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

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] 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]

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]

  1. The general sense of change over time.
  2. All life forms have descended with modifications from ancestors in a process of common descent.
  3. 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

Main article: Scientific 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

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."[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:

  1. populations changing over generations
  2. gradual change
  3. speciation
  4. natural selection
  5. common descent[24]

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]

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][28] 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.

Evolution as fact

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:

Evolution as fact and not theory

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:

Evolution as a collection of theories not fact

Related concepts and terminology

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.

Graham Bell, Selection: The Mechanism of Evolution (2008)[40]

See also

References

  1. Gould 1994, pp. 253–262.
  2. See section 2
  3. Coyne 2009, p. 3.
  4. Mayr 1982, p. 400.
  5. Wright 1984.
  6. 1 2 3 4 Futuyma 1998.
  7. Moran, Laurence (1993). "What is Evolution?". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-01-19.
  8. 1 2 Thomson, Keith Stewart (September–October 1982). "Marginalia: The meanings of evolution". American Scientist (Research Triangle Park, NC: Sigma Xi) 70 (5): 529–531. ISSN 0003-0996. JSTOR 27851662.
  9. 1 2 Moran, Laurence (1993). "Evolution is a Fact and a Theory". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-01-19.
  10. Spencer, Herbert (January–April 1857). "Progress: its Law and Cause" (PDF). Westminster Review. New Series (London: John Chapman) 11: 446. OCLC 26747836.
  11. Spencer 1865, p. 176.
  12. James 1911, Chapter 6: "Herbert Spencer's Autobiography".
  13. Brady, Michael (March 12, 2011). Evolutionary Opponents: William James and Herbert Spencer (DOC). Society for the Advancement of American Philosophy. Cheney, WA. Retrieved 2013-06-10. Paper presented at the 38th Annual Meeting.
  14. "scientific fact". WordNet. Princeton, NJ: Princeton University. 2010. Retrieved 2013-02-21. (an observation that has been confirmed repeatedly and is accepted as true (although its truth is never final))
  15. "Fact". Views of the National Park Service Glossary. Washington, D.C.: National Park Service. Retrieved 2013-02-21. 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)
  16. Webster's Encyclopedic Unabridged Dictionary of the English Language (1996) gives a third meaning of the word "fact" as "(3) A truth known by actual experience or observation; something known to be true: ‘Scientists gather facts about plant growth.’"
  17. Dawkins, Richard; Coyne, Jerry (September 1, 2005). "One side can be wrong". The Guardian (London: Guardian Media Group). Retrieved 2015-01-16.
  18. 1 2 Muller, Hermann Joseph (April 1959). "One Hundred Years Without Darwinism Are Enough". School Science and Mathematics 59 (4): 304–305. doi:10.1111/j.1949-8594.1959.tb08235.x. ISSN 0036-6803. Reprinted in: Zetterberg 1983, p. 33
  19. NAS 1999, p. 28
  20. Gould 1981, citing Darwin, Charles (1871). The Descent of Man, and Selection in Relation to Sex. pp. 152–153. The book is available from The Complete Work of Charles Darwin Online. Retrieved 2015-01-17.
  21. Wilkins, John S. (1997). "Evolution and Philosophy: Is Evolution Science, and What Does 'Science' Mean?". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2009-08-17.
  22. NAS 1999, p. 2
  23. van Wyhe, John (May 22, 2007). "Mind the gap: Did Darwin avoid publishing his theory for many years?". Notes and Records of the Royal Society (London: Royal Society) 61 (2): 177–205. doi:10.1098/rsnr.2006.0171. ISSN 0035-9149. Retrieved 2015-01-17.
  24. Bock, Walter J. (May 2007). "Explanations in evolutionary theory". Journal of Zoological Systematics and Evolutionary Research (Hoboken, NJ: Wiley-Blackwell) 45 (2): 89–103. doi:10.1111/j.1439-0469.2007.00412.x. ISSN 0947-5745.
  25. 1 2 Fitzhugh, Kirk (January 2008). "Fact, theory, test and evolution" (PDF). Zoologica Scripta (Hoboken, NJ: Wiley-Blackwell on behalf of the Norwegian Academy of Science and Letters and the Royal Swedish Academy of Sciences) 37 (1): 109–113. doi:10.1111/j.1463-6409.2007.00308.x. ISSN 0300-3256.
  26. Wilson 1998.
  27. Wiley & Lieberman 2011, p. 300.
  28. Schuh 2000.
  29. Robinson, Bruce A. "Is the theory of evolution merely a 'theory'?". Religioustolerance.org. Kingston, Ontario: Ontario Consultants on Religious Tolerance. Retrieved 2007-10-18.
  30. Miller 2007.
  31. Mayr 1988.
  32. Wells, Huxley & Wells 1931, pp. 429, 600.
  33. Gould 1981.
  34. Lenski, Richard E. (September 2000). "Evolution: Fact and Theory". actionbioscience. Washington, D.C.: American Institute of Biological Sciences. Retrieved 2014-08-27.
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  36. Lewontin, Richard C. (September 1981). "Evolution/Creation Debate: A Time for Truth". BioScience (Oxford: Oxford University Press on behalf of the American Institute of Biological Sciences) 31 (8): 559. doi:10.1093/bioscience/31.8.559. ISSN 0006-3568. Reprinted in: Zetterberg 1983, p. 31
  37. Dawkins, Richard (December 2005). "The Illusion of Design". Natural History (Research Triangle Park, NC: Natural History Magazine, Inc.) 114 (9): 35–37. ISSN 0028-0712. Retrieved 2015-01-19.
  38. Campbell 1990, p. 434.
  39. "Recent Research by J. Kirk Fitzhugh, Ph.D.". Polychaetous Annelids Research Studies. Los Angeles, CA: Natural History Museum of Los Angeles County. Retrieved 2014-02-14.
  40. Bell 2008, p. 1.
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  43. Gould 2002.
  44. Bell 2008, p. 492.
  45. Platt, John R. (October 16, 1964). "Strong Inference". Science (Washington, D.C.: American Association for the Advancement of Science) 146 (3642): 347–353. doi:10.1126/science.146.3642.347. ISSN 0036-8075.
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Bibliography

Further reading

External links

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