould is stating the prevailing view of the scientific community. In other words, the experts on evolution consider it to be a fact. This is not an idea that originated with Gould. The same point was made by Theodosius Dobzhansky eight years earlier—and by others before him.

Let me try to make crystal clear what is established beyond reasonable doubt, and what needs further study, about evolution. Evolution as a process that has always gone on in the history of the earth can be doubted only by those who are ignorant of the evidence or are resistant to evidence, owing to emotional blocks or to plain bigotry. By contrast, the mechanisms that bring evolution about certainly need study and clarification. There are no alternatives to evolution as history that can withstand critical examination. Yet we are constantly learning new and important facts about evolutionary mechanisms.

Theodosius Dobzhansky "Nothing in Biology Makes Sense Except in the Light of Evolution" American Biology Teacher vol. 35 (March 1973) reprinted in Evolution versus Creationism, J. Peter Zetterberg ed., ORYX Press, Phoenix AZ 1983
Dobzhansky's point is that evolution is occurring and has occurred in the
past. These facts are not questioned by anyone who is familiar with the evidence for evolution. He and Gould both knew that there are people who do have doubts about evolution. The scientists' goal is not so much to convince the doubters but to stop them from claiming that evolution is "only" a theory.

It is absolutely safe to say that if you meet somebody who claims not to believe in evolution, that person is ignorant, stupid or insane (or wicked, but I’d rather not consider that).

Richard Dawkins
This concept isn't so hard to understand when you are talking about gravity. Gravity is a fact but scientists still have a theory of gravity to explain how gravity works. The anti-evolutionists will not concede that the same distinction applies to the facts of evolution and evolutionary theory. They continue to proclaim that evolution (the fact) is "only a theory." Dobzhansky suggests that these anti-evolutionists are either ignorant of the evidence or resistant to the evidence. I think he should have entertained the possibility that some of the knowledgeable anti-evolutionists are well aware of the distinction but choose to lie to their audience.

Note that I used the term "evolutionary theory" rather than "the theory of evolution." That's because there's no such thing as "the theory of evolution" and it's time we stopped using that phrase. In its place we talk about evolutionary theory that encompasses a wide variety of ideas ranging from the theories of population genetics to models of how speciation occurs.

What are some of the facts that demonstrate evolution? Lewontin explains,

It is time for students of the evolutionary process, especially those who have been misquoted and used by the creationists, to state clearly that evolution is a fact, not theory, and that what is at issue within biology are questions of details of the process and the relative importance of different mechanisms of evolution. It is a fact that the earth with liquid water, is more than 3.6 billion years old. It is a fact that cellular life has been around for at least half of that period and that organized multicellular life is at least 800 million years old. It is a fact that major life forms now on earth were not at all represented in the past. There were no birds or mammals 250 million years ago. It is a fact that major life forms of the past are no longer living. There used to be dinosaurs and Pithecanthropus, and there are none now. It is a fact that all living forms come from previous living forms. Therefore, all present forms of life arose from ancestral forms that were different. Birds arose from nonbirds and humans from nonhumans. No person who pretends to any understanding of the natural world can deny these facts any more than she or he can deny that the earth is round, rotates on its axis, and revolves around the sun.

The controversies about evolution lie in the realm of the relative importance of various forces in molding evolution.


R. C. Lewontin "Evolution/Creation Debate: A Time for Truth"
Bioscience 31, 559 (1981) reprinted in Evolution
versus Creationism, op cit.
The sad thing about the Gould, Dobzhansky, and Lewontin articles is that they were written so long ago but they continue to be relevant today. An entire generation has grown up since Dobzhansky's 1973 essay was published in American Biology Teacher yet we still have political leaders who question the scientific fact of evolution.

This ignorance can't be due to the lack of education since the fact/theory concept has been explained in introductory biology books that are used in colleges and universities (and in some of the better high schools). For example, in some of the best such textbooks from twenty years ago, we found:
Today, nearly all biologists acknowledge that evolution is a fact. The term theory is no longer appropriate except when referring to the various models that attempt to explain how life evolves... it is important to understand that the current questions about how life evolves in no way implies any disagreement over the fact of evolution.

Neil A. Campbell, Biology 2nd ed., 1990, Benjamin/Cummings, p. 434
Also:

Since Darwin's time, massive additional evidence has accumulated supporting the fact of evolution--that all living organisms present on earth today have arisen from earlier forms in the course of earth's long history. Indeed, all of modern biology is an affirmation of this relatedness of the many species of living things and of their gradual divergence from one another over the course of time. Since the publication of The Origin of Species, the important question, scientifically speaking, about evolution has not been whether it has taken place. That is no longer an issue among the vast majority of modern biologists. Today, the central and still fascinating questions for biologists concern the mechanisms by which evolution occurs.

Helena Curtis and N. Sue Barnes, Biology 5th ed. 1989,
Worth Publishers, p. 972
One of the best introductory books on evolution (as opposed to introductory biology) is that by Douglas J. Futuyma. In the 2nd edition back in 1986 he makes the following comment:
A few words need to be said about the "theory of evolution," which most people take to mean the proposition that organisms have evolved from common ancestors. In everyday speech, "theory" often means a hypothesis or even a mere speculation. But in science, "theory" means "a statement of what are held to be the general laws, principles, or causes of something known or observed." as the Oxford English Dictionary defines it. The theory of evolution is a body of interconnected statements about natural selection and the other processes that are thought to cause evolution, just as the atomic theory of chemistry and the Newtonian theory of mechanics are bodies of statements that describe causes of chemical and physical phenomena. In contrast, the statement that organisms have descended with modifications from common ancestors--the historical reality of evolution--is not a theory. It is a fact, as fully as the fact of the earth's revolution about the sun. Like the heliocentric solar system, evolution began as a hypothesis, and achieved "facthood" as the evidence in its favor became so strong that no knowledgeable and unbiased person could deny its reality. No biologist today would think of submitting a paper entitled "New evidence for evolution;" it simply has not been an issue for a century.

Douglas J. Futuyma, Evolutionary Biology, 2nd ed., 1986, Sinauer Associates, p. 15
This is not an argument from authority. I'm simply pointing out that the distinction between evolutionary facts and evolutionary theory has been explained over and over for the past half century and there's no excuse for not knowing what scientists think about evolution.

Evolutionary Theory

There are several possible mechanisms of evolution.
There are many people who reject evolution for religious reasons. In general these readers oppose both the fact of evolution and evolutionary theory, although some anti-evolutionists have come to realize that there is a difference between the two concepts. That is why we see some leading anti-evolutionists admitting to the fact of "microevolution"—they know that evolution can be demonstrated. These people will not be convinced of the "facthood" of (macro)evolution by any logical argument and it is a waste of time to make the attempt. The best that we can hope for is that they understand the argument that they oppose. Even this simple hope is rarely fulfilled.

There are some people who are not anti-science but still claim that evolution is "only" a theory that can't be proven. This group needs to distinguish between the fact that evolution occurs and evolutionary theories about the mechanisms of evolution. But there's an additional point that needs to be emphasized. Some of the proposed mechanisms of evolution, such as natural selection and random genetic drift, are facts, not speculations. We know for a fact that both these mechanisms occur in living populations. These are not "theoretical models" of evolution, they actually occur. They are part of evolutionary theory because, in many cases, we don't know for sure which one predominates in a particular case—or even if there might be another mechanism such as Lamarckian inheritance, molecular drive, or mutationism.

Fact of Evolution

Chimps and humans share a common ancestor.
Similarly, there are degrees of facthood. Some facts that are easy to demonstrate and others are more circumstantial. Examples of evolution that are readily apparent include the fact that modern populations are evolving and the fact that two closely related species share a common ancestor. The evidence that Homo sapiens and chimpanzees share a recent common ancestor falls into this category. There is so much evidence in support of this aspect of primate evolution that it qualifies as a fact by any common definition of the word "fact."

This is an important point. It is not, by any stretch of the imagination, a "theory" that humans and chimps share a common ancestor. We know enough about the history of life to state that this is a scientific fact.

In other cases, the available evidence is less strong. For example, the relationships of some of the major phyla are still being worked out. Also, the statement that all organisms have descended from a single common ancestor is strongly supported by the available evidence, and there is no opposing evidence. However, it is not yet appropriate to call this a "fact" since it's possible that there were several independent life forms that exchanged genes early on in evolution. This would mean that modern species have descended from more than one common ancestor. Most of us don't think this is very likely but the possibility exists. Common descent may not be a hard fact but it's not part of evolutionary theory either. Evolutionary theory is silent about most aspects of the unique history of life on this planet just as gravitational theory is silent about the unique formation of an eight-planet solar system around an average star at the edge of the Milky Way galaxy. The history of life and the formation of our solar system have to be consistent with what we know about evolution and gravity but neither evolutionary theory nor gravitational theory predict what that history should be.

Finally, there is an epistemological argument against evolution as fact. Some philosophers point out that nothing in science can ever be "proven" and this includes evolution. According to this argument, the probability that evolution is the correct explanation of the origin of chimps and humans may approach 99.9999...9% but it will never be 100%. Thus, the common ancestry of chimps and humans can never be a fact. This kind of argument might be appropriate in a philosophy class (it is essentially correct) but it won't do in the real world. A "fact," as Stephen J. Gould pointed out (see above), means something that is so highly probable that it would be silly not to accept it. This point has also been made by others who contest the nit-picking epistemologists.

The honest scientist, like the philosopher, will tell you that nothing whatever can be or has been proved with fully 100% certainty, not even that you or I exist, since we might be dreaming the whole thing. Thus there is no sharp line between speculation, hypothesis, theory, principle, and fact, but only a difference along a sliding scale, in the degree of probability of the idea. When we say a thing is a fact, then, we only mean that the probability of it being true is high—so high that we are not bothered by doubt about it and are ready to act accordingly. By this use of the term "fact"—the only proper definition—evolution is a fact. For the evidence in favor of it is as voluminous, diverse, and convincing as in the case of any other well established fact of science concerning the existence of things that cannot be directly seen, such as atoms, neutrons, or solar gravitation ....
So enormous, ramifying, and consistent has the evidence for evolution become that if anyone could now disprove it, I should have my conception of the orderliness of the universe so shaken as to lead me to doubt even my own existence. If you like, then, I will grant you that in an absolute sense evolution is not a fact, or rather, that it is no more a fact than that you are hearing or reading these words.

H. J. Muller, "One Hundred Years Without Darwin Are Enough"
School Science and Mathematics 59, 304-305. (1959)
reprinted in Evolution versus Creationism op cit.
In any meaningful sense evolution is a fact, but there are various theories concerning the mechanism of evolution.

Creationists will never accept that evolution is a fact and they will continue to lump the history of life into "the theory of evolution." They will never accept that evolutionary theory includes many models and many proven mechanisms. They insist that it's all "Darwinism." I don't expect to change their minds—I'm not that naive—but I do expect them to learn the truth about what scientists are saying, even if it's only to criticize the science. Surely that's not too much to ask?


1. Republished in Hen's Teeth and Horse's Toes, New York: W. W. Norton & Company, 1983, pp. 253-262.

14 comments:

  1. I read Gould very differently than I'm inferring that you have read him. To me Gould is saying that Evolution - that species change through time and are related by common descent - is both a fact AND theory. You imply that Gould meant that evolution as I have defined is fact and that the mechanisms are theory. At the end of your essay you even seem to make the mistake Gould castigated at the beginning of his essay by ranking fact and theory in a hierarchy of probability. I think my reading of Gould, which boils down to basically any fact is also a theory (it is a fact that cells have membranes. This is also a theory. It is a fact that muscle contract by sliding filaments. This is also a theory), is what troubles non-scientists.

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  2. @middle.professor,

    You are correct when you say that you read Gould very differently than I do.

    You are incorrect when you say that I rank fact and theory as part of a hierarchy or probability. Some parts of evolutionary theory are facts—natural selection is a fact. Some parts of the history of life are not much more than speculations. Some facts, such as chimps and humans sharing a common ancestor, are factual even though we don't know how much of a contribution natural selection has made in each lineage (i.e. we don't know the mechanism.)

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  3. I have always understood "facts" to be those things that can be directly observed, while "theories" are explanations of those observations. Thus, "Apples fall to the ground" is a fact, but even today the mechanism is poorly understood, and thus gravity really is "only a theory," and the jury is out on whether it is the curvature of space-time or the exchange of graviton force particles.

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  4. For many years, I have thought it a shame that we cannot call it the "Law of Evolution". After all, we have never encountered a life form that has demonstrably not evolved from earlier life. Indeed, we have not encountered an organism that clearly did not evolve from the Last Universal Common Ancestor. I have encountered Creationists who have been a bit obsessed with Laws because they "cannot be broken", and so it might shut them up. (Of course, Laws can be broken and are updated or re-scaled in the light of new data but I suspect that anyone savvy enough to know this would also to be savvy enough to understand what a scientific "theory" really is.

    Great post, well said.

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  5. @cabbagesofdoom,

    Have you every tried to formulate a "Law of Evolution" that qualifies as a law?

    I've tried. It's not possible.

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  6. Very nicely explained. Great post.

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  7. Spell check observations, for posterity's sake:

    1) "In it's place we talk about evolutionary theory [...]"-- the possessive is incorrectly spelt.

    2) "The sad thing about the Gould [...] conitinue to be relevant today."-- "continue" is incorrectly typed?

    that is all

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  8. I learned a lot when I read the earlier version. I don't agree with everything and as a physicist I would press more on the ubiquity of natural processes, but that is small quibbles.

    @ Anonymous:

    "I have always understood "facts" to be those things that can be directly observed, while "theories" are explanations of those observations."

    But nothing is "directly observed" nor do we have "explanations"!

    - Already Newton realized in spite of having realism we can only use causality to probe nature, we can only learn from a systems reactions on our actions.

    Quantum mechanics revealed further that it is meaningless to entertain the notion of properties between states, that observation can only tell a subset of reactions (observables), and that it changes the system we observe.

    Finally special relativity took away the ability to do immediate observations "here" and "now", we have only access to the lightcone of probe particles so see "there" and "when".

    There are, as in the case of observable facts vs testable theories, a quantitative but no qualitative difference in remoteness (using instruments, using instruments on other planets, using instruments that captures probe particles (light) from the early universe, et cetera).

    - Theories are predictive, so we can test them for validity. But gravity doesn't tell you the exact pathway (as the article notes), so doesn't provide a minute explanation for your apple fall. You can't get from a statistical description of apple trees & winds to a prediction of an individual apples fall.

    On the other hand with the new standard cosmology, general relativity has been tested to the extreme limits of our observable universe by observation of more or less "direct" nature.

    I would say that the observation of processes are facts for everyone, and that processes are theories for realists, i.e. for a realist there is a well defined mapping between the process and its validated theory. We know curvature of space-time exists, what we don't know is if curvature changes by particle mediation.

    @ Larry Moran:

    "I've tried. It's not possible."

    What a challenge! Especially since "laws" are falling out of fashion, to be replaced by "universal facts" and "general principles" instead.

    Not being a biologist, I would have though that Fisher's fundamental theorem of natural selection and Hardy-Weinberg equilibrium ("Hardy-Weinberg Law") would qualify.

    But maybe you mean the larger theory. Well, I would think that the observation that "descent by modification" results in phylogenies, trees or bushes (nested sets), would be a law. (I hear that you may need "super-matrix" methods to resolve both vertical and horizontal gene transfer, but it seems doable. Even if not, trees are more common than not.)

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  9. "I have always understood "facts" to be those things that can be directly observed,"
    is far too limited. I have an oak tree in my yard, about a hundred years old. No one, so far as I know, saw it sprout, and certainly I did not, yet I affirm with all confidence that it's source was an acorn. How can I know that?

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  10. @Larry Moran. No, I don't think it's possible, which is why it isn't a Law, although I suspect that some individual aspects, like selection and drift, could be formulated as Laws. To be perfectly honest, I'm not sure when something becomes eligible to be a Law with a big L. In hindsight, I think I was really meaning "law of evolution" with a small L, as in " law of biogenesis" - an observation that has never been falsified and is accepted as fact. (It's semantics, not science - but Creationists do love their semantics, especially when wearing an ID hat.)

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  11. alnitak said...
    "I have always understood "facts" to be those things that can be directly observed,"
    is far too limited. I have an oak tree in my yard, about a hundred years old. No one, so far as I know, saw it sprout, and certainly I did not, yet I affirm with all confidence that it's source was an acorn. How can I know that?"

    I think the key phrase in what you said is:
    "can be directly observed".

    The growth of a tree from an acorn CAN be directly observed. You personally may not have observed it but we all agree that it CAN be observed.

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  12. That we have witnessed evolution based on natural selection is no problem for me. The method, however, is a big problem. I would love to hear more about a theory that explains how elements occurring billions of years ago became organic compounds. And how those elements became DNA. And how DNA became able to be an architect and provider of raw materials and develop multiple complex systems with interdependence such that without all the parts and connections, the system would not function. I'm not doubting that it happened but please help explain how.
    Just someone thinking about the subject.

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  13. Might it be useful to substitute fact by data. It should be clear that mere data are nothing but noise without a theory to give them any meaning. That should make it quite clear that having a theory is more than having mere facts, not less. Alas, creationists will probably be daft enough to claim they've data of god.

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  14. I use the term "The Theory of Evolution", but while I don't see a problem with it at all, I also do not have a problem with using "evolutionary theory" instead. Where I personally think the problem lies is in the usage of the word "evolution" by itself to refer to the scientific theory. I think that's very problematic, including making it easier for anti-science people to radically cloud the whole subject. We don't do this with other parts of science. For example, no one would ever refer to gravitational theory as "gravity". That would just be very confusing, wouldn't it? So why do we refer to evolutionary theory or The Theory of Evolution as evolution? We have to stop doing that, in my opinion. The word "evolution" by itself should primarily and perhaps only be used to refer to the idea that life forms change over time, usually into more advanced forms. That idea is a publically observable occurrence (a fact), and should always be treated as such, without using all the ridiculous language creationists want us to use in order to confuse everyone and make it seem like there is some kind of legitimate debate with regard to whether evolution happens or not. There isn't, and we've got to stop helping the anti-science people with their agenda. Thanks for reading. :)

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