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.
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 theoryEdit
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.
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:
- 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.[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.
- 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."
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.
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.
- 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
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.
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 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.
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:
- populations changing over generations
- gradual change
- 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.[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.
Evolution as theory and fact in the literatureEdit
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 factEdit
- American zoologist and paleontologist George Gaylord Simpson stated that "Darwin... finally and definitely established evolution as a fact."
- Hermann Joseph Muller wrote, "So enormous, ramifying, and consistent 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 theoryEdit
"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:
- Writing in 1930, biologist Julian Huxley entitled the third book of the wide-ranging series The Science of Life, which dealt with the fossil record and the evidence of plant and animal structures, The Incontrovertible Fact of Evolution. He also says "Natural Selection...is not a theory but a fact. But does it...suffice to account for the whole spectacle of Evolution?...There we come to speculative matter, to theories." But he concludes that "the broad positions of Darwinism re-emerge from a scrutiny of the most exacting sort essentially unchanged." In 1932, a portion of the book was republished under the title Evolution, Fact and Theory.
- 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 apelike 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 notes, "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 and not theoryEdit
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:
- Richard 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 fact, not theory."
- Douglas J. Futuyma writes in Evolutionary Biology (1998), "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."
Evolution as a collection of theories not factEdit
Evolutionary biologist 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."
Related concepts and terminologyEdit
- "Proof" of a theory has different meanings in science. Proof exists in formal sciences, such as a mathematical proof where symbolic expressions can represent infinite sets and scientific laws having precise definitions and outcomes of the terms. Proof has other meanings as it descends from its Latin roots (provable, probable, probare L.) meaning to test. In this sense a proof is an inference to the best or most parsimonious explanation through a publicly verifiable demonstration (a test) of the factual (i.e., observed) and causal evidence from carefully controlled experiments. Stephen Jay Gould argued that 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. "The classical proof is the improvement of crops and livestock through artificial selection." Natural selection and other evolutionary theories are also represented in various mathematical proofs, such as the Price equation. To remain consistent with the philosophy of science, however, advancement of theory is only achieved through disproofs of hypotheses.
- "Models" are part of the scientific or inferential "tool-kit" that are constructed out of preexistent theory. Model-based science uses idealized structures or mathematical expressions to strategically create simpler representations of complex worldly systems. Models are designed to resemble the relevant aspects of hypothetical relations in the target systems under investigation.
- "Validation is a demonstration that a model within its domain of applicability possesses a satisfactory range of accuracy consistent with the intended application of the model." Models are used in simulation research. For example, evolutionary phylogeneticists run simulations to model the tree like branching process of lineages over time. In turn, this is used to understand the theory of phylogenetics and the methods used to test for relations among genes, species, or other evolutionary units.
- Gould, Stephen Jay (1981) "Evolution as Fact and Theory" Discover 2 (May): 34-37; Reprinted in Hen's Teeth and Horse's Toes New York: W. W. Norton, 1994, pp. 253-262.
- See section 2
- Coyne 2009, p. 3.
- Mayr 1982, p. 400.
- Wright 1984.
- Futuyma 1998.
- Moran, Laurence (1993). "What is Evolution?". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-01-19.
- Thomson, Keith Stewart (September–October 1982). "Marginalia: The meanings of evolution". American Scientist. 70 (5): 529–531. JSTOR 27851662.
- Moran, Laurence (1993). "Evolution is a Fact and a Theory". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-01-19.
- Spencer, Herbert (January–April 1857). "Progress: its Law and Cause" (PDF). Westminster Review. New Series. 11: 446. OCLC 26747836. Archived from the original (PDF) on 2015-01-16. Retrieved 2015-01-16.
- Spencer 1865, p. 176.
- James 1911, Chapter 6: "Herbert Spencer's Autobiography".
- Brady, Michael (March 12, 2011). Evolutionary Opponents: William James and Herbert Spencer. Society for the Advancement of American Philosophy. Cheney, WA. Archived from the original (DOC) on April 3, 2016. Retrieved 2013-06-10. Paper presented at the 38th Annual Meeting.
- "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))
- "Fact". Views of the National Park Service Glossary. Washington, D.C.: National Park Service. Archived from the original on 2016-02-23. 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)
- 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.’"
- Dawkins, Richard; Coyne, Jerry (September 1, 2005). "One side can be wrong". The Guardian. London: Guardian Media Group. Retrieved 2015-01-16.
- 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. Reprinted in: Zetterberg 1983, p. 33
- NAS 1999, p. 28
- 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.
- 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.
- NAS 1999, p. 2
- 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. 61 (2): 177–205. doi:10.1098/rsnr.2006.0171. Retrieved 2015-01-17.
- Bock, Walter J. (May 2007). "Explanations in evolutionary theory". Journal of Zoological Systematics and Evolutionary Research. 45 (2): 89–103. doi:10.1111/j.1439-0469.2007.00412.x.
- Fitzhugh, Kirk (January 2008). "Fact, theory, test and evolution" (PDF). Zoologica Scripta. 37 (1): 109–113. doi:10.1111/j.1463-6409.2007.00308.x.
- Wilson 1998.
- Wiley & Lieberman 2011, p. 300.
- Schuh 2000.
- Robinson, Bruce A. "Is the theory of evolution merely a 'theory'?". Religioustolerance.org. Kingston, Ontario: Ontario Consultants on Religious Tolerance. Retrieved 2007-10-18.
- Miller 2007.
- Mayr 1988.
- Wells, Huxley & Wells 1931, pp. 429, 600.
- Gould 1981.
- Lenski, Richard E. (September 2000). "Evolution: Fact and Theory". actionbioscience. Washington, D.C.: American Institute of Biological Sciences. Retrieved 2014-08-27.
- Gregory, T. Ryan (January 2008). "Evolution as Fact, Theory, and Path" (PDF). Evolution: Education and Outreach. 1 (1): 46–52. doi:10.1007/s12052-007-0001-z. Archived from the original (PDF) on 2015-01-18. Retrieved 2015-01-18.
- Lewontin, Richard C. (September 1981). "Evolution/Creation Debate: A Time for Truth". BioScience. 31 (8): 559. doi:10.1093/bioscience/31.8.559. Reprinted in: Zetterberg 1983, p. 31
- Dawkins, Richard (December 2005). "The Illusion of Design". Natural History. 114 (9): 35–37. Retrieved 2015-01-19.
- Campbell 1990, p. 434.
- "Recent Research by J. Kirk Fitzhugh, Ph.D." Polychaetous Annelids Research Studies. Los Angeles, CA: Natural History Museum of Los Angeles County. Archived from the original on 2016-04-01. Retrieved 2014-02-14.
- Fitzhugh, Kirk (2007). "Fact, theory, test and evolution". Letter to the Editor. Zoologica Scripta: 071027215047001––. doi:10.1111/j.1463-6409.2007.00308.x.
- Robertson, William C. (2009-01-01). Answers to Science Questions from the Stop Faking It! Guy. NSTA Press. ISBN 9781936137992.
- Bell 2008, p. 1.
- Sundholm, Göran (July 1994). "Proof-Theoretical Semantics and Fregean Identity Criteria for Propositions" (PDF). The Monist. 77 (3): 294–314. doi:10.5840/monist199477315. hdl:1887/11990.
- Bissell, Derek (September 1996). "Statisticians have a Word for it" (PDF). Teaching Statistics. 18 (3): 87–89. CiteSeerX 10.1.1.385.5823. doi:10.1111/j.1467-9639.1996.tb00300.x.
- Gould 2002.
- Bell 2008, p. 492.
- Platt, John R. (October 16, 1964). "Strong Inference". Science. 146 (3642): 347–353. doi:10.1126/science.146.3642.347.
- Gorelick, Root (2011). "What is theory?" (PDF). Ideas in Ecology and Evolution. 4: 1–10. doi:10.4033/iee.2011.4.1.c. Archived from the original (PDF) on 2012-05-25. Retrieved 2015-01-19.
- Godfrey-Smith, Peter (November 2006). "The strategy of model-based science" (PDF). Biology and Philosophy. 21 (5): 725–740. doi:10.1007/s10539-006-9054-6.
- Rykiel, Edward J., Jr. (November 1, 1996). "Testing ecological models: the meaning of validation" (PDF). Ecological Modelling. 90 (3): 229–244. doi:10.1016/0304-3800(95)00152-2.
- Rohlf, F. James; Chang, W. S.; Sokal, Robert R.; Kim, Junhyong (September 1990). "Accuracy of Estimated Phylogenies: Effects of Tree Topology and Evolutionary Model". Evolution. 44 (6): 1671–1684. doi:10.2307/2409346. JSTOR 2409346.
- Bell, Graham (2008). Selection: The Mechanism of Evolution (2nd ed.). Oxford; New York: Oxford University Press. ISBN 978-0-19-856972-5. LCCN 2007039692. OCLC 170034792.
- Campbell, Neil A. (1990). Biology (2nd ed.). Redwood City, CA: Benjamin Cummings. ISBN 978-0-8053-1800-5. LCCN 89017952. OCLC 20352649.
- Coyne, Jerry A. (2009). Why Evolution is True. New York: Viking. ISBN 978-0-670-02053-9. LCCN 2008033973. OCLC 233549529.
- Futuyma, Douglas J. (1998). Evolutionary Biology (3rd ed.). Sunderland, MA: Sinauer Associates. ISBN 978-0-87893-189-7. LCCN 97037947. OCLC 37560100.
- Gould, Stephen Jay (1994) [Originally published 1983]. "Evolution as Fact and Theory". Hen's Teeth and Horse's Toes: Further Reflections in Natural History (Reissue ed.). New York: W. W. Norton & Company. ISBN 978-0-393-31103-7. LCCN 82022259. OCLC 785709315.
- —— (May 1981). "Evolution as Fact and Theory". Discover. 2 (5): 34–37.
- Gould, Stephen Jay (2002). The Structure of Evolutionary Theory. Cambridge, MA: Belknap Press of Harvard University Press. ISBN 978-0-674-00613-3. LCCN 2001043556. OCLC 47869352.
- James, William (1911). "Herbert Spencer's Autobiography". Memories and Studies. New York: Longmans, Green & Co. LCCN 11026966. OCLC 1573711.
- Mayr, Ernst (1982). The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Translation of John Ray by E. Silk. Cambridge, MA: Belknap Press. ISBN 978-0-674-36445-5. LCCN 81013204. OCLC 7875904.
- Mayr, Ernst (1988). Toward a New Philosophy of Biology: Observations of an Evolutionist. Cambridge, MA: Belknap Press of Harvard University Press. ISBN 978-0-674-89665-9. LCCN 87031892. OCLC 17108004.
- Miller, Kenneth R. (2007) [Originally published 1999; New York: Cliff Street Books]. Finding Darwin's God: A Scientist's Search for Common Ground Between God and Evolution. New York: Harper Perennial. ISBN 978-0-06-123350-0. LCCN 99016754. OCLC 813854733.
- National Academy of Sciences (1999). Science and Creationism: A View from the National Academy of Sciences (2nd ed.). Washington, D.C.: National Academy Press. ISBN 978-0-309-06406-4. LCCN 99006259. OCLC 43803228. Retrieved 2015-01-17.
- Schuh, Randall T. (2000). Biological Systematics: Principles and Applications. Ithaca, NY: Cornell University Press. ISBN 978-0-8014-3675-8. LCCN 99042377. OCLC 42027466.
- Spencer, Herbert (1865). First Principles of a New System of Philosophy. New York: D. Appleton & Company. LCCN 15024188. OCLC 3015414.
- Wells, H. G.; Huxley, Julian; Wells, G. P. (1931) [Originally published 1929–1930]. The Science of Life. Garden City, NY: Doubleday, Doran & Company, Inc. LCCN 31003561. OCLC 968712.
- Wiley, Edward O.; Lieberman, Bruce S. (2011). Phylogenetics: Theory and Practice of Phylogenetic Systematics (2nd ed.). Hoboken, NJ: Wiley-Blackwell. doi:10.1002/9781118017883. ISBN 978-0-470-90596-8. LCCN 2010044283. OCLC 741259265.
- Wilson, Edward O. (1998). Consilience: The Unity of Knowledge. New York: Knopf. ISBN 978-0-679-45077-1. LCCN 97002816. OCLC 36528112.
- Wright, Sewall (1984) [Originally published 1968]. Genetic and Biometric Foundations. Evolution and the Genetics of Populations. 1. Chicago, IL: University of Chicago Press. ISBN 978-0-226-91038-3. LCCN 67025533. OCLC 246124737.
- Zetterberg, J. Peter, ed. (1983). Evolution Versus Creationism: The Public Education Controversy. Phoenix, AZ: Oryx Press. ISBN 978-0-89774-061-6. LCCN 82018795. OCLC 9393689.
- Branch, Glenn; Mead, Louise (July 2008). "'Theory' in Theory and Practice". Evolution: Education and Outreach. 1 (3): 287–289. doi:10.1007/s12052-008-0056-5. ISSN 1936-6426.
- Dobzhansky, Theodosius (March 1973). "Nothing in Biology Makes Sense Except in the Light of Evolution" (PDF). The American Biology Teacher. 35 (3): 125–129. CiteSeerX 10.1.1.324.2891. doi:10.2307/4444260. ISSN 0002-7685. JSTOR 4444260.
- Lewis, Ralph W. (Winter 1987–1988). "Theory and the Fact of Evolution" (PDF). Creation/Evolution Journal. 8 (22): 34–37. ISSN 0738-6001. Retrieved 2015-01-21.
- National Academy of Sciences; Institute of Medicine (2008). Science, Evolution, and Creationism. Washington, D.C.: National Academies Press. ISBN 978-0-309-10586-6. LCCN 2007015904. OCLC 123539346. Retrieved 2015-01-19.
- "Evolving Ideas: Isn't Evolution Just a Theory?". Evolution Library (Web resource). Evolution. Boston, MA: WGBH Educational Foundation; Clear Blue Sky Productions, Inc. 2001. OCLC 48165595. Retrieved 2015-01-21. Video requires QuickTime or RealPlayer plugin for viewing.
- Isaak, Mark, ed. (September 7, 2006). "Index to Creationist Claims: Claim CB910: No new species have been observed". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-01-21. Response to the claim that no examples of speciation have been observed.
- "Is Evolution a Theory or a Fact?". Evolution Resources from the National Academies. Washington, D.C.: National Academy of Sciences. Retrieved 2015-01-21.
- Howard, Jules (2016-02-05). "Do whales have nipples? Why discussing evolution in schools can occasionally be tricky". the Guardian. Retrieved 2016-02-05.