Evolution as fact and theory

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The statement "evolution is both a theory and a fact" is often seen in biological literature.^{[1][2][3][4][5][6][7]} Evolution is a "theory" in the scientific sense of the term "theory"; it is an established scientific model of a portion of the universe that generates propositions with observational consequences. Such a model both helps generate new research and helps us understand observed phenomena.

When scientists say "evolution is a fact", they are using one of two meanings of the word "fact". One meaning is empirical: evolution can be observed through changes in allele frequencies or traits of a population over successive generations.

Another way "fact" is used is to refer to a certain kind of theory, one that has been so powerful and productive for such a long time that it is universally accepted by scientists. When scientists say evolution is a fact in this sense, they mean it is a fact that all living organisms have descended from a common ancestor (or ancestral gene pool) ^[8] even though this cannot be directly observed. This implies more tangibly that it is a fact that humans share a common ancestor with all living organisms.

Evolution has been described as "fact and theory", "fact not theory", and "only a theory, not a fact". This illustrates a confusion in terminology that hampers discussion.^[9][10] The meanings of the terms "evolution", "fact", and "theory" are described below.

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.^[11]

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.

Fact is often used by scientists to refer to experimental or empirical data or objective verifiable observations.^{[12][13][14][15]} "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^[16]

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.^[16][17] 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.^[18]

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.^[1][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.

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.

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."^[3]
• 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."^[7]

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]
• 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]

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."^[24]
• 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."^[11]
• 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."^[25]
• 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]

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. 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.^[26]
• 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.^[26]
• 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.^[27]
• 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.^[28] In an ongoing experiment, Richard Lenski observed that E. coli evolved the ability to metabolize citrate, which constitutes a novel invention, and an increase in the information of the DNA of the E. coli.^[29]

• 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."^[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."

Template:Wikipedia books

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

• 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).

• 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 (2008). ""Theory" in Theory and Practice". Evo Edu Outreach. 1 (3): 287–289. doi:10.1007/s12052-008-0056-5. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)