Evolution as fact and theory

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 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.^[1]

Each of the words "evolution," "fact" and "theory" has several meanings in different contexts. Evolution means change over time, as in stellar evolution. In biology it refers to observed changes in organisms, 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 that all can agree on; 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, 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."^[2] 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.

Evolution

Life timeline

−4500 —

–

—

–

−4000 —

–

—

–

−3500 —

–

—

–

−3000 —

–

—

–

−2500 —

–

—

–

−2000 —

–

—

–

−1500 —

–

—

–

−1000 —

–

—

–

−500 —

–

—

–

0 —

P
l
a
n
t
s

H
a
d
e
a
n

A
r
c
h
e
a
n

P
r
o
t
e
r
o
z
o
i
c

P
h
a
n
e
r
o
z
o
i
c

←

←

←

←

←

←

←

←

←

←

←

Earliest multicellular life

←

←

←

←

←

←

←

Hirnantian glaciation*

←

Earliest tetrapods

←

Karoo ice age*

←

Earliest apes / humans

←

Quaternary ice age*

(million years ago)

*Ice Ages

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.^[3]

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.^[4] 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.^[5]^{[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:

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.^[6]^{[page needed]}

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.^[7]

To Keith Stewart Thomson, the word evolution has at least three distinct meanings:^[8]

The general sense of change over time.
All life forms have descended with modifications from ancestors in a process of common descent.
The cause or mechanisms of these process of change, that are examined and explained by evolutionary theories.

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."^[8]

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.^[9]

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.^[10] Even Spencer by 1865 was admitting that his definition was imperfect,^[11] but it remained popular throughout the nineteenth century before declining under the criticisms of William James and others.^[12]^[13]

Fact

Fact is often used by scientists to refer to experimental or empirical data or objective verifiable observations.^[14]^[15] "Fact" is also used in a wider sense to mean any theory for which there is overwhelming evidence.^[16]

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 J. Futuyma^[6]

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.^[6]^[17] 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.^[18]

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.^[19]

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."^[20] 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.^[9]^[21]

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."^[22] 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 around 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.^[23]

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.^[25]^[26]^{[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.^[27]^{[page number verification needed]}^[28]^{[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.

Evolution as theory and fact in the literature

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.