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.
Evolution Professor of biology
Jerry Coyne sums up biological evolution succinctly: 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. Subsequent work into genetics,
mutation, paleontology, and
developmental biology expanded the applicability and scope of Darwin's original theory. According to
Douglas J. Futuyma: 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: • 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." 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.
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, 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. The
National Academy of Sciences (U.S.) makes a similar point: 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.
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 , 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. Similar to the term "theory of evolution", the word "theory" is also evident in the names given for other scientific theories, as in "
atom theory", "
germ theory of diseases" or "
cell theory". The "theory of evolution" is actually a network of theories that created the
research program of biology. Specifically Darwin, for example, proposed five separate theories in his original formulation, which included mechanistic explanations for: •
populations changing over generations •
gradual change •
speciation •
natural selection •
common descent 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.
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. 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 ==