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Evolutionary theory unifies observations from fossils, DNA sequences, systematics, biogeography, and laboratory experiments into a testable explanatory scheme. In this sense, the scientific (as opposed to the vernacular) definition of theory refers to an overarching framework that makes sense of otherwise disconnected observations; this includes, for example, the theory of gravity. Theodosius Dobzhansky, a key contributor to the modern evolutionary synthesis, articulated the unifying power of evolutionary theory in a famous paper entitled: "Nothing in biology makes sense except in the light of evolution".[4]
The scientific theory of evolution explains the causes of evolution, as distinct from the more straightforward factual claim that the process of evolution occurs. Natural selection and the neutral theory are examples of theories of evolution. These and many other causal evolutionary theories can be expressed in the mathematical framework of population genetics. Since Darwin, the theory of evolution by means of natural selection has not only been expressed mathematically, but has also been rigorously tested and corroborated empirically by scientific evidence from countless studies. Evolutionary theories continue to generate new testable hypotheses within paleontology, genetics, ecology, and developmental biology.
A fact is not a statement of certainty, but through repeated confirmation the things or processes they refer to are generally accepted as true according to the reliability of inference (inductive, deductive, and abductive). Facts refer to "events that occur" or "the state of being of things" that can be publicly verified, proven through experiment, or witnessed by direct observation.[3][5] That all forms of life on Earth are related by common descent with modification is one of the most reliable and empirically tested theories in science that continues to explain vast numbers of facts in biology.[2]
Evolution, fact and theory
The early developmental stages of most sciences have been characterized by continual competition between a number of distinct views of nature, each partially derived from, and all roughly compatible with, the dictates of scientific observation and method.[6]:4
"...scientific knowledge is tentative (subject to change); empirically based (based on and/or derived from observations of the natural world); subjective (theory-laden); partly the product of human inference, imagination, and creativity (involves the invention of explanation); and socially and culturally embedded...Although there is overlap and interaction between science processes and the nature of science, it is nevertheless important to distinguish the two."[10] :418
Evolution
Main articles: Introduction to evolution and Evolution
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. Other definitions of evolution cover a much broader scale overarching multiple levels of biological organisation, from macroevolutionary phenomena that occur during species formation and divergence, to microevolutionary processes within individual organisms, cells, and biomolecules such as DNA and proteins.[12][13] Evolution also refers to Darwin's theory of natural selection, which is the only known mechanism that can lead to adaptations and is only one of multiple mechanisms of evolutionary change. Natural selection is a process that acts on the heritable characteristics of individuals that interact and reproduce to form lineages of biological populations. Genetic drift, gene flow, vicariance biogeography, and niche construction are examples of other evolutionary mechanisms.[14][15] Evolution leads to the following additional claims:- Differences in trait composition between isolated populations over many generations may result in the origin of new species.
- All living organisms alive today have descended from a common ancestor (or ancestral gene pool).
Evolutionary theory may also refer to cultural evolution.[13] Evolutionary science provides an overarching framework in biology for identifying relationships and providing a coherent understanding of otherwise disconnected natural observations.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.[16]
[edit] Fact
Main article: Scientific fact
Hypotheses and theories are fallible, theory-laden constructs. Such fallibility does not, however, apply to facts, since facts merely exist regardless of their being perceived. The basis for testing is to evaluate the veracity of our claims regarding facts.[3]:2
- 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 Futuyma[22]
- 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.[24]
- 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 fact. Scientists no longer question whether descent with modification occurred because the evidence is so strong.[25]
Theory
Main article: Scientific theory
The scientific definition of the word "theory" is different from the colloquial sense of the word. 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. Scientific theories also contain speculation at first as scientists necessarily reach past the threshold of current knowledge, but they develop heuristically or through axillary claims as observations from experiments are explained and cause-effect relations are understood. Theories are constructs having both explanatory and predictive capacities that are built on inferential sets of logic (consilience of inductions, abductions, and deductions), models, and syllogistic schemes or laws that can be falsified through well designed experiments. In this way, theories that survive and develop through critical testing, such as Charles Darwin's theories on evolution, become richly informative as they explain cause-effect relations among many observable phenomena.[1][3][11][28]Theories give more general explanations of systems in nature than the hypotheses they generate. Hypotheses are explanatory statements for testing specific instances, inferences, or examples of the theory. Hypotheses make experimental claims about specific events in the past that produce effects that can be observed in the present. In contrast, theories apply universally as spatio-temporally unrestricted accounts of cause-effect relations. Theoretical models are one of many kinds of scientific methods that can used to communicate accurate and precise depictions of particular systems or organisms that are continually and repetitively investigated.[3][11][28][29][30]
The "theory of evolution" is actually a network of theories that have developed through the practice and understanding of the science involved. Charles 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, and (5) common descent.[13] 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.[3][11]
Evolutionary theories include theories about inheritance. Under the blending inheritance theory, evolution by natural selection is exceedingly difficult, since genetic variation is rapidly lost. A theoretical advance known as the Hardy–Weinberg principle shows that, under the alternative inheritance theory of Mendelian genetics, variation is not easily lost. The Hardy-Weinberg genotype frequencies also facilitate the population genetics study of natural selection using diploid replicator equations.[31]
The neutral theory of molecular evolution, for example, is used to study evolution as a null model against which tests for natural selection can be applied. Phylogenetic theory is another 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.[14][30]
Evolution compared with gravity
See also: Gravity and History of gravitational theory
The application of the terms "fact" and "theory" to evolution is comparable to their use in describing gravity.[18] The most obvious fact of gravity is that objects in our everyday experience always fall downwards when not otherwise prevented from doing so. People throughout history have wondered what causes this effect. Many explanations have been proposed over the centuries. Aristotle, Galileo, Newton, and Einstein developed models of gravity, each of which constitutes a theory of gravity. Newton, for example, realized that the fact of gravity can be extended to the tendency of any two masses to attract one another. The word "gravity", therefore, can be used to refer to the observed facts (i.e., that masses attract one another) and the theory used to explain the facts (the reason why masses attract one another). In this way, gravity is both a theory and a fact.[edit] Evolution as theory and fact in the literature
The confusion over the word evolution and the distinction between "fact" and "theory" is largely due to authors using evolution to refer to three related yet distinct ideas: first, the changes that occur within species over generations; second, the mechanism thought to drive change; and third, the concept of common descent. However, among biologists there is a consensus that evolution is a fact:- American zoologist and paleontologist George Simpson stated that "Darwin... finally and definitely established evolution as a fact."[32]
- H. J. 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."[24]
- Kenneth R. Miller writes, "evolution is as much a fact as anything we know in science."[33]
- 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."[34]
Commonly "fact" is 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:
- Paleontologist 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 ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered."[1]
- 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."[35]
- Biologist T. Ryan Gregory says, "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."[18]
Other commentators, focusing on the changes in species over generations and in some cases common ancestry have stressed that evolution is a fact to emphasize the weight of supporting evidence while denying it is helpful to use the term "theory":
- R. C. 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 a fact, not theory."[36]
- Douglas Futuyma writes in his Evolutionary Biology book, "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."[16]
- 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."[37]
- 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."[38]
- Evolutionary scientist Kirk Fitzhugh[39] wrote, "‘Evolution’ cannot be both a theory and a fact. Theories are concepts stating cause–effect relations...One might argue that it is conceivable to speak of ‘evolution’ as a fact by way of it being the subject of reference in explanatory hypotheses...In the strictest sense then, ‘evolution’ cannot be regarded as a fact even in the context of hypotheses since the causal points of reference continue to be organisms, and no amount of confirming instances for those hypotheses will transform them into facts...While evolution is not a fact, it is also not a single theory, but a set of theories applied to a variety of causal questions...An emphasis on associating ‘evolution’ with ‘fact’ presents the misguided connotation that science seeks certainty."[3]
A central tenet in science is that a scientific theory is supposed to have predictive power, and verification of predictions are seen as an important and necessary support for the theory. The theory of evolution has provided such predictions.[40] Four examples are:
- Genetic information must be transmitted in a molecular way that will be almost exact but permit slight changes. Since this prediction was made, biologists have discovered the existence of DNA, which has a mutation rate of roughly 10−9 per nucleotide per cell division; this provides just such a mechanism.[41]
- Some DNA sequences are shared by very different organisms. It has been predicted by the theory of evolution that the differences in such DNA sequences between two organisms should roughly resemble both the biological difference between them according to their anatomy and the time that had passed since these two organisms have separated in the course of evolution, as seen in fossil evidence. The rate of accumulating such changes should be low for some sequences, namely those that code for critical RNA or proteins, and high for others that code for less critical RNA or proteins; but for every specific sequence, the rate of change should be roughly constant over time. These results have been experimentally confirmed. Two examples are DNA sequences coding for rRNA, which is highly conserved, and DNA sequences coding for fibrinopeptides (amino acid chains that are discarded during the formation of fibrin), which are highly non-conserved.[41]
- Prior to 2004, paleontologists had found fossils of amphibians with necks, ears, and four legs, in rock no older than 365 million years old. In rocks more than 385 million years old they could only find fish, without these amphibian characteristics. Evolutionary theory predicted that since amphibians evolved from fish, an intermediate form should be found in rock dated between 365 and 385 million years ago. Such an intermediate form should have many fish-like characteristics, conserved from 385 million years ago or more, but also have many amphibian characteristics as well. In 2004, an expedition to islands in the Canadian arctic searching specifically for this fossil form in rocks that were 375 million years old discovered fossils of Tiktaalik.[42]
- Evolutionary theory predicts that novel inventions can arise, while creationists predict that new "information" cannot arise, and that the Second Law of Thermodynamics only allows for "information" to be lost.[43] In an ongoing experiment, Richard Lenski observed that some strains of E. coli evolved the ability to metabolize citrate after tens of thousands of generations.[44]
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.[45]:1
- "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.[46][47] 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. Charles 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.[48] "The classical proof is the improvement of crops and livestock through artificial selection."[45]:492 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.[49]
- "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.[50][51]
- "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."[52]:233 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.[53]
- A scientific law is a generalized formulation of the recurring observable tendencies of nature. Laws are based observations of events or processes that occur regularly and repeatedly under a defined set of conditions. Evolutionary laws are the demonstrated consequences of theoretical mechanisms, such as natural selection, neutral theory, niche construction, or other scientific theories. Branches in the diversity of life are the particular outcomes of the laws of evolution.[45][54]
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