Creode

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Creode or chreod is a neologistic portmanteau term coined by the English 20th century biologist C. H. Waddington to represent the developmental pathway followed by a cell as it grows to form part of a specialized organ. [1] Combining the Greek roots for "necessary" and "path," the term was inspired by the property of regulation. [2] When development is disturbed by external forces, the embryo attempts to regulate its growth and differentiation by returning to its normal developmental trajectory.

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Developmental biology

Waddington used the term along with canalisation and homeorhesis, which describes a system that returns to a steady trajectory, in contrast to homeostasis, which describes a system which returns to a steady state. Waddington explains development with the metaphor of a ball rolling down a hillside, where the hill's contours channel the ball in a particular direction. In the case of a pathway or creode which is deeply carved in the hillside, external disturbance is unlikely to prevent normal development. He notes that creodes tend to have steeper sides earlier in development, when external disturbance rarely suffices to alter the developmental trajectory. [3] Small differences in placement atop the hill can lead to dramatically different results by the time the ball reaches the bottom. This represents the tendency of neighboring regions of the early embryo to develop into different organs with radically different structures. Since intermediate structures rarely exist between organs, each ball that rolls down the hill is "canalised" to a region distinct from other regions, just as an eye, for instance, is distinct from an ear. [4]

Waddington refers to the network of creodes carved into the hillside as an "epigenetic landscape," meaning that the formation of the body depends on not only its genetic makeup but the different ways genes are expressed in different regions of the embryo. [5] He expands his metaphor by describing the underside of the epigenetic landscape. Here we see that the "landscape" is really more like a giant sheet that would blow away except that a series of tension-bearing cables holds it down. The pegs that connect the cables to the ground are the genes. The cables themselves are the epigenetic factors that influence gene expression in various regions of the embryo. The depth and direction of the channels is thus determined by a combination of genetic makeup and the epigenetic feedback loops by which genes are regulated. [6]

While Waddington does assert that the process of development is genetically driven, he makes no attempt to explain how this works and even offers evidence to the contrary. [7] He observes, for instance, that genes ordinarily determine peripheral traits, such as eye color, rather than "focal" traits, such as the structure of the eye itself. Moreover, when genetic mutation influences basic structures, the result tends to be the complete transformation of a structure into another rather than piecemeal change, which Waddington illustrates with the developmental ball rolling out of one creode into another. [8] Thus his account gives the impression that genes influence development, perhaps altering the course of a region of cells, without determining the endpoints toward which the embryo develops.

This interpretation is further reinforced by Waddington's discussion of the organization of the gene pool, where he points out that "the epigenetic process occurring during the development of the organism might be so buffered or canalized that the optimum end-result is produced irrespective of the genes which the individual contains." [9] The more deeply creodes are carved into the epigenetic landscape, the weaker the influence of genes over development. He also argues that deep creodes will resist not only genetic but environmental pressures to change course. This phenomenon, which he calls "stabilizing selection," puts genes and environment on a par in secondary importance compared to the epigenetic system. [10]

Waddington's emphasis on epigenetics over genes prefigured the current interest in evolutionary developmental biology. As Sean B. Carroll and others have explained, genes involved in development are roughly the same in all animal species, from insect to primate. Instead of mutations in developmental genes, evolution has been driven by changes in gene expression, namely which genes are expressed at which times and locations in the developing organism. [11]

Architecture

Architectural theorist Sanford Kwinter described the concept of the chreod as "the most important concept of the 20th century." [12] The word "chreod" also closely describes paths of decision within what Christopher Alexander has called configuration space, his term for what he notes that Stuart Kaufmann calls "fitness landscape." By Alexander's theory, because conscious human design decisions do not need to follow these chreods, conscious human design can lead to mixed results. Therefore, he proposes that discovering ways to allow architecture to follow these paths is the best way to get good results in the built environment. Alexander sees his theories of "The Fundamental Process", "structure preserving transformations" and "15 fundamental properties" which he outlines in his work The Nature of Order as instrumentally shaping paths through configuration space. [13]

See also

Related Research Articles

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Embryo Multicellular diploid eukaryote in its earliest stage of development

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Epigenetics study of heritable DNA and histone modifications that affect the expression of a gene without a change in its nucleotide sequence.

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Evolutionary developmental biology Field of research that compares the developmental processes of different organisms to infer the ancestral relationships

Evolutionary developmental biology is a field of biological research that compares the developmental processes of different organisms to infer the ancestral relationships between them and how developmental processes evolved.

Homology (biology) Shared ancestry between a pair of structures or genes in different taxa

In biology, homology is similarity due to shared ancestry between a pair of structures or genes in different taxa. A common example of homologous structures is the forelimbs of vertebrates, where the wings of bats and birds, the arms of primates, the front flippers of whales and the forelegs of four-legged vertebrates like dogs and crocodiles are all derived from the same ancestral tetrapod structure. Evolutionary biology explains homologous structures adapted to different purposes as the result of descent with modification from a common ancestor. The term was first applied to biology in a non-evolutionary context by the anatomist Richard Owen in 1843. Homology was later explained by Charles Darwin's theory of evolution in 1859, but had been observed before this, from Aristotle onwards, and it was explicitly analysed by Pierre Belon in 1555.

Embryology Branch of biology studying prenatal biology

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<i>Drosophila</i> embryogenesis Embryogenesis of the fruit fly Drosophila, a popular model system

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C. H. Waddington

Conrad Hal Waddington CBE FRS FRSE was a British developmental biologist, paleontologist, geneticist, embryologist and philosopher who laid the foundations for systems biology, epigenetics, and evolutionary developmental biology.

Facilitated variation

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Canalisation (genetics)

Canalisation is a measure of the ability of a population to produce the same phenotype regardless of variability of its environment or genotype. It is a form of evolutionary robustness. The term was coined in 1942 by C. H. Waddington to capture the fact that "developmental reactions, as they occur in organisms submitted to natural selection...are adjusted so as to bring about one definite end-result regardless of minor variations in conditions during the course of the reaction". He used this word rather than robustness to take into account that biological systems are not robust in quite the same way as, for example, engineered systems.

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.

Developmental systems theory (DST) is an overarching theoretical perspective on biological development, heredity, and evolution. It emphasizes the shared contributions of genes, environment, and epigenetic factors on developmental processes. DST, unlike conventional scientific theories, is not directly used to help make predictions for testing experimental results; instead, it is seen as a collection of philosophical, psychological, and scientific models of development and evolution. As a whole, these models argue the inadequacy of the modern evolutionary synthesis on the roles of genes and natural selection as the principle explanation of living structures. Developmental systems theory embraces a large range of positions that expand biological explanations of organismal development and hold modern evolutionary theory as a misconception of the nature of living processes.

Transgenerational epigenetic inheritance

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Rosa Susan Penelope Beddington FRS was a British biologist whose career had a major impact on developmental biology.

von Baer's laws (embryology)

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Probabilistic epigenesis is a way of understanding human behavior based on the relationship between experience and biology. It is a variant form of epigenetics, proposed by American psychologist Gilbert Gottlieb in 1991. Gottlieb’s model is based on Conrad H. Waddington's idea of developmental epigenesis. Both theories examine the complexity of the ways in which the brain develops and explore factors that occur outside the genome. However, probabilistic epigenesis differs from Waddington’s model as it relies much more heavily on the potential developmental impacts of experience and environment and how they interact with an individual’s genes. Probabilistic epigenesis takes into account developmental, hormonal, environmental, neuropsychological, and genetic factors in order to explain various forms of behavior.

In evolutionary biology, developmental bias refers to the production against or towards certain ontogenetic trajectories which ultimately influence the direction and outcome of evolutionary change by affecting the rates, magnitudes, directions and limits of trait evolution. Historically, the term was synonymized with developmental constraint, however, the latter has been more recently interpreted as referring solely to the negative role of development in evolution.

References

  1. ↑ Waddington, pp 19–30
  2. ↑ Waddington, p 32
  3. ↑ Waddington, p 23
  4. ↑ Waddington, p 19
  5. ↑ Waddington, pp 30–33
  6. ↑ Waddington, pp 34–37
  7. ↑ Waddington, p 37
  8. ↑ Waddington, pp 51-52
  9. ↑ Waddington, p 120
  10. ↑ Waddington, p 123
  11. ↑ Sean B Carroll, Endless Forms Most Beautiful, WW Norton & Company, 2005, pp 9, 64–71
  12. ↑ Sandford Kwinter, "Lecture Excerpt: What Is Life?," GSD 08 Platform, Harvard University Graduate School of Design, page 40.
  13. ↑ Christopher Alexander, New Concepts in Complexity Theory Arising from Studies in the Field of Architecture, p 17.

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