Evolution as theory and fact

Evolution is often said to be both theory and fact. This statement, or something similar, is frequently seen in biological literature. The point of this statement is to differentiate the concept of the "fact of evolution", namely the observed changes in populations of organisms over time, from the "theory of evolution", namely the current scientific explanation of how those changes came about.

The terminology of science
Scientists use many specialized terms, frequently attributing to common words meanings foreign to the layperson. In particular,


 * A fact is an observation or a piece of data. Facts can include objective measurements which can be either pieces of verifiable evidence, or the results of an experiment which can be repeated over and over again by different people.    For example, the gravitational force can be measured and observed so can therefore be described by scientists as a fact. Every time an apple is dropped and it falls, an observation of gravity is made.


 * Theories in science are different from facts. Scientific theories describe the coherent framework into which observable data fit. Predictions can be made and tested based on this theory. There have been many theories that attempt to explain the fact of gravity. That is, scientists ask what is gravity, 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.

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 the theory that has no equally acceptable alternate theory, and has not been falsified. 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. Should new observations contradict the current theory, the theory must be revised or a new theory needs to be developed to explain the current findings (see paradigm shift or scientific revolution). Falsification of a theory does not falsify the facts on which it is based.

Confusion of the terms can arise when a single word is used to describe both the observed facts and the theory to explain those facts. 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 (gravity is the reason why masses attract each other). Thus gravity is both a theory and a fact.

Scientific terminology applied to evolution
The terms fact and theory can be applied to evolution, just as they are to gravity. Misuse and misunderstanding of how those terms are applied to evolution have been used to construct arguments disputing the validity of evolution.

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. The observation of fruit fly populations changing character is also an example of a fact, using the scientific meaning of the word fact. So evolution is a fact, at least using the scientific meaning of the word fact. These facts require an explanation, just as the observations of gravity did.

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 explanation for all relevant observations regarding the development of life is called the Theory of Evolution. The theory of evolution is 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.

Evolution as theory and fact in the literature
This confusion between "fact" and "theory" in the study of evolution was explored in a well-known quote by paleontologist Stephen Jay Gould:

"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."

Similarly, biologist Richard Lenski writes,

"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."

Other commentators have stressed that evolution is a fact, and not all of them are quite so clear on the distinction between fact and theory. For example, 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."

and biologist Ernst Mayr states,

"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."

The factual nature of evolution arises over and over again in the biological literature in different guises. Carl Sagan wrote "Evolution is a fact, not a theory". American zoologist and paleontologist George Simpson, stated that "Darwin...finally and definitely established evolution as a fact." 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." Douglas Futuyama writes in his 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." H. J. Muller states, "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." Kenneth R. Miller writes, "evolution is as much a fact as anything we know in science."

Nonetheless, there are those that refuse to accept this basic dualism. House Bill HB1504, in the Oklahoma state legislature, provides a requirement for an "evolution disclaimer" in state-approved textbooks stating that evolution is a "controversial theory" and that "any statement about life’s origins should be considered as theory, not fact." . The creationist website Answers in Genesis claims that evolution should not be classed as a "theory" in the proper scientific sense; rather, they claim that "it would be better to say that particles-to-people evolution is an unsubstantiated hypothesis or conjecture."

The issue was brought before the courts In 1986, when an amicus curiae brief asking the US Supreme Court to reject a Louisiana state law requiring the teaching of creationism in the case Edwards v. Aguillard was signed by 72 US Nobel Prize winners, 17 state academies of science and 7 other scientific societies. The brief provides a detailed argument which it summarises in the following statement. "The Act's unconstitutional purpose is also evident in its requirement that both 'creation-science' and 'evolution-science' be taught as 'theory' and not as 'proven scientific fact.' To a scientist or a science educator, the distinction between scientific theories and scientific facts is well understood. A 'fact' is a property of a natural phenomenon. A 'theory' is a naturalistic explanation for a body of facts. That distinction permeates all fields of scientific endeavor. It is no more relevant to discussions of the origin of the universe and life than to any other area of research. By singling out one topic in science – 'origins' – for special treatment, the legislature conveys the false message that the prevailing theory of 'origins' – evolutionary theory – is less robust and reliable than all other scientific concepts. This misleadingly disparaging treatment of evolution confirms that the Act favors a particular religious belief."

Predictive power
A central tenet in science is that a scientific theory is supposed to have a 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. Two examples are:


 * Genetic information must be transmitted in a molecular way that will be almost exact but permit slight changes. Indeed, 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.


 * 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 indeed been found experimentally. 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.

Related concepts and terminology

 * Speculative or conjectural explanations are called hypotheses. Well-tested explanations are called theories.


 * "Theories" are not "true" in science, at least in the regular sense of the word "true". "True" "theories" only are "confirmed to such a degree that it would be perverse to withhold provisional assent."


 * "Proof" of a theory does not exist in science. Proof only exists in 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."