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 underdeveloped. 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 is in accord with Irreducible complexity by explaining that mutation do not randomly cause 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.
- Charlesworth, Brian (2005). "EVOLUTION: on the Origins of Novelty and Variation". Science 310 (5754): 1619–1620. doi:10.1126/science.1119727.
- Marc W. Kirschner, John C. Gerhart: The Plausibility of Life: Resolving Darwin's Dilemma (Yale University Press 2005) ISBN 0-300-10865-6, released in 2005.
- Gerhart J, Kirschner M (2007). "The theory of facilitated variation". Proc. Natl. Acad. Sci. U.S.A. 104 (suppl_1): 8582–9. doi:10.1073/pnas.0701035104. PMC 1876433. PMID 17494755.