The extended evolutionary synthesis is a set of extensions of the earlier modern synthesis of evolutionary biology that took place between 1918 and 1942. An extension to the modern 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 then relaunched in 2007 by Massimo Pigliucci. It is part of an on-going robust discussion in biology that involves topics such as group selection and individual selection, the reach of the 'extended phenotype' and the
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, and evolvability.
Not all biologists have agreed on the need for, or the scope of, an extended synthesis. Many have collaborated on a different synthesis in evolutionary developmental biology, which integrates embryology with molecular genetics and evolution to understand developmental processes and deep homologies between organisms at the level of highly conserved genes.
The preceding "modern synthesis"
The so-called '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 on a mathematical basis. It established evolution as a central paradigm in biology. The 19th-century ideas of natural selection by Darwin and Mendelian genetics were united by researchers who included Ronald Fisher, one of the three founders of population genetics, and J. B. S. Haldane and Sewall Wright, between 1918 and 1932. Julian Huxley introduced the phrase "modern synthesis" in his 1942 book, Evolution: The Modern Synthesis.
During the 1950s, the English biologist C. H. Waddington called for an extended synthesis based from his research on epigenetics and genetic assimilation. An extended synthesis was also proposed by the Austrian zoologist Rupert Riedl, with the study of evolvability. 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. This was 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. The ethologist John Endler wrote a paper in 1988 discussing processes of evolution that he felt had been neglected.
A different synthesis: evolutionary developmental biology
Researchers in the field of evolutionary developmental biology created a different extended synthesis. They argued that the modern and extended syntheses were mostly centered on genes and ignored the development of morphology. They collaborated on a different synthesis in evolutionary developmental biology, which integrates embryology with 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 idea of an extended synthesis was relaunched in 2007 by Massimo Pigliucci, 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 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 channels evolutionary pathways.
- How organisms modify the environments they belong to through niche construction.
Other processes such as evolvability, phenotypic plasticity, reticulate evolution, sex evolution and symbiogenesis are said by proponents to have been excluded or missed from the modern synthesis. The goal of Piglucci'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 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."
Pigliucci and colleagues make the following predictions:
- change in phenotype can precede change in genotype
- changes in phenotype are predominantly positive, rather than neutral (see: neutral theory of molecular evolution)
- changes in phenotype are induced in many organisms, rather than one organism
- revolutionary change in phenotype can occur through mutation or facilitated variation
- "repeated evolution in isolated populations may be due to convergent selection and/or developmental bias"
- adaptation occur due to natural selection, environmental induction, non-genetic inheritance, learning and cultural transmission (see: Baldwin effect, meme, transgenerational epigenetic inheritance, ecological inheritance, non-Mendelian inheritance)
- rapid evolution can result from the simultaneous induction and natural selection
- biodiversity can be affected by features of developmental systems (evolvability, constraints)
- heritable variation is directed towards variants that are adaptive and integrated with phenotype
- "niche construction will be systematically biased towards environmental changes that are well suited to the constructor's phenotype, or that of its descendants, and enhance the constructor's, or its descendant's, fitness"
- multilevel selection
- kin selection
Biologists disagree on the need for an extended synthesis. Opponents contend that the modern synthesis is able to fully account for the newer observations, while proponents think that the conceptions of evolution at the core of the modern synthesis are too narrow. Proponents argue 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 distort 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.
The principal focus of dispute with Neo-Darwinists is over the source of variation on which Natural Selection can operate (supporters of the Extended Synthesis deny that these always arise from "random copying errors" in DNA replication). In fact they often assert that development of an improved organism by a non-standard mechanism - even occasionally - would violate Shannon's principles of Information Theory.