The extended evolutionary synthesis consists of a set of theoretical concepts argued to be more comprehensive than the earlier modern synthesis of

evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...

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 In evolutionary biology, punctuated equilibrium (also called punctuated equilibria) is a theory that proposes that once a species appears in the fossil record, the population will become stable, showing little evolutionary change for most of i ...

by Stephen Jay Gould and Niles Eldredge in the 1980s, and was reconceptualized in 2007 by

Massimo Pigliucci Massimo Pigliucci (; born January 16, 1964) is Professor of Philosophy at the City College of New York, former co-host of the '' Rationally Speaking Podcast'', and former editor in chief for the online magazine ''Scientia Salon''. He is a critic o ...

and Gerd B. Müller. Notably, Dr. Müller concluded from this research that Natural Selection has no way of explaining speciation, saying: “selection has no innovative capacity...the generative and the ordering aspects of morphological evolution are thus absent from evolutionary theory.” 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. It includes multilevel selection, transgenerational epigenetic inheritance, niche construction,

evolvability Evolvability is defined as the capacity of a system for adaptive evolution. Evolvability is the ability of a population of organisms to not merely generate genetic diversity, but to generate ''adaptive'' genetic diversity, and thereby evolve throu ...

, and several concepts from evolutionary developmental biology. Not all biologists have agreed on the need for, or the scope of, an extended synthesis. Many have collaborated on another synthesis in evolutionary developmental biology, which concentrates on developmental

molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...

and

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

to understand how

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...

operated on developmental processes and deep homologies between organisms at the level of highly conserved genes.

The preceding "modern synthesis"

The modern synthesis was the widely accepted early-20th-century synthesis reconciling

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

's theory of evolution by

and

Gregor Mendel Gregor Johann Mendel, OSA (; cs, Řehoř Jan Mendel; 20 July 1822 – 6 January 1884) was a biologist, meteorologist, mathematician, Augustinian friar and abbot of St. Thomas' Abbey in Brünn (''Brno''), Margraviate of Moravia. Mendel was ...

's theory of

genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...

in a joint mathematical framework. It established

biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...

's central paradigm. The 19th-century ideas of

by Darwin and

Mendelian genetics Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later populari ...

were united by researchers who included

Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who ...

, one of the three founders of

population genetics Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as Adaptation (biology), adaptation, ...

, and J. B. S. Haldane and

Sewall Wright Sewall Green Wright FRS(For) Honorary FRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. He was a founder of population genetics alongsi ...

, between 1918 and 1932. Julian Huxley introduced the phrase "modern synthesis" in his 1942 book, '' Evolution: The Modern Synthesis''.

Early history

During the 1950s, the English biologist C. H. Waddington called for an extended synthesis based on his research on

epigenetics In biology, epigenetics is the study of stable phenotypic changes (known as ''marks'') that do not involve alterations in the DNA sequence. The Greek prefix '' epi-'' ( "over, outside of, around") in ''epigenetics'' implies features that are ...

and genetic assimilation. An extended synthesis was also proposed by the Austrian zoologist

Rupert Riedl Rupert Riedl (22 February 1925 – 18 September 2005) was an Austrian zoologist. Biography Riedl was a scientist with broad interests, whose influence in epistemology grounded in evolutionary theory was notable, although less in English-speakin ...

, with the study of

. In 1978, Michael J. D. White wrote about an extension of the modern synthesis based on new research from speciation.

1980s: punctuated equilibrium

In the 1980s, the American palaeontologists Stephen Jay Gould and Niles Eldredge argued for an extended synthesis based on their idea of

, the role of species selection shaping large scale evolutionary patterns and

working on multiple levels extending from genes to species. The ethologist

John Endler John Arthur Endler (born 1947) is a Canadian ethologist and evolutionary biologist noted for his work on the adaptation of vertebrates to their unique perceptual environments, and the ways in which animal sensory capacities and colour patterns ...

wrote a paper in 1988 discussing processes of evolution that he felt had been neglected.

Contributions from evolutionary developmental biology

Some researchers in the field of evolutionary developmental biology proposed another synthesis. They argue that the modern and extended syntheses should mostly center on

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s and suggest an integration of

embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos ...

with

and

, aiming to understand how

operates on

gene regulation Regulation of gene expression, or gene regulation, includes a wide range of mechanisms that are used by cells to increase or decrease the production of specific gene products (protein or RNA). Sophisticated programs of gene expression are wi ...

and deep homologies between organisms at the level of highly conserved genes,

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The f ...

s and signalling pathways. By contrast, a different strand of evo-devo following an organismal approach contributes to the extended synthesis by emphasizing (amongst others) developmental bias (both through facilitation and constraint), evolvability, and inherency of form as primary factors in the evolution of complex structures and phenotypic novelties.

Recent history

The idea of an extended synthesis was relaunched in 2007 by

, and Gerd B. Müller, with a book in 2010 titled ''Evolution: The Extended Synthesis'', which has served as a launching point for work on the extended synthesis. This includes: * The role of prior configurations, genomic structures, and other traits in the organism in generating evolutionary variations. * How increasing dimensionality of

fitness landscapes Fitness may refer to: * Physical fitness, a state of health and well-being of the body * Fitness (biology), an individual's ability to propagate its genes * Fitness (cereal), a brand of breakfast cereals and granola bars * ''Fitness'' (magazine) ...

affects our view of speciation. * The role of multilevel selection in the major evolutionary transitions. * New types of inheritance, including cultural and epigenetic inheritance. * The way that organismal development and developmental plasticity channel evolutionary pathways and generates phenotypic novelty * How organisms modify the environments they belong to through niche construction. Other processes such as

phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompa ...

reticulate evolution Reticulate evolution, or network evolution is the origination of a lineage through the partial merging of two ancestor lineages, leading to relationships better described by a phylogenetic network than a bifurcating tree. Reticulate patterns ca ...

, sex evolution and

symbiogenesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory,) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and pos ...

are said by proponents to have been excluded or missed from the modern synthesis. 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. The biologist

Eugene Koonin Eugene Viktorovich Koonin (Russian: Евге́ний Ви́кторович Ку́нин; born October 26, 1956) is a Russian-American biologist and Senior Investigator at the National Center for Biotechnology Information (NCBI). He is a recognise ...

wrote in 2009 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."

Predictions

The extended synthesis is characterized by its additional set of predictions that differ from the standard modern synthesis theory: # change in

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pr ...

can precede change in genotype # changes in phenotype are predominantly positive, rather than neutral (see:

neutral theory of molecular evolution The neutral theory of molecular evolution holds that most evolutionary changes occur at the molecular level, and most of the variation within and between species are due to random genetic drift of mutant alleles that are selectively neutral. The ...

) # changes in phenotype are induced in many organisms, rather than one organism # revolutionary change in phenotype can occur through mutation,

facilitated variation The theory of facilitated variation demonstrates how seemingly complex biological systems can arise through a limited number of regulatory genetic changes, through the differential re-use of pre-existing developmental components. The theory was p ...

or threshold events # repeated evolution in isolated populations can be by

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

or developmental bias # adaptation can be caused by natural selection, environmental induction, non-genetic inheritance, learning and cultural transmission (see:

Baldwin effect In evolutionary biology, the Baldwin effect, a phenotype-first theory of evolution, describes the effect of learned behaviour on evolution. James Mark Baldwin and others suggested during the eclipse of Darwinism in the late 19th century that an ...

, meme, transgenerational epigenetic inheritance,

ecological inheritance Ecological inheritance is the passing on to descendants of inherited resources and conditions, and associated modified selection pressures, through niche construction. For instance, many organisms build, choose or provision nursery environments, su ...

non-Mendelian inheritance Non-Mendelian inheritance is any pattern in which traits do not segregate in accordance with Mendel's laws. These laws describe the inheritance of traits linked to single genes on chromosomes in the nucleus. In Mendelian inheritance, each paren ...

) # rapid evolution can result from simultaneous induction, natural selection and developmental dynamics # biodiversity can be affected by features of developmental systems such as differences in

# heritable variation is directed towards variants that are adaptive and integrated with phenotype # niche construction is biased towards environmental changes that suit the constructor's phenotype, or that of its descendants, and enhance their fitness #

kin selection Kin selection is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution i ...

# multilevel selection #

self-organization Self-organization, also called spontaneous order in the social sciences, is a process where some form of overall order arises from local interactions between parts of an initially disordered system. The process can be spontaneous when suff ...

Testing

During 2016–2019 the extended evolutionary synthesis was tested by a group of scientists from eight institutions in Britain, Sweden and the United States. The £7.7 million project was supported by a £5.7 million grant from the

John Templeton Foundation The John Templeton Foundation (Templeton Foundation) is a philanthropic organization that reflects the ideas of its founder, John Templeton, who became wealthy via a career as a contrarian investor, and wanted to support progress in religious a ...

. In 2019 a final report of the 2016–2019 consortium was published, ''Putting the Extended Evolutionary Synthesis to the Test''. The project was headed by Kevin N. Laland at the

University of St Andrews (Aien aristeuein) , motto_lang = grc , mottoeng = Ever to ExcelorEver to be the Best , established = , type = Public research university Ancient university , endowment ...

and Tobias Uller at

Lund University , motto = Ad utrumque , mottoeng = Prepared for both , established = , type = Public research university , budget = SEK 9 billion

Status

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. Proponents think that the conceptions of evolution at the core of the modern synthesis are too narrow 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 Denis Noble (born 16 November 1936) is a British biologist who held the Burdon Sanderson Chair of Cardiovascular Physiology at the University of Oxford from 1984 to 2004 and was appointed Professor Emeritus and co-Director of Computational Ph ...

says that using terms and categories of the modern synthesis distorts the picture of biology that modern experimentation has discovered. 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.

References

External links

Extended Evolutionary SynthesisShould Evolutionary Theory Evolve?, By Bob Grant, January 1, 2010
'' The Scientist''. {{evolution Evolution Biology theories History of biology