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Many scientists and philosophers of science have described evolution as fact and theory, a phrase which was used as the title of an article by paleontologist Stephen Jay Gould in 1981. He describes fact in science as meaning data, not known with absolute certainty but "confirmed to such a degree that it would be perverse to withhold provisional assent". A scientific theory is a well-substantiated explanation of such facts. The facts of evolution come from observational evidence of current processes, from imperfections in organisms recording historical common descent, and from transitions in the fossil record. Theories of evolution provide a provisional explanation for these facts.
Each of the words evolution, fact and theory has several meanings in different contexts. In biology, evolution refers to observed changes in organisms over successive generations, to their descent from a common ancestor, and at a technical level to a change in gene frequency over time; it can also refer to explanatory theories (such as Charles Darwin's theory of natural selection) which explain the mechanisms of evolution. To a scientist, fact can describe a repeatable observation capable of great consensus; it can refer to something that is so well established that nobody in a community disagrees with it; and it can also refer to the truth or falsity of a proposition. To the public, theory can mean an opinion or conjecture (e.g., "it's only a theory"), but among scientists it has a much stronger connotation of "well-substantiated explanation". With this number of choices, people can often talk past each other, and meanings become the subject of linguistic analysis.
Evidence for evolution continues to be accumulated and tested. The scientific literature includes statements by evolutionary biologists and philosophers of science demonstrating some of the different perspectives on evolution as fact and theory.
Evolution has been described as "fact and theory"; "fact, not theory"; "only a theory, not a fact"; "multiple theories, not fact"; and "neither fact, nor theory." The disagreements among these statements, however, have more to do with the meaning of words than the substantial issues and this controversy is discussed below.
Professor of biology Jerry Coyne sums up biological evolution succinctly:
Life on Earth evolved gradually beginning with one primitive species – perhaps a self-replicating molecule – that lived more than 3.5 billion years ago; it then branched out over time, throwing off many new and diverse species; and the mechanism for most (but not all) of evolutionary change is natural selection.
This shows the breadth and scope of the issue, incorporating the scientific fields of zoology, botany, genetics, geology, and paleontology, among many others.
But the central core of 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. Natural selection is only one of several mechanisms in the theory of evolutionary change that explains how organisms historically adapt to changing environments. The principles of heredity were re-discovered in 1900, after Darwin's death, in Gregor Mendel's research on the inheritance of simple trait variations in peas.[page needed] Subsequent work into genetics, mutation, paleontology, and developmental biology expanded the applicability and scope of Darwin's original theory.
According to Douglas J. Futuyma:[page needed]
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.
The word evolution in a broad sense refers to processes of change, from stellar evolution to changes in language. In biology, the meaning is more specific: heritable changes which accumulate over generations of a population. Individual organisms do not evolve in their lifetimes, but variations in the genes they inherit can become more or less common in the population of organisms. Any changes during the lifetime of organisms which are not inherited by their offspring are not part of biological evolution.
To Keith Stewart Thomson, the word evolution has at least three distinct meanings:
Thomson remarks: "Change over time is a fact, and descent from common ancestors is based on such unassailable logic that we act as though it is a fact. Natural selection provides the outline of an explanatory theory."
Biologists consider it to be a scientific fact that evolution has occurred in that modern organisms differ from past forms, and evolution is still occurring with discernible differences between organisms and their descendants. There is such strong quantitative support for the second that scientists regard common descent as being as factual as the understanding that in the Solar System the Earth orbits the Sun, although the examination of the fundamentals of these processes is still in progress. There are several theories about the mechanisms of evolution, and there are still active debates about specific mechanisms.
There is a fourth meaning for the word evolution that is not used by biologists today. In 1857, the philosopher Herbert Spencer defined it as "change from the homogeneous to the heterogeneous." He claimed (before Darwin) that this was "settled beyond dispute" for organic evolution and applied it to the evolution of star systems, geology and human society. Even Spencer by 1865 was admitting that his definition was imperfect, but it remained popular throughout the nineteenth century before declining under the criticisms of William James and others.
Main article: Scientific fact
The word fact is often used by scientists to refer to experimental or empirical data or objective verifiable observations. Fact is also used in a wider sense to mean any theory for which there is overwhelming evidence. According to Douglas J. Futuyma,
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.
In the sense that evolution is overwhelmingly validated by the evidence, it is a fact. It is frequently said to be a fact in the same way as the Earth's revolution around the Sun is a fact. The following quotation from Hermann Joseph Muller's article, "One Hundred Years Without Darwinism 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 Sciences (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 a fact. Scientists no longer question whether descent with modification occurred because the evidence supporting the idea is so strong.
Stephen Jay Gould also points out that "Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory – natural selection – to explain the mechanism of evolution." These two aspects are frequently confused. Scientists continue to argue about particular explanations or mechanisms at work in specific instances of evolution – but the fact that evolution has occurred, and is still occurring, is undisputed.
A common misconception is that evolution cannot be reliably observed because it all happened millions of years ago and the science therefore is not dependent on facts (in the initial sense above). However, both Darwin and Alfred Russel Wallace, the co-founders of the theory, and all subsequent biologists depend primarily on observations of living organisms; Darwin concentrated largely on the breeding of domesticated animals whereas Wallace started from the biogeographical distribution of species in the Amazon and Malay Archipelago. In the early twentieth century, population genetics had centre stage, and more recently DNA has become the main focus of observation and experimentation.
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.
Main article: Scientific theory
The scientific definition of the word theory is different from the definition of the word in colloquial use. In the vernacular, theory can refer to guesswork, a simple conjecture, an opinion, or a speculation that does not have to be based on facts and need not be framed for making testable predictions.
In science, however, the meaning of theory is more rigorous. A scientific theory is "a well-substantiated explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses." Theories are formed from hypotheses that have been subjected repeatedly to tests of evidence which attempt to disprove or falsify them. In the case of evolution through natural selection, Darwin conceived the hypothesis c. 1839, and made a first draft of the concept three years later in 1842. He discussed this widely with many of his intellectual companions, and conducted further research in the background to his other writings and work. After years of development, he finally published his evidence and theory in On the Origin of Species in 1859.
The "theory of evolution" is actually a network of theories that created the research program of biology. Darwin, for example, proposed five separate theories in his original formulation, which included mechanistic explanations for:
Since Darwin, evolution has become a well-supported body of interconnected statements that explains numerous empirical observations in the natural world. Evolutionary theories continue to generate testable predictions and explanations about living and fossilized organisms.[page needed]
Phylogenetic theory is an 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 inferred through the practice of phylogenetic theory. They depict relations among individuals that can speciate and diverge from one another. The evolutionary process of speciation creates groups that are linked by a common ancestor and all its descendants. Species inherit traits, which are then passed on to descendants. Evolutionary biologists use systematic methods and test phylogenetic theory to observe and explain 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 independent distributions of traits and their various forms to provide explanations of observed patterns in relation to their evolutionary history and biology.[page number verification needed][page needed] The neutral theory of molecular evolution is used to study evolution as a null model against which tests for natural selection can be applied.
The following sections provide specific quotable references from evolutionary biologists and philosophers of science demonstrating some of the different perspectives on evolution as fact and theory.
Fact is commonly 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:
Other commentators – focusing on the changes in species over generations and in some cases common ancestry – have stressed, in order to emphasize the weight of supporting evidence, that evolution is a fact, arguing that the use of the term "theory" is not useful:
The curator at the Natural History Museum of Los Angeles County, Kirk J. Fitzhugh writes that scientists must be cautious to "carefully and correctly" describe the nature of scientific investigation at a time when evolutionary biology is under attack from creationists and proponents of intelligent design. Fitzhugh writes that while facts are states of being in nature, theories represent efforts to connect those states of being by causal relationships:
"Evolution" cannot be both a theory and a fact. Theories are concepts stating cause–effect relations. Regardless of one's certainty as to the utility of a theory to provide understanding, it would be epistemically incorrect to assert any theory as also being a fact, given that theories are not objects to be discerned by their state of being.
Fitzhugh recognizes that the "theory" versus "fact" debate is one of semantics. He nevertheless contends that referring to evolution as a "fact" is technically incorrect and distracts from the primary "goal of science, which is to continually acquire causal understanding through the critical evaluation of our theories and hypotheses." Fitzhugh concludes that the "certainty" of evolution "provides no basis for elevating any evolutionary theory or hypothesis to the level of fact."
Dr William C. Robertson writing for National Science Teachers Association writes, "I have heard too many scientists claim that evolution is a fact, often in retort to the claim that it is just a theory. Evolution isn't a fact. Rather than claiming so, I think scientists would be better served to agree that evolution is a theory and then proceed to explain what a theory is – a coherent explanation that undergoes constant testing and often revision over a period of time."
The main purpose of evolutionary biology is to provide a rational explanation for the extraordinarily complex and intricate organization of living things. To explain means to identify a mechanism that causes evolution and to demonstrate the consequences of its operation. These consequences are then the general laws of evolution, of which any given system or organism is a particular outcome.
(an observation that has been confirmed repeatedly and is accepted as true (although its truth is never final))
In science, a fact is an observation that has been repeatedly confirmed and for all practical purposes is accepted as 'true.' Truth is science, however, is never final, and what is accepted as a fact today may be modified or even discarded at some point in the future. Source: National Academy of Sciences (1999)