Evolution as theory and fact
Encyclopedia
"Evolution is both fact and theory" is a statement that appears in numerous publications on biological evolution
. The statement is framed to clarify misconceptions about the philosophy
of evolution primarily in response to creationist statements that "evolution is only a theory". In the context of the creationist claim, theory is used in its vernacular
meaning as an imperfect fact or an unsubstantiated speculation. The purported intent is to discredit or reject the scientific credibility of evolution. However, this claim cannot be substantiated.
In the statement "evolution is both fact and theory", evolution as theory refers to the scientific
(as opposed to the vernacular) meanings of theory. In the first scientific meaning, a theory is an overarching framework that makes sense of otherwise disconnected observations. For example, the theory of gravity unifies astronomical observations with observations about the acceleration with which an object falls to earth. Similarly, the theory of evolution unifies observations from fossils, DNA sequences, systematics, biogeography, and laboratory experiments. 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
".
In the second scientific meaning, a scientific theory of evolution describes the causes of evolution, as distinct from the more straightforward factual claim that evolution occurs. Natural selection
and the neutral theory
are examples of theories of evolution in the second scientific sense. 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 it is generally accepted as true according to the reliability of inference (inductive
, deductive
, and abductive
). Facts are "events that occur" or "the state of being of things" that can be publicly verified, proven through experiment, or witnessed by direct observation. That all forms of life on Earth are related by common descent with modification is one of the most reliable facts in the biological sciences.
Evolutionary science is part of larger network of scientific theory that is used to confront and deal with the world. Science is a collective enterprise that evolves through the progress of experimentation and rejection of obsolete theories. By necessity, scientific ideas begin as speculation, because scientists lack foresight on correct solutions as they push the boundaries of knowledge and discovery. Scientific research has been called the "...interplay between imagination (hypothesis formulation) and experiment." In the process, scientist gather facts, conduct experiments, analyze, probe, prod, scrutinize, communicate on, and raise questions about the natural world. Over time, methods are developed and improved to further scientific knowledge related to the theory. Unexpected discoveries are not immediately integrated as fact. Science is developmentally enriched through the novelties of fact and theory. In this context, the meanings of the terms "evolution", "fact", and "theory" are described below.
phenomena that occur during species
formation and divergence, to microevolutionary
processes within individual organism
s, cells, and biomolecules
such as DNA
and protein
s. 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
population
s. Genetic drift
, gene flow
, vicariance
biogeography
, and niche construction
are examples of other evolutionary mechanisms. Evolution leads to the following additional claims:
According to Douglas Futuyma:
Evolutionary theory may also refer to cultural evolution. Evolutionary science provides an overarching framework in biology for identifying relationships and providing a coherent understanding of otherwise disconnected natural observations.
verifiable observations. However, scientists do not accept facts as absolute truth and in some cases even observations have been defined "as low-level hypotheses that exist only as interpretations of the facts of nature in light of present theories, not as the facts of nature themselves." According to this view, all concepts are theory-laden meaning that there is always an element of subjective interpretation embedded in our language. The US National Academy of Science defines a scientific fact as "an observation that has been repeatedly confirmed, and for all practical purposes, is accepted as ‘true’." Facts are also established through the process of experimentation:
Evolution is a fact in the sense that it is overwhelmingly validated by the evidence. Frequently, evolution is said to be a fact in the same way as the Earth revolving around the Sun is a fact. The following quotation from H. J. Muller, "One Hundred Years Without Darwin Are Enough" explains the point.
The National Academy of Science (U.S.) makes a similar point:
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.
, an opinion
, or a speculation
that does not have to be based on facts and it need not be framed for making testable predictions. Scientific theories
also contain speculation at first, but they develop over time and many are rejected as they are specifically crafted for the purpose or function of being testable. In this way, theories can be constructed using logic, models, or schemes for generating testable hypotheses
with precision.
The "theory of evolution" is actually a network of theories that develop with the science over time. Since Darwin, evolution has developed into a well-supported body of interconnected statements that explains countless observations in the natural world. Evolutionary theories generate testable predictions about nature. Theories offer more general explanations of system
s in nature than the hypotheses they generate, which are used to test specific instances or examples of the theory. Theoretical models are one of many kinds of scientific methods that can used to communicate accurate and precise depictions of systems, such as evolutionary systems, that are continually and repetitively investigated.
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 equation
s.
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 generated from the practice of phylogenetic theory. They depict relations among individuals that can speciate and diverge from one another. The evolutionary process of speciation necessarily creates groups that are linked by a common ancestor and all its descendants
. As species inherit traits they pass these onto their descendants. More closely allied species, of closer genealogical descent, have more common sets of traits that share a greater degree of similarity
in their morphology
and genetic composition. Evolutionary biologists use systematic
methods and test phylogenetic theory to observe 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 distributions of traits and their various forms to provide explanations of observed patterns in relation to their evolutionary history.
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 have all developed useful 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.
In the study of biological species, the facts include the existence of many different species in existence today, some very similar to each other and some very dissimilar, the remains of extinct species in the fossil record, and so forth. In species that rapidly reproduce, for example fruit flies
, the process of change from generation to generation — that is, evolutionary change — has been observed in the laboratory. The observation of fruit fly populations changing over time is also an example of a fact. So evolution is a fact just as observations of gravity are factual.
There have been many attempts to explain these biological observations over the years. Lamarckism
, transmutationism and orthogenesis
were all non-Darwinian theories that attempted to explain the observations of species and fossils, as well as other evidence. However, the modern theory of evolution is the explanation for all relevant observations regarding the development of life, based on a model that explains all the available data and observations (and provides testable predictions). Thus, evolution is not only a fact but also a theory, just as gravity is both a fact and a theory.
. However, among biologists there is a consensus that evolution is a fact:
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 . Four examples are:
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
. The statement is framed to clarify misconceptions about the philosophy
Philosophy of science
The philosophy of science is concerned with the assumptions, foundations, methods and implications of science. It is also concerned with the use and merit of science and sometimes overlaps metaphysics and epistemology by exploring whether scientific results are actually a study of truth...
of evolution primarily in response to creationist statements that "evolution is only a theory". In the context of the creationist claim, theory is used in its vernacular
Vernacular
A vernacular is the native language or native dialect of a specific population, as opposed to a language of wider communication that is not native to the population, such as a national language or lingua franca.- Etymology :The term is not a recent one...
meaning as an imperfect fact or an unsubstantiated speculation. The purported intent is to discredit or reject the scientific credibility of evolution. However, this claim cannot be substantiated.
In the statement "evolution is both fact and theory", evolution as theory refers to the scientific
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...
(as opposed to the vernacular) meanings of theory. In the first scientific meaning, a theory is an overarching framework that makes sense of otherwise disconnected observations. For example, the theory of gravity unifies astronomical observations with observations about the acceleration with which an object falls to earth. Similarly, the theory of evolution unifies observations from fossils, DNA sequences, systematics, biogeography, and laboratory experiments. Theodosius Dobzhansky
Theodosius Dobzhansky
Theodosius Grygorovych Dobzhansky ForMemRS was a prominent geneticist and evolutionary biologist, and a central figure in the field of evolutionary biology for his work in shaping the unifying modern evolutionary synthesis...
, a key contributor to the modern evolutionary synthesis
Modern evolutionary synthesis
The modern evolutionary synthesis is a union of ideas from several biological specialties which provides a widely accepted account of evolution...
, articulated the unifying power of evolutionary theory in a famous paper entitled: "Nothing in biology makes sense except in the light of evolution
Nothing in Biology Makes Sense Except in the Light of Evolution
"Nothing in Biology Makes Sense Except in the Light of Evolution" is a 1973 essay by the evolutionary biologist and Russian Orthodox Christian Theodosius Dobzhansky, criticising anti-evolution creationism and espousing theistic evolution...
".
In the second scientific meaning, a scientific theory of evolution describes the causes of evolution, as distinct from the more straightforward factual claim that evolution occurs. Natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
and the neutral theory
Neutral theory of molecular evolution
The neutral theory of molecular evolution states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants . The theory was introduced by Motoo Kimura in the late 1960s and early 1970s...
are examples of theories of evolution in the second scientific sense. These and many other causal evolutionary theories can be expressed in the mathematical framework of population genetics
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
. Since Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
, the theory of evolution by means of natural selection has not only been expressed mathematically, but has also been rigorously tested and corroborated
Corroborating evidence
Corroborating evidence is evidence that tends to support a proposition that is already supported by some evidence, therefore confirming the proposition. For example, W, a witness, testifies that she saw X drive his automobile into a green car...
empirically
Empirical research
Empirical research is a way of gaining knowledge by means of direct and indirect observation or experience. Empirical evidence can be analyzed quantitatively or qualitatively...
by scientific evidence
Scientific evidence
Scientific evidence has no universally accepted definition but generally refers to evidence which serves to either support or counter a scientific theory or hypothesis. Such evidence is generally expected to be empirical and properly documented in accordance with scientific method such as is...
from countless studies. Evolutionary theories continue to generate new testable hypotheses within paleontology
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, genetics
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....
, ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
, and developmental biology
Evolutionary developmental biology
Evolutionary developmental biology is a field of biology that compares the developmental processes of different organisms to determine the ancestral relationship between them, and to discover how developmental processes evolved...
.
A fact is not a statement of certainty, but through repeated confirmation it is generally accepted as true according to the reliability of inference (inductive
Inductive reasoning
Inductive reasoning, also known as induction or inductive logic, is a kind of reasoning that constructs or evaluates propositions that are abstractions of observations. It is commonly construed as a form of reasoning that makes generalizations based on individual instances...
, deductive
Deductive reasoning
Deductive reasoning, also called deductive logic, is reasoning which constructs or evaluates deductive arguments. Deductive arguments are attempts to show that a conclusion necessarily follows from a set of premises or hypothesis...
, and abductive
Abductive reasoning
Abduction is a kind of logical inference described by Charles Sanders Peirce as "guessing". The term refers to the process of arriving at an explanatory hypothesis. Peirce said that to abduce a hypothetical explanation a from an observed surprising circumstance b is to surmise that a may be true...
). Facts are "events that occur" or "the state of being of things" that can be publicly verified, proven through experiment, or witnessed by direct observation. That all forms of life on Earth are related by common descent with modification is one of the most reliable facts in the biological sciences.
Evolution, fact and theory
Evolution has been described as "fact and theory", "fact not theory", "only a theory, not a fact", and "multiple theories, not fact". Science cannot achieve absolute "certainty" nor is it a continuous march toward an objective truth as the vernacular meaning of the terms "proof" or "fact" might imply. A proof, fact, theory, hypothesis, and other words of science are hobbled by multiple meanings but are used nonetheless because they invigorate research methods and lead to discovery in all branches of scientific research. The philosophy of scientific inquiry solves problems of novelty as discoveries are made. Scientific knowledge is shared, incorporated, and tested across disciplines. Charles Darwin, for example, not only advanced theory and hypotheses in evolution, but experimented and tested his ideas across disciplines, including and not limited to geology, botany, psychology, and ecology.Evolutionary science is part of larger network of scientific theory that is used to confront and deal with the world. Science is a collective enterprise that evolves through the progress of experimentation and rejection of obsolete theories. By necessity, scientific ideas begin as speculation, because scientists lack foresight on correct solutions as they push the boundaries of knowledge and discovery. Scientific research has been called the "...interplay between imagination (hypothesis formulation) and experiment." In the process, scientist gather facts, conduct experiments, analyze, probe, prod, scrutinize, communicate on, and raise questions about the natural world. Over time, methods are developed and improved to further scientific knowledge related to the theory. Unexpected discoveries are not immediately integrated as fact. Science is developmentally enriched through the novelties of fact and theory. In this context, the meanings of the terms "evolution", "fact", and "theory" are described below.
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 macroevolutionaryMacroevolution
Macroevolution is evolution on a scale of separated gene pools. Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes within a species or population.The process of speciation may fall...
phenomena that occur during species
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
formation and divergence, to microevolutionary
Microevolution
Microevolution is the changes in allele frequencies that occur over time within a population. This change is due to four different processes: mutation, selection , gene flow, and genetic drift....
processes within individual organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
s, cells, and biomolecules
Molecular evolution
Molecular evolution is in part a process of evolution at the scale of DNA, RNA, and proteins. Molecular evolution emerged as a scientific field in the 1960s as researchers from molecular biology, evolutionary biology and population genetics sought to understand recent discoveries on the structure...
such as DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s. 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
Heritability
The Heritability of a population is the proportion of observable differences between individuals that is due to genetic differences. Factors including genetics, environment and random chance can all contribute to the variation between individuals in their observable characteristics...
characteristics of individuals that interact and reproduce to form lineages of biological
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
s. Genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
, gene flow
Gene flow
In population genetics, gene flow is the transfer of alleles of genes from one population to another.Migration into or out of a population may be responsible for a marked change in allele frequencies...
, vicariance
Vicariance
Vicariance is a process by which the geographical range of an individual taxon, or a whole biota, is split into discontinuous parts by the formation of a physical barrier to gene flow or dispersal. Vicariance of whole biotas occurs following large-scale geophysical events such as the uplift of a...
biogeography
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
, and niche construction
Niche construction
Niche construction is the process in which an organism alters its own environment, often but not always in a manner that increases its chances of survival...
are examples of other evolutionary mechanisms. Evolution leads to the following additional claims:
- Differences in trait composition between isolated populations over many generations may result in the origin of new speciesSpeciationSpeciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
. - All living organisms alive today have descended from a common ancestorCommon descentIn evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
(or ancestral gene pool).
According to Douglas Futuyma:
Evolutionary theory may also refer to cultural evolution. Evolutionary science provides an overarching framework in biology for identifying relationships and providing a coherent understanding of otherwise disconnected natural observations.
Fact
Facts are "events that occur" or "the state of being of things" that can be publicly verified, proven through experiment, or witnessed by direct observation. Unlike other terms of science, facts in science are similar in definition and interpretation relative to their use in common language. Fact is often used by scientists to refer to experimental or empirical data or objectiveObjectivity (science)
Objectivity in science is a value that informs how science is practiced and how scientific truths are created. It is the idea that scientists, in attempting to uncover truths about the natural world, must aspire to eliminate personal biases, a priori commitments, emotional involvement, etc...
verifiable observations. However, scientists do not accept facts as absolute truth and in some cases even observations have been defined "as low-level hypotheses that exist only as interpretations of the facts of nature in light of present theories, not as the facts of nature themselves." According to this view, all concepts are theory-laden meaning that there is always an element of subjective interpretation embedded in our language. The US National Academy of Science defines a scientific fact as "an observation that has been repeatedly confirmed, and for all practical purposes, is accepted as ‘true’." Facts are also established through the process of experimentation:
- 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
Evolution is a fact in the sense that it is overwhelmingly validated by the evidence. Frequently, evolution is said to be a fact in the same way as the Earth revolving around the Sun is a fact. The following quotation from H. J. Muller, "One Hundred Years Without Darwin 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 Science (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 fact. Scientists no longer question whether descent with modification occurred because the evidence is so strong.
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 colloquial sense of the word. Colloquially or in the vernacular sense of the term, "theory" can refer to guesswork, a simple conjectureConjecture
A conjecture is a proposition that is unproven but is thought to be true and has not been disproven. Karl Popper pioneered the use of the term "conjecture" in scientific philosophy. Conjecture is contrasted by hypothesis , which is a testable statement based on accepted grounds...
, an opinion
Opinion
In general, an opinion is a subjective belief, and is the result of emotion or interpretation of facts. An opinion may be supported by an argument, although people may draw opposing opinions from the same set of facts. Opinions rarely change without new arguments being presented...
, or a speculation
Speculation
In finance, speculation is a financial action that does not promise safety of the initial investment along with the return on the principal sum...
that does not have to be based on facts and it need not be framed for making testable predictions. Scientific theories
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...
also contain speculation at first, but they develop over time and many are rejected as they are specifically crafted for the purpose or function of being testable. In this way, theories can be constructed using logic, models, or schemes for generating testable hypotheses
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
with precision.
The "theory of evolution" is actually a network of theories that develop with the science over time. Since Darwin, evolution has developed into a well-supported body of interconnected statements that explains countless observations in the natural world. Evolutionary theories generate testable predictions about nature. Theories offer more general explanations of system
System
System is a set of interacting or interdependent components forming an integrated whole....
s in nature than the hypotheses they generate, which are used to test specific instances or examples of the theory. Theoretical models are one of many kinds of scientific methods that can used to communicate accurate and precise depictions of systems, such as evolutionary systems, that are continually and repetitively investigated.
Evolutionary theories include theories about inheritance. Under the blending inheritance
Blending inheritance
Many biologists and other academics held to the idea of blending inheritance during the 19th century, prior to the discovery of genetics. Blending inheritance was merely a widespread hypothetical model, rather than a formalized scientific theory , in...
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
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
study of natural selection using diploid replicator equation
Replicator equation
In mathematics, the replicator equation is a deterministic monotone non-linear and non-innovative game dynamic used in evolutionary game theory. The replicator equation differs from other equations used to model replication, such as the quasispecies equation, in that it allows the fitness landscape...
s.
The neutral theory of molecular evolution
Neutral theory of molecular evolution
The neutral theory of molecular evolution states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants . The theory was introduced by Motoo Kimura in the late 1960s and early 1970s...
, 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
Phylogenetic tree
A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical and/or genetic characteristics...
are hypotheses that are generated from the practice of phylogenetic theory. They depict relations among individuals that can speciate and diverge from one another. The evolutionary process of speciation necessarily creates groups that are linked by a common ancestor and all its descendants
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
. As species inherit traits they pass these onto their descendants. More closely allied species, of closer genealogical descent, have more common sets of traits that share a greater degree of similarity
Homology
Homology may refer to:* Homology , analogy between human beliefs, practices or artifacts owing to genetic or historical connections* Homology , any characteristic of biological organisms that is derived from a common ancestor....
in their morphology
Morphology
Morphology may mean:*Morphology , the study of the structure and content of word forms*Morphology , the study of the form or shape of an organism or part thereof...
and genetic composition. Evolutionary biologists use systematic
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
methods and test phylogenetic theory to observe 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 distributions of traits and their various forms to provide explanations of observed patterns in relation to their evolutionary history.
Evolution compared with gravity
The application of the terms "fact" and "theory" to evolution is similar to their use in describing gravity.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
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...
, Galileo, Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...
, and Einstein have all developed useful 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.
In the study of biological species, the facts include the existence of many different species in existence today, some very similar to each other and some very dissimilar, the remains of extinct species in the fossil record, and so forth. In species that rapidly reproduce, for example fruit flies
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
, the process of change from generation to generation — that is, evolutionary change — has been observed in the laboratory. The observation of fruit fly populations changing over time is also an example of a fact. So evolution is a fact just as observations of gravity are factual.
There have been many attempts to explain these biological observations over the years. Lamarckism
Lamarckism
Lamarckism is the idea that an organism can pass on characteristics that it acquired during its lifetime to its offspring . It is named after the French biologist Jean-Baptiste Lamarck , who incorporated the action of soft inheritance into his evolutionary theories...
, transmutationism and orthogenesis
Orthogenesis
Orthogenesis, orthogenetic evolution, progressive evolution or autogenesis, is the hypothesis that life has an innate tendency to evolve in a unilinear fashion due to some internal or external "driving force". The hypothesis is based on essentialism and cosmic teleology and proposes an intrinsic...
were all non-Darwinian theories that attempted to explain the observations of species and fossils, as well as other evidence. However, the modern theory of evolution is the explanation for all relevant observations regarding the development of life, based on a model that explains all the available data and observations (and provides testable predictions). Thus, evolution is not only a fact but also a theory, just as gravity is both a fact and a theory.
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 descentCommon descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
. However, among biologists there is a consensus that evolution is a fact:
- American zoologist and paleontologist George SimpsonGeorge Gaylord SimpsonGeorge Gaylord Simpson was an American paleontologist. Simpson was perhaps the most influential paleontologist of the twentieth century, and a major participant in the modern evolutionary synthesis, contributing Tempo and mode in evolution , The meaning of evolution and The major features of...
stated that "Darwin... finally and definitely established evolution as a fact." - H. J. Muller wrote, "So enormous, ramifying, and consistant 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. MillerKenneth R. MillerKenneth Raymond Miller is a biology professor at Brown University. Miller, who is Roman Catholic, is particularly known for his opposition to creationism, including the intelligent design movement...
writes, "evolution is as much a fact as anything we know in science." - Ernst MayrErnst MayrErnst Walter Mayr was one of the 20th century's leading evolutionary biologists. He was also a renowned taxonomist, tropical explorer, ornithologist, historian of science, and naturalist...
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 theory
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:- PaleontologistPaleontologyPaleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
Stephen Jay GouldStephen Jay GouldStephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
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." - Similarly, biologist Richard LenskiRichard LenskiRichard E. Lenski is an American evolutionary biologist. He is the son of sociologist Gerhard Lenski. He earned his BA from Oberlin College in 1976, and his PhD from the University of North Carolina in 1982...
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 GregoryT. Ryan GregoryDr. T. Ryan Gregory is a Canadian evolutionary biologist and genome biologist and a tenured Associate Professor in the and the at the University of Guelph in Guelph, Ontario, Canada....
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."
Evolution as fact not theory
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."
- 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."
- Richard DawkinsRichard DawkinsClinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
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 CampbellNeil Campbell (scientist)Neil A. Campbell was an American scientist known best for his textbook Biology. First published in 1987, the text is currently in its 9th edition and co-authored by Jane Reece...
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." - Evolutionary scientist Kirk Fitzhugh penned, ‘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.
Predictive power
A central tenet in scienceScience
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
is that a scientific theory is supposed to have predictive power
Predictive power
The predictive power of a scientific theory refers to its ability to generate testable predictions. Theories with strong predictive power are highly valued, because the predictions can often encourage the falsification of the theory...
, and verification of predictions are seen as an important and necessary support for the theory. The theory of evolution has provided such predictions . 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 DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
, which has a mutation rate of roughly 10−9 per nucleotideNucleotideNucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
per cell divisionCell divisionCell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...
; this provides just such a mechanism. - 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 anatomyAnatomyAnatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
and the time that had passed since these two organisms have separated in the course of evolution, as seen in fossilFossilFossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
evidence. The rate of accumulating such changes should be low for some sequences, namely those that code for critical RNARNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
or proteinProteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, 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 acidAmino acidAmino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
chains that are discarded during the formation of fibrinFibrinFibrin is a fibrous, non-globular protein involved in the clotting of blood. It is a fibrillar protein that is polymerised to form a "mesh" that forms a hemostatic plug or clot over a wound site....
), which are highly non-conserved. - 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 TiktaalikTiktaalikTiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...
. - 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. In an ongoing experiment, Richard LenskiRichard LenskiRichard E. Lenski is an American evolutionary biologist. He is the son of sociologist Gerhard Lenski. He earned his BA from Oberlin College in 1976, and his PhD from the University of North Carolina in 1982...
observed that some strains of E. coli evolved the ability to metabolize citrateCitrateA citrate can refer either to the conjugate base of citric acid, , or to the esters of citric acid. An example of the former, a salt is trisodium citrate; an ester is triethyl citrate.-Other citric acid ions:...
after tens of thousands of generations.
Related concepts and terminology
- Speculative or conjectural explanations are called hypothesesHypothesisA hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
. Well-tested and corroborated (or validated) explanations are called theories. - "Fact" does not mean "absolute certainty". In the words of Stephen J. Gould: In science, "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional assent."
- "Proof" of a theory does not exist in natural sciences. Proof only exists in formal sciences, such as mathematicsMathematical proofIn mathematics, a proof is a convincing demonstration that some mathematical statement is necessarily true. Proofs are obtained from deductive reasoning, rather than from inductive or empirical arguments. That is, a proof must demonstrate that a statement is true in all cases, without a single...
. Experimental observation of the predictions made by a hypothesis or theory is called validation. - A scientific law is a concept related to a scientific theory. Very well-established "theories" that rely on a simple principle are often called scientific "laws". For example, it is common to encounter reference to "the law of gravity", "the law of natural selection", or the "laws of evolution."
See also
- Epistemology
- Evidence of common descent
- Evolution is just a theory, not a fact (in objections to evolutionObjections to evolutionObjections to evolution have been raised since evolutionary ideas came to prominence in the 19th century. When Charles Darwin published his 1859 book On the Origin of Species, his theory of evolution by natural selection initially met opposition from scientists with different theories, but came to...
) - Misconceptions about evolution (in List of misconceptions)
- Theory vs. Fact (in Creation-evolution controversyCreation-evolution controversyThe creation–evolution controversy is a recurring cultural, political, and theological dispute about the origins of the Earth, humanity, life, and the universe....
)
External links
- Not Just a Theory Discredits the assertion that evolution is "just a theory", with an explanation of the meaning of the word 'theory' in a scientific context.
- Talk Origins Response to the claim that no examples of speciation have been observed.