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The Extended Evolutionary Synthesis (EES) consists of a set of theoretical concepts argued to be more comprehensive than the earlier modern synthesis of evolutionary biology that took place between 1918 and 1942. The extended evolutionary synthesis was called for in the 1950s by C. H. Waddington, argued for on the basis of punctuated equilibrium by Stephen Jay Gould and Niles Eldredge in the 1980s, and was reconceptualized in 2007 by Massimo Pigliucci and Gerd B. Müller.
The extended evolutionary synthesis revisits the relative importance of different factors at play, examining several assumptions of the earlier synthesis, and augmenting it with additional causative factors. [1] [2] It includes multilevel selection, transgenerational epigenetic inheritance, niche construction, evolvability, and several concepts from evolutionary developmental biology. [3] [4] [5] [6]
Not all biologists have agreed on the need for, or the scope of, an extended synthesis. [7] [8] [9] Many have collaborated on another synthesis in evolutionary developmental biology, which concentrates on developmental molecular genetics and evolution to understand how natural selection operated on developmental processes and deep homologies between organisms at the level of highly conserved genes.
The modern synthesis was the widely accepted early-20th-century synthesis reconciling Charles Darwin's theory of evolution by natural selection and Gregor Mendel's theory of genetics in a joint mathematical framework. It established evolution as biology's central paradigm. The 19th-century ideas of natural selection by Darwin and Mendelian genetics were united by researchers who included Ronald Fisher, J. B. S. Haldane and Sewall Wright, the three founders of population genetics, between 1918 and 1932. [10] [11] [12] [13] Julian Huxley introduced the phrase "modern synthesis" in his 1942 book, Evolution: The Modern Synthesis . [14] [15] [16]
During the 1950s, English biologist C. H. Waddington called for an extended synthesis based on his research on epigenetics and genetic assimilation. [17] [18] [19]
In 1978, Michael J. D. White wrote about an extension of the modern synthesis based on new research from speciation. [20] In the 1980s, entomologist Ryuichi Matsuda coined the term "pan-environmentalism" as an extended evolutionary synthesis which he saw as a fusion of Darwinism with neo-Lamarckism. [21] He held that heterochrony is a main mechanism for evolutionary change and that novelty in evolution can be generated by genetic assimilation. [21] [22] An extended synthesis was also proposed by the Austrian zoologist Rupert Riedl, with the study of evolvability. [23]
Gordon Rattray Taylor in his 1983 book The Great Evolution Mystery called for an extended synthesis, noting that the modern synthesis is only a subsection of a more comprehensive explanation for biological evolution still to be formulated. [24] In 1985, biologist Robert G. B. Reid authored Evolutionary Theory: The Unfinished Synthesis, which argued that the modern synthesis with its emphasis on natural selection is an incomplete picture of evolution, and emergent evolution can explain the origin of genetic variation. [25] [26] [27]
In 1988, ethologist John Endler wrote about developing a newer synthesis, discussing processes of evolution that he felt had been neglected. [28]
In 2000, Robert L. Carroll called for an "expanded evolutionary synthesis" due to new research from molecular developmental biology, systematics, geology and the fossil record. [29]
In the 1980s, the American palaeontologists Stephen Jay Gould and Niles Eldredge argued for an extended synthesis based on their idea of punctuated equilibrium, the role of species selection shaping large scale evolutionary patterns and natural selection working on multiple levels extending from genes to species. [30] [31] [32] [33]
Some researchers in the field of evolutionary developmental biology proposed another synthesis. They argue that the modern and extended syntheses should mostly center on genes and suggest an integration of embryology with molecular genetics and evolution, aiming to understand how natural selection operates on gene regulation and deep homologies between organisms at the level of highly conserved genes, transcription factors and signalling pathways. [34] [5] By contrast, a different strand of evo-devo following an organismal approach [35] [36] [37] [38] [39] [40] contributes to the extended synthesis by emphasizing (amongst others) developmental bias [41] (both through facilitation [42] and constraint [43] ), evolvability, [44] [45] and inherency of form [46] [47] as primary factors in the evolution of complex structures and phenotypic novelties. [48] [49]
The idea of an extended synthesis was relaunched in 2007 by Massimo Pigliucci, [50] [51] [52] and Gerd B. Müller, [38] [52] with a book in 2010 titled Evolution: The Extended Synthesis, which has served as a launching point for work on the extended synthesis. [52] This includes:
Other processes such as evolvability, phenotypic plasticity, reticulate evolution, horizontal gene transfer, symbiogenesis are said by proponents to have been excluded or missed from the modern synthesis. [59] [60] The goal of Piglucci's and Müller's extended synthesis is to take evolution beyond the gene-centered approach of population genetics to consider more organism- and ecology-centered approaches. Many of these causes are currently considered secondary in evolutionary causation, and proponents of the extended synthesis want them to be considered first-class evolutionary causes. [61]
Michael R. Rose and Todd Oakley have called for a postmodern synthesis, they commented that "it is now abundantly clear that living things often attain a degree of genomic complexity far beyond simple models like the "gene library" genome of the Modern Synthesis". [62] Biologist Eugene Koonin has suggested that the gradualism of the modern synthesis is unsustainable as gene duplication, horizontal gene transfer and endosymbiosis play a pivotal role in evolution. [63] Koonin commented that "the new developments in evolutionary biology by no account should be viewed as refutation of Darwin. On the contrary, they are widening the trails that Darwin blazed 150 years ago and reveal the extraordinary fertility of his thinking." [63]
Arlin Stoltzfus and colleagues advocate mutational and developmental bias in the introduction of variation as an important source of orientation or direction in evolutionary change. [64] [65] [66] [67] They argue that bias in the introduction of variation was not formally recognized throughout the 20th century, due to the influence of neo-Darwinism on thinking about causation. [68]
The early biologists of the organicist movement have influenced the modern extended evolutionary synthesis. Recent research has called for expanding the population genetic framework of evolutionary biology by a more organism-centered perspective. [69] [70] This has been described as "organism-centered evolution" which looks beyond the genome to the ways that individual organisms are participants in their own evolution. [70] [71] [72] Philip Ball has written a research review on organism-centered evolution. [73] [74]
Rui Diogo has proposed a revision of evolutionary theory, which he has termed ONCE: Organic Nonoptimal Constrained Evolution. [75] According to ONCE, evolution is mainly driven by the behavioural choices and persistence of organisms themselves, whilst natural selection plays a secondary role. [75] [76] [77] ONCE cites examples of reciprocal causation between organism and the environment, Baldwin effect, organic selection, developmental bias and niche construction. [76] [77] [78]
The extended synthesis is characterized by its additional set of predictions that differ from the standard modern synthesis theory:
From 2016 to 2019, there was an organized project entitled "Putting The Extended Evolutionary Synthesis To The Test" supported by a 7.5 million USD grant from the John Templeton Foundation, supplemented with further money from participating instutitions including Clark University, Indiana University, Lund University, Stanford University, University of Southampton and University of St Andrews. [84]
Publications from the project include over 200 papers, a special issue, [85] and an anthology on Evolutionary Causation. [86] In 2019 a final report of the 2016–2019 consortium was published, Putting the Extended Evolutionary Synthesis to the Test. [87]
The project was headed by Kevin N. Laland at the University of St Andrews and Tobias Uller at Lund University. According to Laland what the extended synthesis "really boils down to is recognition that, in addition to selection, drift, mutation and other established evolutionary processes, other factors, particularly developmental influences, shape the evolutionary process in important ways." [88]
Biologists disagree on the need for an extended synthesis. Opponents contend that the modern synthesis is able to fully account for the newer observations, whereas others criticize the extended synthesis for not being radical enough. [89] Proponents think that the conceptions of evolution at the core of the modern synthesis are too narrow [90] and that even when the modern synthesis allows for the ideas in the extended synthesis, using the modern synthesis affects the way that biologists think about evolution. For example, Denis Noble says that using terms and categories of the modern synthesis distorts the picture of biology that modern experimentation has discovered. [91] Proponents therefore claim that the extended synthesis is necessary to help expand the conceptions and framework of how evolution is considered throughout the biological disciplines. [2] [92] In 2022, the John Templeton Foundation published a review of recent literature. [93]
Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every level of biological organisation.
Heredity, also called inheritance or biological inheritance, is the passing on of traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cells or organisms acquire the genetic information of their parents. Through heredity, variations between individuals can accumulate and cause species to evolve by natural selection. The study of heredity in biology is genetics.
Evolutionary developmental biology is a field of biological research that compares the developmental processes of different organisms to infer how developmental processes evolved.
The modern synthesis was the early 20th-century synthesis of Charles Darwin's theory of evolution and Gregor Mendel's ideas on heredity into a joint mathematical framework. Julian Huxley coined the term in his 1942 book, Evolution: The Modern Synthesis. The synthesis combined the ideas of natural selection, Mendelian genetics, and population genetics. It also related the broad-scale macroevolution seen by palaeontologists to the small-scale microevolution of local populations.
Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism can pass on to its offspring physical characteristics that the parent organism acquired through use or disuse during its lifetime. It is also called the inheritance of acquired characteristics or more recently soft inheritance. The idea is named after the French zoologist Jean-Baptiste Lamarck (1744–1829), who incorporated the classical era theory of soft inheritance into his theory of evolution as a supplement to his concept of orthogenesis, a drive towards complexity.
Evolutionary biology is the subfield of biology that studies the evolutionary processes that produced the diversity of life on Earth. It is also defined as the study of the history of life forms on Earth. Evolution holds that all species are related and gradually change over generations. In a population, the genetic variations affect the phenotypes of an organism. These changes in the phenotypes will be an advantage to some organisms, which will then be passed on to their offspring. Some examples of evolution in species over many generations are the peppered moth and flightless birds. In the 1930s, the discipline of evolutionary biology emerged through what Julian Huxley called the modern synthesis of understanding, from previously unrelated fields of biological research, such as genetics and ecology, systematics, and paleontology.
Niche construction is the ecological process by which an organism alters its own local environment. These alterations can be a physical change to the organism’s environment, or it can encompass the active movement of an organism from one habitat to another where it then experiences different environmental pressures. Examples of niche construction include the building of nests and burrows by animals, the creation of shade, the influencing of wind speed, and alternations to nutrient cycling by plants. Although these modifications are often directly beneficial to the constructor, they are not necessarily always. For example, when organisms dump detritus, they can degrade their own local environments. Within some biological evolutionary frameworks, niche construction can actively beget processes pertaining to ecological inheritance whereby the organism in question “constructs” new or unique ecologic, and perhaps even sociologic environmental realities characterized by specific selective pressures.
In evolutionary biology, the Baldwin effect describes an effect of learned behaviour on evolution. James Mark Baldwin and others suggested that an organism's ability to learn new behaviours will affect its reproductive success and will therefore have an effect on the genetic makeup of its species through natural selection. It posits that subsequent selection might reinforce the originally learned behaviors, if adaptive, into more in-born, instinctive ones. Though this process appears similar to Lamarckism, that view proposes that living things inherited their parents' acquired characteristics. The Baldwin effect only posits that learning ability, which is genetically based, is another variable in / contributor to environmental adaptation. First proposed during the Eclipse of Darwinism in the late 19th century, this effect has been independently proposed several times, and today it is generally recognized as part of the modern synthesis.
Mutationism is one of several alternatives to evolution by natural selection that have existed both before and after the publication of Charles Darwin's 1859 book On the Origin of Species. In the theory, mutation was the source of novelty, creating new forms and new species, potentially instantaneously, in sudden jumps. This was envisaged as driving evolution, which was thought to be limited by the supply of mutations.
Origination of Organismal Form: Beyond the Gene in Developmental and Evolutionary Biology is an anthology published in 2003 edited by Gerd B. Müller and Stuart A. Newman. The book is the outcome of the 4th Altenberg Workshop in Theoretical Biology on "Origins of Organismal Form: Beyond the Gene Paradigm", hosted in 1999 at the Konrad Lorenz Institute for Evolution and Cognition Research. It has been cited over 200 times and has a major influence on extended evolutionary synthesis research.
In biology, saltation is a sudden and large mutational change from one generation to the next, potentially causing single-step speciation. This was historically offered as an alternative to Darwinism. Some forms of mutationism were effectively saltationist, implying large discontinuous jumps.
Genetic assimilation is a process described by Conrad H. Waddington by which a phenotype originally produced in response to an environmental condition, such as exposure to a teratogen, later becomes genetically encoded via artificial selection or natural selection. Despite superficial appearances, this does not require the (Lamarckian) inheritance of acquired characters, although epigenetic inheritance could potentially influence the result. Waddington stated that genetic assimilation overcomes the barrier to selection imposed by what he called canalization of developmental pathways; he supposed that the organism's genetics evolved to ensure that development proceeded in a certain way regardless of normal environmental variations.
Biological or process structuralism is a school of biological thought that objects to an exclusively Darwinian or adaptationist explanation of natural selection such as is described in the 20th century's modern synthesis. It proposes instead that evolution is guided differently, by physical forces which shape the development of an animal's body, and sometimes implies that these forces supersede selection altogether.
Marion Julia Lamb was Senior Lecturer at Birkbeck, University of London, before her retirement. She studied the effect of environmental conditions such as heat, radiation and pollution on metabolic activity and genetic mutability in the fruit fly Drosophila. From the late 1980s, Lamb collaborated with Eva Jablonka, researching and writing on the inheritance of epigenetic variations, and in 2005 they co-authored the book Evolution in Four Dimensions, considered by some to be in the vanguard of an ongoing revolution within evolutionary biology.
Evolutionary thought, the recognition that species change over time and the perceived understanding of how such processes work, has roots in antiquity—in the ideas of the ancient Greeks, Romans, Chinese, Church Fathers as well as in medieval Islamic science. With the beginnings of modern biological taxonomy in the late 17th century, two opposed ideas influenced Western biological thinking: essentialism, the belief that every species has essential characteristics that are unalterable, a concept which had developed from medieval Aristotelian metaphysics, and that fit well with natural theology; and the development of the new anti-Aristotelian approach to modern science: as the Enlightenment progressed, evolutionary cosmology and the mechanical philosophy spread from the physical sciences to natural history. Naturalists began to focus on the variability of species; the emergence of palaeontology with the concept of extinction further undermined static views of nature. In the early 19th century prior to Darwinism, Jean-Baptiste Lamarck (1744–1829) proposed his theory of the transmutation of species, the first fully formed theory of evolution.
Gerd B. Müller is an Austrian biologist who is emeritus professor at the University of Vienna where he was the head of the Department of Theoretical Biology in the Center for Organismal Systems Biology. His research interests focus on vertebrate limb development, evolutionary novelties, evo-devo theory, and the Extended Evolutionary Synthesis. He is also concerned with the development of 3D based imaging tools in developmental biology.
Transgenerational epigenetic inheritance is the transmission of epigenetic markers and modifications from one generation to multiple subsequent generations without altering the primary structure of DNA. Thus, the regulation of genes via epigenetic mechanisms can be heritable; the amount of transcripts and proteins produced can be altered by inherited epigenetic changes. In order for epigenetic marks to be heritable, however, they must occur in the gametes in animals, but since plants lack a definitive germline and can propagate, epigenetic marks in any tissue can be heritable.
Alternatives to Darwinian evolution have been proposed by scholars investigating biology to explain signs of evolution and the relatedness of different groups of living things. The alternatives in question do not deny that evolutionary changes over time are the origin of the diversity of life, nor that the organisms alive today share a common ancestor from the distant past ; rather, they propose alternative mechanisms of evolutionary change over time, arguing against mutations acted on by natural selection as the most important driver of evolutionary change.
In biology, reciprocal causation arises when developing organisms are both products of evolution as well as causes of evolution. Formally, reciprocal causation exists when process A is a cause of process B and, subsequently, process B is a cause of process A, with this feedback potentially repeated. Some researchers, particularly advocates of the extended evolutionary synthesis, promote the view that causation in biological systems is inherently reciprocal.
In biology, constructive development refers to the hypothesis that organisms shape their own developmental trajectory by constantly responding to, and causing, changes in both their internal state and their external environment. Constructive development can be contrasted with programmed development, the hypothesis that organisms develop according to a genetic program or blueprint. The constructivist perspective is found in philosophy, most notably developmental systems theory, and in the biological and social sciences, including developmental psychobiology and key themes of the extended evolutionary synthesis. Constructive development may be important to evolution because it enables organisms to produce functional phenotypes in response to genetic or environmental perturbation, and thereby contributes to adaptation and diversification.
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