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The potentially confusing statement that "evolution is both a theory and a fact" is often seen in biological literature.^{[1][2][3][4][5][4][6][7]} This statement arises because evolution is used in two ways. First, the "fact of evolution" refers to the changes in populations of biological organisms over time, which are known to have occurred through scientific observations and experimentations. Second, the "theory of evolution" refers to the modern evolutionary synthesis, which is the current scientific explanation of how these changes occur. Misuse and misunderstanding of how these terms are applied to evolution have been used to construct arguments disputing the validity of evolution.

The distinction between fact and theory is not limited to the study of evolution. Consider, for example, gravity. The law of gravity is a scientific fact that bodies of mass attract each other, but there are different theories of gravity that attempt to explain how bodies of mass are attracted to each other. In this way, gravity is both a scientific fact and a scientific theory.

On its own, the word "evolution" often refers to the combination of the underlying facts, and the theory that explains them. However, it is also frequently used to refer to one or the other. Readers should take care to determine an author's meaning.

Contents 1 Evolution, fact and theory 1.1 Evolution 1.2 Fact 1.3 Theory 2 Evolution compared with gravity 3 Evolution as theory and fact in the literature 3.1 Evolution as fact and theory 3.2 Evolution as fact not theory 4 Predictive power 5 Related concepts and terminology 6 See also 7 Notes 8 References 9 External links

[edit] Evolution, fact and theory

Evolution has been described as "fact and theory", "fact not theory" and, "only a theory, not a fact". This illustrates a terminological confusion that hampers discussion.^[8][9] The meanings of the terms "evolution" and "fact" and "theory", are described below:

[edit] Evolution

Evolution is usually defined simply as changes in trait or gene frequency in a population of organisms from one generation to the next. However, "evolution" is often used to include 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).

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.' ^[10]

The term "evolution", especially when referred to as a "theory", is also used more broadly to incorporate processes such as natural selection and genetic drift.

[edit] Fact

Fact is often used by scientists to refer to experimental data or objective verifiable observations. "Fact" is also used in a wider sense to mean any hypothesis for which there is overwhelming evidence.

"Fact"
Observation	Established Hypothesis
Facts are empirical data, objective verifiable observations.[11][12][13][14]	'A fact is hypothesis that is so firmly supported by evidence that we assume it is true, and act as if it were true.' Douglas Futyuma. [15]

Evolution is a fact in the sense of it being 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.^[15][16] 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.^[3]

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 anything 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.

[edit] Theory

"Theory"
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 conjecture, an opinion, or a speculation that does not have to be based on facts or make testable predictions. In science, the meaning of theory is more rigorous: a theory must be based on observed facts and make testable predictions.

In science, a current theory is a theory that has no equally acceptable or more acceptable alternative theory, and has survived attempts at falsification. That is, there have been no observations made which contradict it to this point and, indeed, every observation ever made either supports the current theory or at least does not falsify it by contradicting it completely. A revision of the current theory, or the generation of a new theory is necessary if new observations contradict the current theory, as the current findings are in need of a new explanation (see scientific revolution or paradigm shift). However, the falsification of a theory does not falsify the facts on which the theory is based.

[edit] Evolution compared with gravity

The terms "fact" and "theory" can be applied to evolution, just as they are to gravity.^[1]

There have been many theories that attempt to explain the fact of gravity. That is, scientists ask what gravity is, and what causes it. They develop a model to explain gravity, a theory of gravity. Many explanations of gravity that qualify as a Theory of Gravity have been proposed over the centuries: Aristotle's, Galileo's, Newton's, and now Einstein's. Confusion of the terms can arise when we use a single word to describe both the observed facts and the theory that explains it. The word ‘’gravity’’ can be used to refer to the observed facts (i.e., the observed attraction of masses) and the theory used to explain it (i.e., the reason why masses attract each other). Thus, gravity is both a "theory" and a "fact."

In the study of biological species, the facts include fossils and measurements of these fossils. The location of a fossil is an example of a fact (using the scientific meaning of the word fact). In species that rapidly reproduce, for example fruit flies, the process of evolutionary change has been observed in the laboratory.^[18] The observation of fruit fly populations changing character is also an example of a fact. So evolution is a fact just as the observations of gravity are a fact.

In biology, there have been many attempts to explain these observations over the years. Lamarckism, Transmutationism and Orthogenesis were all non-Darwinian theories that attempted to explain the observations of species and fossils and other evidence. However, the 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. Thus, evolution is not only a fact but also a theory, just as gravity is both a fact and a theory.

Gravity	Evolution
Things falling is an observation of the pull of bodies towards each other.	Fruit flies changing generation to generation is an observation of generational organism change.
Bodies pulling towards each other is called gravity.	Organisms changing generation to generation is called evolution.
Gravity is a fact.	Evolution is a fact.
Explanations for the facts of gravity.	Explanations for the facts of evolution.
Aristotle and Galileo created explanations of the fact of gravity. These are now obsolete explanations.	Lamarckism, Transmutationism and Orthogenesis were created as explanations of the fact of evolution. These are now discredited explanations.
Newton's explanation of gravity is approximately correct but required refinement.	Darwin's explanation of evolution is approximately correct, but required refinement.
Einstein's explanation is a refinement of Newton's explanation of gravity. Einstein's explanation is currently the most accepted explanation of the fact of gravity.	The modern evolutionary synthesis is a refinement of Darwin's explanation of evolution, which did not include genes in its explanation. This modern synthesis is currently the most accepted explanation of the fact of evolution.
Einstein's explanation of the fact of gravity is called The General theory of relativity.	The explanation of the fact of evolution provided by the modern synthesis is the latest and most widely accepted Theory of Evolution.
Gravity is a fact and a theory.	Evolution is a fact and a theory.

[edit] Evolution as theory and fact in the literature

The confusion between "fact" and "theory" and the use of the word "evolution" is largely due to some authors using evolution to refer to the changes that occur within species over generations and common descent, while others use the term more generally to include the mechanisms driving the change. However, among biologists at least, there seems to be consensus that evolution is a fact:

• American zoologist and paleontologist George Simpson stated that 'Darwin...finally and definitely established evolution as a fact.' ^[19]
• H. J. Muller has written, '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.' ^[3]
• Kenneth R. Miller writes, 'evolution is as much a fact as anything we know in science.' ^[20]
• Ernst Mayr has 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.' ^[7]

[edit] 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.' ^[2]
• 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.' ^[6]

[edit] 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.' ^[21]
• 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.' ^[10]
• 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.' ^[22]
• 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.' ^[4]

[edit] 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 did provide such predictions. Three 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.^[23]

• 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, which 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 through evolution. 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 which are discarded during the formation of fibrin), which are highly non-conserved.^[23]

• 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.^[24]

[edit] Related concepts and terminology

• Speculative or conjectural explanations are called hypotheses. Well-tested 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." ^[2]

• "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."

[edit] See also

• Theory vs. Fact (in Creation-evolution controversy)
• Evolution is just a theory, not a fact (in objections to evolution)
• Misconceptions about evolution (in List of misconceptions)
• Evidence of common descent
• Epistemology

[edit] Notes

[edit] References

• 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).

[edit] 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.
• Parody website satirizing theory and fact statements
• Talk Origins Response to the claim that no examples of speciation have been observed.
• Glenn Branch; Louise S. Mead (2008-06-06). "“Theory” in Theory and Practice" (pdf). Evo Edu Outreach (2008) 1:287–289. Springer Science + Business Media. http://www.springerlink.com/content/fr258627q2x3t378/fulltext.pdf. Retrieved on 2008-07-21.