Facilitated variation
Encyclopedia
Facilitated variation is a new theory that has been presented by Marc W. Kirschner, a professor and chair at the Department of Systems Biology, Harvard Medical School
, and John C. Gerhart, a professor at the Graduate School, University of California, Berkeley
.
The theory of facilitated variation addresses the nature and function of variation
in evolution
. Recent advances in cellular
and developmental biology
in the light of evolutionary biology shed light on a number of mechanisms for generating novelty. While the concept and mechanism of natural selection is well understood, the variation component of the evolutionary theory remains under-developed. Variation mechanisms such as mutations, genetic drift, and genetic flow have been studied and documented thoroughly. The theory of facilitated variation is an effort to illustrate that seemingly complex biological systems can arise with a limited number of genes, and a limited number of variation mechanisms.
This is accomplished by exploring the relation between the genotype and phenotype, specifically:
The theory can be summarized in the following points:
In the classical Darwinian view, a large number of successive mutations, each selected for its usefulness to the survival of the organism, is required to produce novel structures such as wings, limbs, or the brain. Alternatively, facilitated variation asserts that the physiological adaptability of core processes and properties such as weak linkage and exploratory processes enable proteins, cells, and body structures to interact in numerous ways that can lead to the creation of novelty with a limited number of genes, and a limited number of mutations.
Therefore, the role of mutations is often to change how, where, and when the genes are expressed during the development of the embryo and adult.
The theory challenges Irreducible complexity
by explaining how mutation
can cause unusual changes within a species. They explain how the individual organism can change from a passive target of natural selection, to a central player in the 3-billion-year history of evolution. By closing the major gap in Darwin
’s theory Kirschner and Gerhart also provide a scientific rebuttal to modern critics of evolution who champion "intelligent design
".
Harvard Medical School
Harvard Medical School is the graduate medical school of Harvard University. It is located in the Longwood Medical Area of the Mission Hill neighborhood of Boston, Massachusetts....
, and John C. Gerhart, a professor at the Graduate School, University of California, Berkeley
University of California, Berkeley
The University of California, Berkeley , is a teaching and research university established in 1868 and located in Berkeley, California, USA...
.
The theory of facilitated variation addresses the nature and function of variation
Genetic variability
Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another. Variability is different from genetic diversity, which is the amount of variation seen in a particular population. The variability of a trait describes how much that trait tends to...
in evolution
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...
. Recent advances in cellular
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...
and developmental biology
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
in the light of evolutionary biology shed light on a number of mechanisms for generating novelty. While the concept and mechanism of natural selection is well understood, the variation component of the evolutionary theory remains under-developed. Variation mechanisms such as mutations, genetic drift, and genetic flow have been studied and documented thoroughly. The theory of facilitated variation is an effort to illustrate that seemingly complex biological systems can arise with a limited number of genes, and a limited number of variation mechanisms.
This is accomplished by exploring the relation between the genotype and phenotype, specifically:
- How are changes in the genotype translated to changes in the phenotype?
- Can environmental conditions affecting the phenotype affect the genotype?
- Since selection operates on the phenotype, how can physiological adaptability affect selection?
The theory can be summarized in the following points:
- Organisms have 'constrained' and 'deconstrained' variations of their phenotype. The constrained processes remain mostly unchanged but they allow 'deconstrained' processes to alter the phenotype without resulting in lethality.
- The constrained elements are called "conserved core processes" that remain in stasis for perhaps millions of years and they endure little if any changes. These core processes, such as the processing of DNA and RNA, are created in rapid 'invention' periods (e.g. from prokaryotes to Eukaryotes), and are shared by all organisms.
- The 'deconstrained' processes are those that change the amount, time, kind, and place of gene expression.
- The 'conserved core processes' are linked via weak regulatory linkage, which is a loose mechanism for signal transcription. This mechanism facilitates new changes in how genes are expressed, thereby leading to variations in the phenotype which are selected upon. Furthermore, changes and combinations of these weak permissive signals can give rise to new novel functions on the structural and molecular level.
- Exploratory processes have the ability to generate many different phenotypical outcomes or states given a limited number of genes. A few of these states are selected for their physiological adaptability in their respective environmental conditions. For example, the vascular system expands to regions with insufficient oxygen supply. There is no predetermined genetical map for the distribution of blood vessels in the body, but the vascular system responds to signals from hypoxic tissues. Exploratory processes are powerful because they provide organisms with a tremendous adaptation scope.
In the classical Darwinian view, a large number of successive mutations, each selected for its usefulness to the survival of the organism, is required to produce novel structures such as wings, limbs, or the brain. Alternatively, facilitated variation asserts that the physiological adaptability of core processes and properties such as weak linkage and exploratory processes enable proteins, cells, and body structures to interact in numerous ways that can lead to the creation of novelty with a limited number of genes, and a limited number of mutations.
Therefore, the role of mutations is often to change how, where, and when the genes are expressed during the development of the embryo and adult.
The theory challenges Irreducible complexity
Irreducible complexity
Irreducible complexity is an argument by proponents of intelligent design that certain biological systems are too complex to have evolved from simpler, or "less complete" predecessors, through natural selection acting upon a series of advantageous naturally occurring, chance mutations...
by explaining how mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...
can cause unusual changes within a species. They explain how the individual organism can change from a passive target of natural selection, to a central player in the 3-billion-year history of evolution. By closing the major gap in 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...
’s theory Kirschner and Gerhart also provide a scientific rebuttal to modern critics of evolution who champion "intelligent design
Intelligent design
Intelligent design is the proposition that "certain features of the universe and of living things are best explained by an intelligent cause, not an undirected process such as natural selection." It is a form of creationism and a contemporary adaptation of the traditional teleological argument for...
".
Further reading
- Marc W. Kirschner, John C. Gerhart: The Plausibility of Life: Resolving Darwin's Dilemma (Yale University PressYale University PressYale University Press is a book publisher founded in 1908 by George Parmly Day. It became an official department of Yale University in 1961, but remains financially and operationally autonomous....
2005) ISBN 0-300-10865-6, released in 2005.