Definitions of "evolution"
The word evolution has many usages. Even when only biological evolution is meant, there are many various processes, any or all of which may be meant. Clear communication (whether you are the writer / speaker or the reader / listener) requires awareness of and attention to these various meanings, to minimize ambiguity and patent misunderstanding. In anti-evolutionist writings, the concept is considerably muddled by the addition of several extra concepts that have no place in scientific usage.
Note that Darwin used the word "evolved" only once in his Origin of Species, where it was the very last word of the text. He did not even once use the word "evolution".
Perhaps wisely, the authors of our textbook (Stearns & Hoekstra) do not even try to define evolution.
The Oxford English Dictionary (the standard reference for word usage in the English language) presents 12 different definitions for "evolution", including:
- Unfolding, opening out, emergence.
- Growth according to inherent tendencies.
- Rise or origination of anything by natural development.
- "The process of developing, or working out in detail, what is implicitly or potentially contained in an idea or principle."
- Biological development (i.e. ontogeny),
(specifically, the "Theory of Evolution" by Bonnet, 1762, now referred to as "preformation").- Formation of the heavenly bodies
- Origin of species
Note that several of these meanings include connotations of pre-existing plan or goal.
Also note that all but the last of these usages predate the modern application of the term to describe biological evolution.
These prior meanings have served to contaminate and mislead popular understanding of the biological processes first presented by Darwin (who avoided the word "evolution", presumably in a futile attempt to avoid such misinterpretations). As a result, popular understanding of evolution commonly includes elements of predestination, goal-directedness, or progress which have no place in conventional current scientific understanding of biological evolution.
(More: Essay by Stephen Jay Gould)
In reference to modern understanding of biological evolution, casual use of the word "evolution" can refer to any of a variety of patterns and processes, including:
- History of life
"Evolution" can refer simply to the description of life's changing diversity over time, as recorded by fossils in geological strata, with no implication of specific mechanism. In this sense, many scientists who opposed Darwin and Darwinism during the 19th and early 20th centuries were nevertheless evolutionists.
- Descent with Modification
"Evolution" can refer to a process of change from ancestor to descendant over many generations, whether or not adaptation ensues. Another label for this process is anagenesis. As in the broader "history of life" meaning (above), this carries no necessary implication of a specific mechanism. Lamarck just as much as Darwin believed in descent with modification.
- Adaptation
"Evolution" can refer to the acquisition and/or adjustment of traits which confer fitness for life in a particular niche (presumably over the course of descent with modification). Note that "adaptation" itself has two distinct usages:
- Evolutionary adaptation, in which fitness is acquired by natural selection of heritable variation.
- Physiological adaptation, in which traits are adjusted by inate mechanisms within the lifespan of each individual.
Exclusive or excessive emphasis on evolution as adaptation is sometimes called adaptationism.
- Speciation
"Evolution" can refer to the formation of new species, through the splitting of an ancestral population into two or more reproductively isolated taxa. Over the long term, this process may be called phylogenesis (literally, the origin of tribes) and cladogenesis (literally, the origin of branches). Note that speciation is not a requirement for either adaptation or descent with modification. And Darwin had remarkably little to say about speciation, in his book titled Origin of Species.
- Adaptative radiation
In a usage that incorporates BOTH adaptation AND speciation, "evolution" can refer to the divergence of related species into various different niches.
- Macroevolution
"Evolution" can refer to the transformation of body plans (e.g., fish to bird; alga to orchid), including the invention of complex innovations (e.g., organs, behaviors) for new roles. Adequately explaining macroevolution in terms of genomic organization and developmental mechanisms remains an outstanding challenge for modern biology.
- Natural selection
"Evolution" can refer to the process of differential reproductive success among different genetic variants. This is part of an explanatory mechanism for "descent with modification" and for "adaptation". Becaused Darwin introduced and emphasized natural selection, an understanding of evolutionary adaptation operating through natural selection of spontaneous variation is often referred to as Darwinism.
- "Survival of the Fittest"
"Evolution" can refer to the differential survival of individuals with differing traits. "Survival of the fittest" is a popular but inaccurate catch-phrase (attributed to Herbert Spencer) that can promote several misunderstandings:
- "Survival of the fittest" makes evolutionary explanation appear tautological (i.e, based on circular reasoning). Since fitness is often measured by survival, the caricature phrase "survival of the fittest" appears to mean no more than "survival of the survivors".
- "Survival of the fittest" emphasizes survival over reproduction, and thus misrepresents the actual process of natural selection (which is based more essentially on differential reproduction).
- By explicitly mentioning fitness while neglecting to mention genetic variation, "survival of the fittest" can be interpreted to include selection of acquired (rather than heritable) traits.
- Genetic drift
"Evolution" can refer to the process of random sampling which occurs as alleles are passed from one generation to the next. Although this statistical process is an important element in the "modern synthesis", it is quite distinct from natural selection and played little role in early Darwinism. Excessive (or exclusive) emphasis on drift is some characterized as neutralism.
- Change in allele frequencies over time
"Evolution" can refer simply to the change over time in the distribution of allele frequencies within populations. This is one of the most popular text-book definitions. An understanding of the mechanisms which cause changes in allele frequencies is sometimes referred to as neo-Darwinism. Unfortunately, even though this definition embraces the several microevolutionary processes of mutation, selection, migration, and drift, it omits any mention of the significant (and highly interesting) processes of adaptation, speciation, and macroevolution.
- Theory of evolution
"Evolution" can refer to the integration of all of the above patterns and processes into a coherent explanatory framework. In this sense, "evolution" is very much a work in progress, one whose prevailing concepts and emphases are modified and enriched with each passing decade.
Related to the various meanings of "evolution" are various stances, or "-isms", which reflect more-or-less extreme perspectives on what is important in evolution.
- Darwinism refers to explanatory concepts elaborated by Charles Darwin and his followers, emphasizing descent with modification, adaptation, and natural selection. "Darwinism" is commonly but carelessly used as a synonym for any aspect of biological evolution, and is often used derisively by anti-evolutionists. In stricter usage, "Darwinism" does not incorporate genes or statistical genetics (including drift), concepts which were unknown to Darwin.
- Social Darwinism is a simplistic misapplication of Darwinism to human affairs, in which human individuals and societies who succeed over others (whether by intellectual, economic, or military means) are deemed thereby to be "better", more worthy to dominate. It has been widely discredited both as biology and as social philosophy. Many of the traits that enable one human group to oppress others are cultural rather than genetic, hence Darwinian explanations are inappropriate. Furthermore, social darwinism has been used as a justification for unrestrained capitalism and for extreme nationalism, with results that are are widely perceived as evil. For example, the philosophical foundations for German Nazism are commonly laid at the feet of Ernst Haeckel, who in the late 1800s was a principal advocate of darwinism in continental Europe. Note that social-darwinist justification for exploitative economic and militarist policies may have contributed, through righteous dismay at the consequences, to organized religious opposition to evolutionary biology as the supposed cause of such associated evils.
- Neo-Darwinism refers to the understanding established in the first half of the twentieth-century, in which Mendelian genetics and statistical population genetics became integrated with Darwin's understanding of variation, natural selection, and descent with modification. This understanding has been labelled the Modern Synthesis. As commonly used, "neo-Darwinism" is a label for modern evolutionary biology. However, the Modern Synthesis predates our current understanding of DNA and genomic organization, as well as some of the finer points of population genetics. Hence, for some, "neo-Darwinism" has become a label for a rather dated and restrictive understanding of evolutionary process.
- Adaptationism refers to the more-or-less extreme view that evolution optimizes the fitness-conferring value of all traits. An adaptationist attitude is commonly implied (but seldom explicit) in much popular nature-education, which emphasizes the power of "evolution" to accomplish ideal design. The term itself is most often used pejoratively as a criticism of someone's failure to consider critically alternative explanatory mechanisms. But to a first approximation, an adaptationist perspective can provide a useful guide for research. Thus an implicit adaptationism is fairly deeply rooted in various disciplines such as physiology, ethology, anthropology, and evolutionary psychology.
- Neutralism refers to the view that much variation observed at the molecular level results from approximately neutral mutations together with the statistical processes of genetic drift. Neutralism and adaptationism are often presented as antagonistic positions, when they are more usefully appreciated as supplementary perspectives.
- Mutationism is the label for an obsolete view that macro-mutations are responsible for the origin of species by sudden, dramatic change. Saltationism is an approximate synonym.
304 index pageAnti-evolutionist writings often conflate these several meanings, especially the distinction between the many "factual" aspects of evolution (i.e., those patterns and processes which are known by more-or-less-direct observation) and the "theoretical" organization of associated concepts into an explanation of the interrelationships between observable pattern and observable process, as if criticisms relevant in one domain were directly applicable to another. There is also commonly a presumption (or an explicit assertion) that evolution (as understood by the anti-evolutionist) must necessarily be random, purposeless, godless, associated with immorality, and false.
For examples of anti-evolutionist argument (pro and con) see the listings at Talk.Origins.
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