Evolution as fact and theory

"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 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.

In the statement "evolution is both fact and theory", evolution as theory refers to the scientific (as opposed to the vernacular) definition of theory. In the first scientific meaning, a theory is an overarching framework that makes sense of otherwise disconnected observations. For example, the germ theory of disease was similarly highly controversial when first proposed, yet it was validated in the late 19th century and is now a fundamental part of modern medicine and clinical microbiology, leading to such important innovations as antibiotics and hygienic practices. In another example, the theory of gravity unifies astronomical observations with observations about the acceleration 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".

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. 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.

Evolution, fact and theory

The early developmental stages of most sciences have been characterized by continual competition between a number of distinct views of nature, each partially derived from, and all roughly compatible with, the dictates of scientific observation and method.

Evolution has been described as "fact and theory", "fact not theory", "only a theory, not a fact", and "multiple theories, not fact". Science cannot achieve absolute "certainty" nor is it a continuous march toward an objective truth as the vernacular meaning of the terms "proof" or "fact" might imply. A proof, fact, theory, hypothesis, and other words of science are hobbled by multiple meanings but are used nonetheless because they invigorate research methods and lead to discovery in all branches of scientific research. The philosophy of scientific inquiry solves problems of novelty as discoveries are made. Scientific knowledge is shared, incorporated, and tested across disciplines. Charles Darwin, for example, not only advanced theory and hypotheses in evolution, but experimented and tested his ideas across disciplines, including and not limited to geology, botany, psychology, and ecology.

"...scientific knowledge is tentative (subject to change); empirically based (based on and/or derived from observations of the natural world); subjective (theory-laden); partly the product of human inference, imagination, and creativity (involves the invention of explanation); and socially and culturally embedded...Although there is overlap and interaction between science processes and the nature of science, it is nevertheless important to distinguish the two."

Evolutionary science is part of larger network of scientific theory that is used to confront and deal with the world. Science is a collective enterprise that evolves through the progress of experimentation and rejection of obsolete theories. By necessity, scientific ideas begin as speculation, because scientists lack foresight on correct solutions as they push the boundaries of knowledge and discovery. Scientific research has been called the "...interplay between imagination (hypothesis formulation) and experiment." In the process, scientists gather facts, conduct experiments, analyze, probe, prod, scrutinize, communicate on, and raise questions about the natural world. Over time, methods are developed and improved to further scientific knowledge related to the theory. Unexpected discoveries are not immediately integrated as fact. Science is developmentally enriched through the novelties of fact and theory. In this context, the meanings of the terms "evolution", "fact", and "theory" are described below.

Evolution

Main articles: Introduction to evolution and Evolution

Evolution is generally defined as changes in trait or gene frequency in a population of organisms from one generation to the next. This has been dubbed the standard genetic definition of evolution. Other definitions of evolution cover a much broader scale overarching multiple levels of biological organisation, from macroevolutionary phenomena that occur during species formation and divergence, to microevolutionary processes within individual organisms, cells, and biomolecules such as DNA and proteins. Evolution also refers to Darwin's theory of natural selection, which is the only known mechanism that can lead to adaptations and is only one of multiple mechanisms of evolutionary change. Natural selection is a process that acts on the heritable characteristics of individuals that interact and reproduce to form lineages of biological populations. Genetic drift, gene flow, vicariance biogeography, and niche construction are examples of other evolutionary mechanisms. 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).

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.

Evolutionary theory may also refer to cultural evolution. Evolutionary science provides an overarching framework in biology for identifying relationships and providing a coherent understanding of otherwise disconnected natural observations.

Fact

Main article: Scientific fact

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. Unlike other terms of science, facts in science are similar in definition and interpretation relative to their use in common language. Fact is often used by scientists to refer to experimental or empirical data or objective verifiable observations. However, scientists do not accept facts as absolute truth and in some cases even observations have been defined "as low-level hypotheses that exist only as interpretations of the facts of nature in light of present theories, not as the facts of nature themselves." According to this view, all concepts are theory-laden meaning that there is always an element of subjective interpretation embedded in our language. The US National Academy of Science defines a scientific fact as "an observation that has been repeatedly confirmed, and for all practical purposes, is accepted as ‘true’." Facts are also established through the process of experimentation:

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

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. 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.

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.

Many philosophers of science argue that human observers cannot 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 direction of thought, all apparent facts are provisional, in a Platonic sense. In René Descartes' consideration of ontology and epistemology in Discourse on the Method, he attempts to arrive at a fundamental set of principles that one can know as true without any doubt, i.e. non-provisional facts. Descartes' epistemology presupposes a God who does not desire to deceive mankind. Therefore, errors in ontological conclusions are mistakes of the thinker, not deceptions by the Creator.

Theory

Main article: Scientific theory

The scientific definition of the word "theory" is different from the colloquial sense of the word. Colloquially or in the vernacular sense of the term, "theory" can refer to guesswork, a simple conjecture, an opinion, or a speculation that does not have to be based on facts and it need not be framed for making testable predictions. Scientific theories also contain speculation at first, but they develop over time and many are rejected as they are specifically crafted for the purpose or function of being testable. In this way, theories can be constructed using logic, models, or schemes for generating testable hypotheses with precision.

The "theory of evolution" is actually a network of theories that develop with the science over time. Since Darwin, evolution has developed into a well-supported body of interconnected statements that explains countless observations in the natural world. Evolutionary theories generate testable predictions about nature. Theories offer more general explanations of systems in nature than the hypotheses they generate, which are used to test specific instances or examples of the theory. Theoretical models are one of many kinds of scientific methods that can used to communicate accurate and precise depictions of systems, such as evolutionary systems, that are continually and repetitively investigated.

Evolutionary theories include theories about inheritance. Under the blending inheritance theory, evolution by natural selection is exceedingly difficult, since genetic variation is rapidly lost. A theoretical advance known as the Hardy–Weinberg principle shows that under the alternative inheritance theory of Mendelian genetics, variation is not easily lost. The Hardy-Weinberg genotype frequencies also facilitate the population genetics study of natural selection using diploid replicator equations.

The neutral theory of molecular evolution, for example, is used to study evolution as a null model against which tests for natural selection can be applied. Phylogenetic theory is another example of evolutionary theory. It is based on the evolutionary premise of an ancestral descendant sequence of genes, populations, or species. Individuals that evolve are linked together through historical and genealogical ties. Evolutionary trees are hypotheses that are generated from the practice of phylogenetic theory. They depict relations among individuals that can speciate and diverge from one another. The evolutionary process of speciation necessarily creates groups that are linked by a common ancestor and all its descendants. As species inherit traits they pass these onto their descendants. More closely allied species, of closer genealogical descent, have more common sets of traits that share a greater degree of similarity in their morphology and genetic composition. Evolutionary biologists use systematic methods and test phylogenetic theory to observe changes in and among species over time. These methods include the collection, measurement, observation, and mapping of traits onto evolutionary trees. Phylogenetic theory is used to test the distributions of traits and their various forms to provide explanations of observed patterns in relation to their evolutionary history.

Evolution compared with gravity

See also: Gravity and History of gravitational theory

The application of the terms "fact" and "theory" to evolution is comparable to their use in describing gravity. 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 developed 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.

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."
• H. J. Muller wrote, "So enormous, ramifying, and consistant 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."
• Kenneth R. Miller writes, "evolution is as much a fact as anything we know in science."
• 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."

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."
• 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."
• 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."

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."
• 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."
• 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."
• 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."
• Evolutionary scientist Kirk Fitzhugh penned, "‘Evolution’ cannot be both a theory and a fact. Theories are concepts stating cause–effect relations...One might argue that it is conceivable to speak of ‘evolution’ as a fact by way of it being the subject of reference in explanatory hypotheses...In the strictest sense then, ‘evolution’ cannot be regarded as a fact even in the context of hypotheses since the causal points of reference continue to be organisms, and no amount of confirming instances for those hypotheses will transform them into facts...While evolution is not a fact, it is also not a single theory, but a set of theories applied to a variety of causal questions...An emphasis on associating ‘evolution’ with ‘fact’ presents the misguided connotation that science seeks certainty."

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 . 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.
• 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.
• 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.
• 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. 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.

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."
• "Proof" of a theory has different meanings in science. Proof exists in formal sciences, such as a mathematical proof where symbolic expressions can represent an infinite sets with precise definitions and outcomes of the terms. Proof has other meanings stemming from its Latin roots meaning to test. In this sense a proof is a convincing explanation or a verifiable demonstration of the facts usually involving evidence stemming from carefully controlled experiments. Charles Darwin's research, for example, pointed to the coordination of so many pieces of evidence that no other configuration other than his theory could offer a conceivable causal explanation of the facts. In this way natural selection and common ancestry has been proven. Natural selection and other evolutionary theories are also represented in various mathematical proofs. 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

• 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)

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).

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 (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)