Evolution as fact and theory

Evolution as fact and theory
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"Evolution is both fact and theory" is a statement that appears in numerous publications on biological evolution. The statement is framed to clarify misconceptions about the philosophy of evolution primarily in response to creationist statements that "evolution is only a theory". In the context of the creationist claim, theory is used in its vernacular meaning as an imperfect fact or an unsubstantiated speculation. The purported intent is to discredit or reject the scientific credibility of evolution. However, this claim cannot be substantiated.[1][2]

In the statement "evolution is both fact and theory", evolution as theory refers to the scientific (as opposed to the vernacular) meanings of theory. In the first scientific meaning, a theory is an overarching framework that makes sense of otherwise disconnected observations. For example, the theory of gravity unifies astronomical observations with observations about the speed with which an object falls to earth. Similarly, the theory of evolution unifies observations from fossils, DNA sequences, systematics, biogeography, and laboratory experiments. Theodosius Dobzhansky, a key contributor to the modern evolutionary synthesis, articulated the unifying power of evolutionary theory in a famous paper entitled: "Nothing in biology makes sense except in the light of evolution".[3]

In the second scientific meaning, a scientific theory of evolution describes the causes of evolution, as distinct from the more straightforward factual claim that evolution occurs. Natural selection and the neutral theory are examples of theories of evolution in the second scientific sense. These and many other causal evolutionary theories can be expressed in the mathematical framework of population genetics. Since Darwin, the theory of evolution by means of natural selection has not only been expressed mathematically, but has also been rigorously tested and corroborated empirically by scientific evidence from countless studies. Evolutionary theories continue to generate new testable hypotheses within paleontology, genetics, ecology, and developmental biology.

A fact is not a statement of certainty, but through repeated confirmation it is generally accepted as true according to the reliability of inference (inductive, deductive, and abductive). Facts are "events that occur" or "the state of being of things" that can be publicly verified, proven through experiment, or witnessed by direct observation.[4][5] That all forms of life on Earth are related by common descent with modification is one of the most reliable facts in the biological sciences.[2]


Evolution, fact and theory

Evolution has been described as "fact and theory", "fact not theory", "only a theory, not a fact". and "neither theory, nor fact". This illustrates a confusion in terminology.[4][6][7] The meanings of the terms "evolution", "fact", and "theory" are described below.


Evolution is usually defined as changes in trait or gene frequency in a population of organisms from one generation to the next. Evolution also refers to Darwin's theory of natural selection as the process that leads to adaptations and as one of several mechanisms of evolutionary change. Evolution leads to the following additional claims:

  1. Differences in trait composition between isolated populations over many generations may result in the origin of new species.
  2. All living organisms alive today have descended from a common ancestor (or ancestral gene pool).
  3. Evolutionary mechanisms give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.[8]

According to Douglas Futuyma:

Biological evolution may be slight or substantial; it embraces everything from slight changes in the proportion of different alleles within a population (such as those determining blood types) to the successive alterations that led from the earliest proto-organism to snails, bees, giraffes, and dandelions.[9]

Evolutionary theory generally refers to an overarching framework that identifies relationships and a coherent understanding of otherwise disconnected natural observations. In more specific cases, evolutionary theory refers to causes of evolution, such as natural selection, genetic drift, or vicariance.


Fact is often used by scientists to refer to experimental or empirical data or objective verifiable observations.[10][11][12][13] "Fact" is also used in a wider sense to mean any theory for which there is overwhelming evidence.

A fact is a hypothesis that is so firmly supported by evidence that we assume it is true, and act as if it were true. —Douglas Futuyma[14]

Evolution is a fact in the sense that it is overwhelmingly validated by the evidence. Frequently, evolution is said to be a fact in the same way as the Earth revolving around the Sun is a fact.[14][15] The following quotation from H. J. Muller, "One Hundred Years Without Darwin Are Enough" explains the point.

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 its probability is an extremely high one: so high that we are not bothered by doubt about it and are ready to act accordingly. Now in this use of the term fact, the only proper one, evolution is a fact.[16]

The National Academy of Science (U.S.) makes a similar point:

Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is fact. Scientists no longer question whether descent with modification occurred because the evidence is so strong.[17]

Philosophers of science argue that we do not know mind-independent empirical truths with absolute certainty: even direct observations may be "theory laden" and depend on assumptions about our senses and the measuring instruments used. In this sense all facts are provisional.[18][19]


A scientific theory is a well-supported body of interconnected statements that explains observations and can be used to make testable predictions. Scientific theories describe the coherent framework into which observable data fit. The "theory of evolution" is the framework that best explains observed changes of species over time and best predicts the new observations that continue to be made in evolutionary biology and related sciences.

The scientific definition of the word "theory" is different from the colloquial sense of the word. Colloquially, "theory" can mean a hypothesis, a conjecture, an opinion, or a speculation that does not have to be based on facts or make testable predictions. However, in science, the meaning of theory is more rigorous. A theory is hypothesis corroborated by observation of facts which makes testable predictions. In science, a current theory is a theory that has no equally acceptable or more acceptable alternative theory.

Evolution compared with gravity

The application of the terms "fact" and "theory" to evolution is similar to their use in describing gravity.[20]

The most obvious fact of gravity is that objects in our everyday experience always fall downwards when not otherwise prevented from doing so. People throughout history have wondered what causes this effect. Many explanations have been proposed over the centuries. Aristotle, Galileo, Newton, and Einstein have all developed useful models of gravity, each of which constitutes a theory of gravity. (Newton, for example, realized that the fact of gravity can be extended to the tendency of any two masses to attract one another.) The word "gravity", therefore, can be used to refer to the observed facts (i.e., that masses attract one another) and the theory used to explain the facts (the reason why masses attract one another). In this way, gravity is both a theory and a fact.

In the study of biological species, the facts include the existence of many different species in existence today, some very similar to each other and some very dissimilar, the remains of extinct species in the fossil record, and so forth. In species that rapidly reproduce, for example fruit flies, the process of change from generation to generation — that is, evolutionary change — has been observed in the laboratory.[21] The observation of fruit fly populations changing over time is also an example of a fact. So evolution is a fact just as observations of gravity are factual.

There have been many attempts to explain these biological observations over the years. Lamarckism, transmutationism and orthogenesis were all non-Darwinian theories that attempted to explain the observations of species and fossils, as well as other evidence. However, the modern theory of evolution is the explanation for all relevant observations regarding the development of life, based on a model that explains all the available data and observations (and provides testable predictions). Thus, evolution is not only a fact but also a theory, just as gravity is both a fact and a theory.

Evolution as theory and fact in the literature

The confusion over the word evolution and the distinction between "fact" and "theory" is largely due to authors using evolution to refer to three related yet distinct ideas: first, the changes that occur within species over generations; second, the mechanism thought to drive change; and third, the concept of common descent. However, among biologists there is a consensus that evolution is a fact:

  • American zoologist and paleontologist George Simpson stated that "Darwin... finally and definitely established evolution as a fact."[22]
  • H. J. Muller wrote, "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."[16]
  • Kenneth R. Miller writes, "evolution is as much a fact as anything we know in science."[23]
  • Ernst Mayr observed, "The basic theory of evolution has been confirmed so completely that most modern biologists consider evolution simply a fact. How else except by the word evolution can we designate the sequence of faunas and floras in precisely dated geological strata? And evolutionary change is also simply a fact owing to the changes in the content of gene pools from generation to generation."[24]

Evolution as fact and theory

Commonly "fact" is used to refer to the observable changes in organisms' traits over generations while the word "theory" is reserved for the mechanisms that cause these changes:

  • Paleontologist Stephen Jay Gould writes, "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 do not go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air, 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."[1]
  • Similarly, biologist Richard Lenski says, "Scientific understanding requires both facts and theories that can explain those facts in a coherent manner. Evolution, in this context, is both a fact and a theory. It is an incontrovertible fact that organisms have changed, or evolved, during the history of life on Earth. And biologists have identified and investigated mechanisms that can explain the major patterns of change."[25]
  • Biologist T. Ryan Gregory says, "biologists rarely make reference to 'the theory of evolution,' referring instead simply to 'evolution' (i.e., the fact of descent with modification) or 'evolutionary theory' (i.e., the increasingly sophisticated body of explanations for the fact of evolution). That evolution is a theory in the proper scientific sense means that there is both a fact of evolution to be explained and a well-supported mechanistic framework to account for it."[20]

Evolution as fact not theory

Other commentators, focusing on the changes in species over generations and in some cases common ancestry have stressed that evolution is a fact to emphasize the weight of supporting evidence while denying it is helpful to use the term "theory":

  • R. C. Lewontin wrote, "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."[26]
  • Douglas Futuyma writes in his Evolutionary Biology book, "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."[9]
  • Richard Dawkins says, "One thing all real scientists agree upon is the fact of evolution itself. It is a fact that we are cousins of gorillas, kangaroos, starfish, and bacteria. Evolution is as much a fact as the heat of the sun. It is not a theory, and for pity's sake, let's stop confusing the philosophically naive by calling it so. Evolution is a fact."[27]
  • Neil Campbell wrote in his 1990 biology textbook, "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."[28]

Predictive power

A central tenet in science is that a scientific theory is supposed to have predictive power, and verification of predictions are seen as an important and necessary support for the theory. The theory of evolution has provided such predictions[29] . Four examples are:

  • Genetic information must be transmitted in a molecular way that will be almost exact but permit slight changes. Since this prediction was made, biologists have discovered the existence of DNA, which has a mutation rate of roughly 10−9 per nucleotide per cell division; this provides just such a mechanism.[30]
  • Some DNA sequences are shared by very different organisms. It has been predicted by the theory of evolution that the differences in such DNA sequences between two organisms should roughly resemble both the biological difference between them according to their anatomy and the time that had passed since these two organisms have separated in the course of evolution, as seen in fossil evidence. The rate of accumulating such changes should be low for some sequences, namely those that code for critical RNA or proteins, and high for others that code for less critical RNA or proteins; but for every specific sequence, the rate of change should be roughly constant over time. These results have been experimentally confirmed. Two examples are DNA sequences coding for rRNA, which is highly conserved, and DNA sequences coding for fibrinopeptides (amino acid chains that are discarded during the formation of fibrin), which are highly non-conserved.[30]
  • Prior to 2004, paleontologists had found fossils of amphibians with necks, ears, and four legs, in rock no older than 365 million years old. In rocks more than 385 million years old they could only find fish, without these amphibian characteristics. Evolutionary theory predicted that since amphibians evolved from fish, an intermediate form should be found in rock dated between 365 and 385 million years ago. Such an intermediate form should have many fish-like characteristics, conserved from 385 million years ago or more, but also have many amphibian characteristics as well. In 2004, an expedition to islands in the Canadian arctic searching specifically for this fossil form in rocks that were 375 million years old discovered fossils of Tiktaalik.[31]
  • Evolutionary theory predicts that novel inventions can arise, while creationists predict that new "information" cannot arise, and that the Second Law of Thermodynamics only allows for "information" to be lost.[32] In an ongoing experiment, Richard Lenski observed that some strains of E. coli evolved the ability to metabolize citrate after tens of thousands of generations.[33]

Related concepts and terminology

  • Speculative or conjectural explanations are called hypotheses. Well-tested and corroborated (or validated) explanations are called theories.
  • "Fact" does not mean "absolute certainty". In the words of Stephen J. Gould: In science, "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional assent."[1]
  • "Proof" of a theory does not exist in natural sciences. Proof only exists in formal sciences, such as mathematics. Experimental observation of the predictions made by a hypothesis or theory is called validation.
  • A scientific law is a concept related to a scientific theory. Very well-established "theories" that rely on a simple principle are often called scientific "laws". For example, it is common to encounter reference to "the law of gravity", "the law of natural selection", or the "laws of evolution."

See also


  1. ^ a b c Gould, Stephen Jay (1981-05-01). "Evolution as Fact and Theory". Discover 2 (5): 34–37. http://www.stephenjaygould.org/library/gould_fact-and-theory.html. 
  2. ^ a b National Academy of Science Institute of Medicine (2008). Science, Evolution, and Creationism. National Academy Press. ISBN 0309105862. http://www.nap.edu/catalog.php?record_id=11876. 
  3. ^ Dobzhansky, T. (1973). "Nothing in biology makes sense except in the light of evolution". The American Biology Teacher 35 (3): 125–129. http://img.signaly.cz/upload/1/0/9a462eb6be1ed7828f57a184cde3c0/Dobzhansky.pdf. 
  4. ^ a b Fitzhugh, K. (2007). "Fact, theory, test and evolution". Zoological Scripta 37 (1): 109–113. doi::10.1111/j.1463-6409.2007.00308.x. http://www.mobot.org/plantscience/ResBot/EvSy/PDF/Fitzhugh%202007%20-%20Zoologica%20Scripta.pdf. 
  5. ^ Evolution vs. Creatinism: An Introduction. Berkeley and Los Angeles, California: University of California Press. 2004. pp. 298. ISBN 0-520-24650-0. http://books.google.ca/books?hl=en&lr=&id=03b_a0monNYC&oi=fnd&pg=PR9&ots=YrEkpRUvLI&sig=BhLV-TVDQm7PoWwYhHZhC84D640#v=onepage&q&f=false. 
  6. ^ Is "Evolution" a "Theory" or "Fact" or Is This Just a Trivial Game of Semantics? by Casey Luskin
  7. ^ Committee for Skeptical Inquiry — Evolution & Creationism: Terminology in Conflict by Richard Joltes
  8. ^ Hall, B. K.; Hallgrímsson, B., eds (2008). Strickberger's Evolution (4th ed.). Jones & Bartlett. pp. 762. ISBN 0763700665. http://www.jblearning.com/catalog/9780763700669/. 
  9. ^ a b Futuyma, Douglas J. (1997). , Evolutionary Biology, 3rd ed.. Sinauer Associates. p. 751. ISBN 0878931899. 
  10. ^ Wordnet entry for phrase "scientific fact"
  11. ^ United States National Park Service Glossary
  12. ^ Webster's New Millennium Dictionary of English, Preview Edition (v 0.9.6), Copyright © 2003–2006 Lexico Publishing Group, LLC
  13. ^ Webster's Encyclopedic Unabridged Dictionary of the English Language (1996) gives a third meaning of the word "fact" as (3) A truth known by actual experience or observation; something known to be true: 'Scientists gather facts about plant growth.'
  14. ^ a b Hypotheses, Facts, and the Nature of Science, —Douglas Futuyma
  15. ^ Guardian article by Richard Dawkins, Jerry Coyne
  16. ^ a b Muller, H. J. (1959). "One hundred years without Darwin are enough". School Science and Mathematics 59 (4): 304–305. doi:10.1111/j.1949-8594.1959.tb08235.x. http://www.skepticfiles.org/evolut/100pcnts.htm.  Reprinted in: *Zetterberg, Peter (ed.) (1983-05-01). Evolution Versus Creationism: The Public Education Controversy. Phoenix AZ: ORYX Press. ISBN 0897740610. 
  17. ^ Science and Creationism: A View from the National Academy of Sciences, Second Edition (1999), National Academy of Sciences (NAS), National Academy Press, Washington DC, 2006.
  18. ^ Moran, Laurence (1993-01-22). "Evolution is a Fact and a Theory". Talk.origins. http://www.talkorigins.org/faqs/evolution-fact.html. Retrieved 2007-10-18. 
  19. ^ Wilkins, JS (1997). "Evolution and Philosophy:Is Evolution Science, and What Does 'Science' Mean?". The TalkOrigins Archive. http://www.talkorigins.org/faqs/evolphil/falsify.html. Retrieved 2009-08-17. 
  20. ^ a b Gregory, T. Ryan (2007). "Evolution as Fact, Theory, and Path". Evolution: Education and Outreach 1 (1): 46–52. doi:10.1007/s12052-007-0001-z. 
  21. ^ Dobzhansky T, Pavlovsky O (1971). "Experimentally created incipient species of Drosophila". Nature 230 (5292): 289–92. doi:10.1038/230289a0. PMID 5549403. 
  22. ^ Robinson, B.A. (2005-08-30). "Is the theory of evolution merely a "theory"?". http://www.religioustolerance.org/ev_stat.htm. Retrieved 2007-10-18. 
  23. ^ "Miller, Kenneth S. (2007). Finding Darwin's God: A Scientist's Search for Common Ground Between God and Evolution (P.S.). New York, N.Y: Harper Perennial. ISBN 0061233501. 
  24. ^ Mayr, Ernst (1988). Toward a New Philosophy of Biology: Observations of an Evolutionist. Cambridge: Harvard University Press. ISBN 0-674-89666-1. 
  25. ^ Lenski, Richard E. (2000). "Evolution: Fact and Theory". American Institute of Biological Sciences. http://www.actionbioscience.org/evolution/lenski.html. Retrieved 2007-10-18. 
  26. ^ Lewontin, R. C. (1981). "Evolution/Creation Debate: a time for truth". Bioscience 31: 559.  Reprinted in:
    • Zetterberg, Peter, ed. (1983-05-01). Evolution Versus Creationism: the public education controversy. Phoenix AZ: Oryx Press. ISBN 0897740610. 
  27. ^ Natural History article : The Illusion of Design by Richard Dawkins
  28. ^ Campbell, Neil A.; Reece, Jane B. (2002-02-05). Biology 6th ed.. Benjamin Cummings. p. 1175. ISBN 0805366245. 
  29. ^ Isaak, Mark. "Claim CA210: Evolution Predictions". The TalkOrigins Archive. Talk Origins. http://www.talkorigins.org/indexcc/CA/CA210.html. Retrieved 1 October 2011. 
  30. ^ a b Bruce Alberts; Alexander Johnson; Julian Lewis; Martin Raff; Keith Roberts; Peter Walter (March, 2002). Molecular Biology of the Cell (4th ed.). Routledge. ISBN 0-8153-3218-1. 
  31. ^ "Shubin, Neil. (2008). Your Inner Fish. Pantheon. ISBN 9780375424472. 
  32. ^ TalkOrigins.org
  33. ^ NS:bacteria make major evolutionary shift in the lab


  • J.P. Franck, et al., "Evolution of a satellite DNA family in tilapia." Annual Meeting Canadian Federation of Biological Societies. Halifax, (1990).
  • M. Losseau-Hoebeke, "The biology of four haplochromine species of Lake Kivu (Zaire) with evolutionary implications." Thesis, Dept. Ichthyology, Rhodes University, Grahamstown, (1992).

External links

  • Not Just a Theory Discredits the assertion that evolution is "just a theory", with an explanation of the meaning of the word 'theory' in a scientific context.
  • Talk Origins Response to the claim that no examples of speciation have been observed.
  • Glenn Branch; Louise S. Mead (2008). ""Theory" in Theory and Practice". Evo Edu Outreach 1 (3): 287–289. doi:10.1007/s12052-008-0056-5. 

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