This glossary contains a vocabulary used by CPER concerning the overall site, regarding: meaning of scientific words, useful definitions, short explanations of some concepts, and references to reliable external sources of information on the Internet or on paper.
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The term Genotype, will be explained together with the term Phenotype.
The genotype is the genetic makeup of a cell, an organism, or an
The genotype of an organism is the inherited instructions it carries within its genetic code. Not all organisms with the same genotype look or act the same way because appearance and behavior are modified by environmental and developmental conditions. Likewise, not all organisms that look alike necessarily have the same genotype.
A phenotype is the composite of an organism's observable characteristics or traits, such as its morphology, development, biochemical or physiological properties, phenology, behaviour, and products of behaviour (such as a bird's nest). A phenotype results from the expression of an organism's genes as well as the influence of environmental factors and the interactions between the two. When two or more clearly different phenotypes exist in the same population of a species, the species is called polymorph.
Phenotypic variation (due to underlying heritable genetic variation) is a fundamental prerequisite for evolution by natural selection. It is the living organism as a whole that contributes (or not) to the next generation, so natural selection affects the genetic structure of a population indirectly via the contribution of phenotypes. Without phenotypic variation, there would be no evolution by natural selection.
The interaction between genotype and phenotype has often been
conceptualized by the following relationship:
The smallest unit of replicators is the gene. Replicators cannot be directly selected upon, but they are selected on by their phenotypic effects. These effects are packaged together in organisms. We should think of the replicator as having extended phenotypic effects. These are all of the ways it affects the world, not just the effects the replicators have on the body in which they reside.
This genotype-phenotype distinction was proposed by Wilhelm Johannsen in 1911 to make clear the difference between an organism's heredity and what that heredity produces. The distinction is similar to that proposed by August Weismann, who distinguished between germ plasm (heredity) and somatic cells (the body). The genotype-phenotype distinction should not be confused with Francis Crick's central dogma of molecular biology, which is a statement about the directionality of molecular sequential information flowing from DNA to protein, and not the reverse.
Evolution of genetic traits
The genotype–phenotype distinction is drawn in genetics. "Genotype" is an organism's full hereditary information. "Phenotype" is an organism's actual observed properties, such as morphology, development, or behavior. This distinction is fundamental in the study of inheritance of traits and their evolution.
It is the organism's physical properties which directly determine its chances of survival and reproductive output, while the inheritance of physical properties occurs only as a secondary consequence of the inheritance of genes. Therefore, to properly understand the theory of evolution via natural selection, one must understand the genotype–phenotype distinction. The genes contribute to a trait, and the phenotype is the observable expression of the genes (and therefore the genotype that affects the trait). Say a white mouse had both recessive genes that cause the colour of the mouse to be inactive (so "cc"). Its genotype would be responsible for its phenotype (the white colour).
The mapping of a set of genotypes to a set of phenotypes is sometimes referred to as the genotype–phenotype map.
An organism's genotype is a major influencing factor in the development of its phenotype, but it is not the only one. Even two organisms with identical genotypes normally differ in their phenotypes. One experiences this in everyday life with monozygous (i.e. identical) twins. Identical twins share the same genotype, since their genomes are identical; but they never have the same phenotype, although their phenotypes may be very similar. This is apparent in the fact that their mothers and close friends can always tell them apart, even though others might not be able to see the subtle differences. Further, identical twins can be distinguished by their fingerprints, which are never completely identical.
The term Genotype is derived from the ancient Greek word genes (γένος) meaning "born" or "race", and týpos (τύπος), meaning "type".
The term Phenotype is derived from the ancient Greek word phainein / phainō (φαίνω) meaning "to show, to bring to light, make to appear", and typos, meaning "type".
Morphogenesis is the biological process that causes an organism to develop its shape. It is one of three fundamental aspects of developmental biology along with the control of cell growth and cellular differentiation. The process controls the organized spatial distribution of cells during the embryonic development of an organism. Morphogenesis can take place also in a mature organism, in cell culture or inside tumor cell masses.
Morphogenesis also describes the development of unicellular life forms that do not have an embryonic stage in their life cycle, or describes the evolution of a body structure within a taxonomic group. Morphogenetic responses may be induced in organisms by hormones, by environmental chemicals ranging from substances produced by other organisms to toxic chemicals or radionuclides released as pollutants, and other plants, or by mechanical stresses induced by spatial patterning of the cells.
The term Morphogenesis is derived from the ancient Greek words morphê (μορφή) meaning "shape" or "form", and genesis (γένεσις) meaning "creation", "origin", "source", or "birth" thus literally: "beginning of the shape".
Negative Feedback Loop
Negative Feedback Loop
Negative feedback occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other disturbances. Whereas positive feedback tends to lead to instability via exponential growth or oscillation, negative feedback generally promotes stability.
Negative feedback tends to promote a settling to equilibrium, and reduces the effects of perturbations. Negative feedback loops in which just the right amount of correction is applied in the most timely manner can be very stable, accurate, and responsive.
In place of the adjective "Negative" before feedback loop, alternative terms are used, such as: degenerative, self-inhibiting, self-correcting, self-dampening, balancing, discrepancy-reducing, or centripetal.
Negative feedback is widely used in mechanical and electronic engineering, but it also occurs naturally within living organisms, and can be seen in many other fields from chemistry and economics to physical systems such as the climate. General negative feedback systems are studied in control systems engineering.
In organisms, feedback enables various measures (eg body temperature, or blood sugar level) to be maintained within precise desired ranges by homeostatic processes. In many physical and biological systems, qualitatively different influences can oppose each other. For example, in biochemistry, one set of chemicals drives the system in a given direction, whereas another set of chemicals drives it in an opposing direction. If one or both of these opposing influences are non-linear, equilibrium point(s) result.
Figure: Both starting points eventuate in equilibrium, during a negative feedback loop.
Early researchers in the area of cybernetics subsequently generalized the idea of negative feedback to cover any goal-seeking or purposeful behavior. All purposeful behavior may be considered to require negative feed-back. If a goal is to be attained, some signals from the goal are necessary at some time to direct the behavior.
Cybernetics pioneer Norbert Wiener helped to formalize the concepts of feedback control, defining feedback in general as "the chain of the transmission and return of information". Wiener defind negative feedback as the case when: "The information fed back to the control center tends to oppose the departure of the controlled from the controlling quantity...".
Confusion arose after BF Skinner introduced the terms positive and negative reinforcement, both of which can be considered negative feedback mechanisms in the sense that they try to minimize deviations from the desired behavior.
In a similar context, Herold and Greller used the term "negative" to refer to the valence of the feedback: that is, cases where a subject receives an evaluation with an unpleasant emotional connotation.
Differerence in Terminology
In biology, this process (in general, biochemical) is often referred to as homeostasis; whereas in mechanics, the more common term is equilibrium.
In engineering, mathematics and the physical, and biological sciences, common terms for the points around which the system gravitates include: attractors, stable states, eigenstates/eigenfunctions, equilibrium points, and setpoints.
In control theory, negative refers to the sign of the multiplier in mathematical models for feedback. In delta notation, −Δoutput is added to or mixed into the input.
In multivariate systems, vectors help to illustrate how several influences can both partially complement and partially oppose each other.
Some authors, in particular with respect to modelling business systems, use negative to refer to the reduction in difference between the desired and actual behavior of a system.
In a psychology context, on the other hand, negative refers to the valence of the feedback – attractive versus aversive, or praise versus criticism.
Negative versus Positive
Negative feedback is feedback in which the system responds so as
to decrease the magnitude of any particular perturbation, leading
to dampening of the original signal, resulting in stabilization
of the process.
Any system in which there is positive feedback together with a
gain greater than one will result in a runaway situation. Both
positive and negative feedback require a feedback loop to
Fields of application
Examples of the use of negative feedback to control its system are: thermostat control, the phase-locked loop oscillator, the ballcock control of water level, and temperature regulation in animals.
The ballcock or float valve uses negative feedback to control the water level in a cistern of a toilette.
Figure: the ballcock control of water level via negative feedback.
A simple and practical example is a thermostat. When the temperature in a heated room reaches a certain upper limit, the room heating is switched off so that the temperature begins to fall. When the temperature drops to a lower limit, the heating is switched on again. Provided the limits are close to each other, a steady room temperature is maintained. Similar control mechanisms are used in cooling systems, such as an air conditioner, a refrigerator, or a freezer.
Biology and chemistry
Control of endocrine hormones by negative feedback. Some biological systems exhibit negative feedback such as the baroreflex in blood pressure regulation and erythropoiesis. Many biological process (e.g., in human physiology) use negative feedback. Examples of this are numerous, from the regulating of body temperature, to the regulating of blood glucose levels.
The disruption of feedback loops can lead to undesirable results: in the case of blood glucose levels, if negative feedback fails, the glucose levels in the blood may begin to rise dramatically, thus resulting in diabetes.
For hormone secretion regulated by the negative feedback loop:
when gland X releases hormone X, this stimulates target cells to
release hormone Y. When there is an excess of hormone Y, gland X
"senses" this and inhibits its release of hormone X.
Figure: Stress hormone Cortisol dampens its own creation indirectly.
Self-organization is the capability of certain systems "of organizing their own behavior or structure". There are many possible factors contributing to this capacity, and most often positive feedback is identified as a possible contributor. However, negative feedback also can play a role.
In economics, automatic stabilisers are government programs that are intended to work as negative feedback to dampen fluctuations in real GDP. Free market economic theorists claim that the pricing mechanism operated to match supply and demand. However Norbert Wiener wrote in 1948: "There is a belief current in many countries and elevated to the rank of an official article of faith in the United States that free competition is itself a homeostatic process... Unfortunately the evidence, such as it is, is against this simple-minded theory."
Negentropy has also been called: negative entropy, syntropy, extropy or entaxy.
The negentropy of a living system is the entropy that it exports to keep its own entropy low. It lies at the intersection of entropy and life. Negentropy has been used by biologists as the basis for purpose or direction in life, namely cooperative or moral instincts.
The concept and phrase "negative entropy" were introduced by Erwin Schrödinger in his 1944 popular-science book 'What is Life?'. Later, Léon Brillouin shortened the phrase to negentropy, to express it in a more "positive" way of resoning: a living system imports negentropy and stores it. In 1974, Albert Szent-Györgyi proposed replacing the term negentropy with syntropy. That term may have originated in the 1940s with the Italian mathematician Luigi Fantappiè, who tried to construct a unified theory of biology and physics. Buckminster Fuller tried to popularize this usage, but negentropy remains common.
In 2009, Mahulikar & Herwig redefined negentropy of a dynamically ordered sub-system as the specific entropy deficit of the ordered sub-system relative to its surrounding chaos. Thus, negentropy has units [J/kg-K] when defined based on specific entropy per unit mass, and [K−1] when defined based on specific entropy per unit energy. This definition enabled: i) scale-invariant thermodynamic representation of dynamic order existence, ii) formulation of physical principles exclusively for dynamic order existence and evolution, and iii) mathematical interpretation of Schrödinger's negentropy debt.
The term Negentropy is not only used in physics and biology, but also in other domains, such as Information Theory, Statistics, Organization management, though with a slightly different meaning, for example: In Risk Management, negentropy is the force that seeks to achieve effective organizational behavior and lead to a steady predictable state.
Extropy is a concept that life will continue to expand throughout the universe as a result of human intelligence and technology.
Ontogenesis is also called: Ontogeny.
The process during which personality and sexual behavior of an individual person mature through a series of stages (Psychosexual development).
Compare with: Phylogenesis , Palingenesis .
Phylogeny relates to genetic development of a species during evolution, while Ontogeny relates to physiological (and psychological) development of an individual during his / her lifetime. Phylogenesis: long-term time scale. Ontogenesis: short-term time scale.
The term Ontogenesis is derived from the ancient Greek
words ontos (ὄντος), meaning "being", plus the term
genesis (γένεσις) meaning "origin", "source", or
In biology, Palingenesis (or palingenesia) is also called Recapitulation.
CPER uses this biological meaning.
Compare with: Ontogenesis , Phylogenesis .
Phylogeny relates to genetic development of a species during evolution, while Ontogeny relates to physiological (and psychological) development of an individual during his / her lifetime. Phylogenesis: long-term time scale. Ontogenesis: short-term time scale.
Palingenesis assumes that a fetus undergoes a condensed reflection of genetic evolutional traits (cf. phylogeny), during the embryonic development of this organism (cf. ontogeny).
The term Palingenesis is derived from the ancient Greek word palingenesia (παλιγγενεσία) that exists of the terms palin (πάλιν), meaning "again", plus the term genesis (γένεσις) meaning "origin", "source", or "birth".
Phenology is the study of periodic plant and animal life cycle events and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation). Phenology has been principally concerned with the dates of first occurrence of biological events in their annual cycle. Examples include the date of emergence of leaves and flowers, the first flight of butterflies and the first appearance of migratory birds, the date of leaf colouring and fall in deciduous trees, the dates of egg-laying of birds and amphibia, or the timing of the developmental cycles of temperate-zone honey bee colonies. In the scientific literature on ecology, the term is used more generally to indicate the time frame for any seasonal biological phenomena, including the dates of last appearance (e.g.: the seasonal phenology of a species may be from April through September).
Because many such phenomena are very sensitive to small variations in climate, especially to temperature, phenological records can be a useful proxy for temperature in historical climatology, especially in the study of climate change and global warming. For example, viticultural records of grape harvests in Europe have been used to reconstruct a record of summer growing season temperatures going back more than 500 years. In addition to providing a longer historical baseline than instrumental measurements, phenological observations provide high temporal resolution of on-going changes related to global warming.
The term Phenology is derived from the Greek phainō (φαίνω), meaning "to show, to bring to light, make to appear" + logos (λόγος), which has many meanings, such as "word, study, discourse, reasoning".
Phylogenesis is also called: Phylogeny.
Compare with: Ontogenesis , Palingenesis .
Phylogeny relates to genetic development of a species during
evolution, while Ontogeny relates to physiological (and
psychological) development of an individual during his / her
The term Phylogenesis is derived from the
words phylé (φυλή) and phulon (φῦλον), meaning
"tribe", "clan", or "race", plus the term genesis
(γένεσις) meaning "origin", "source", or
Positive Feedback Loop
Positive Feedback Loop
Positive feedback is a process that occurs in a feedback loop in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, mechanism A produces more of B which in turn produces more of A. In contrast, a system in which the results of a change act to reduce or counteract it, has negative feedback (see: Negative Feedback Loop).
Figure: Positive feedback loop.
Mathematically, positive feedback is defined as a positive loop gain around a closed loop of cause and effect. That is, positive feedback is in phase with the input, in the sense that it adds to make the input larger. Positive feedback tends to cause system instability. When the loop gain is positive and above 1, there will typically be exponential growth, increasing oscillations or divergences from equilibrium. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system latched into a new stable state. Positive feedback may be controlled by signals in the system being filtered, damped, or limited, or it can be cancelled or reduced by adding negative feedback.
Positive feedback is used in digital electronics to force voltages away from intermediate voltages into '0' and '1' states. On the other hand, thermal runaway is a positive feedback that can destroy semiconductor junctions. Positive feedback in chemical reactions can increase the rate of reactions, and in some cases can lead to explosions. Positive feedback in mechanical design causes tipping-point, or 'over-centre', mechanisms to snap into position, for example in switches and locking pliers. Out of control, it can cause bridges to collapse. Positive feedback in economic systems can cause boom-then-bust cycles. A familiar example of positive feedback is the loud squealing or howling sound produced by audio feedback in public address systems: the microphone picks up sound from its own loudspeakers, amplifies it, and sends it through the speakers again.
In place of the adjective "Positive", alternative terms are used, such as: regenerative, self-stimulating, self-reinforcing, self-amplifying, discrepancy-increasing, or centrifugal.
Figure: Possible results of a Positive feedback loop.
Positive feedback enhances or amplifies an effect by it having an influence on the process which gave rise to it. For example, when part of an electronic output signal returns to the input, and is in phase with it, the system gain is increased. The feedback from the outcome to the originating process can be direct, or it can be via other state variables. Such systems can give rich qualitative behaviours, but whether the feedback is instantaneously positive or negative in sign has an extremely important influence on the results. Positive feedback reinforces and negative feedback moderates the original process. Positive and negative in this sense refer to loop gains greater than or less than zero, and do not imply any value judgements as to the desirability of the outcomes or effects. A key feature of positive feedback is thus that small disturbances get bigger. When a change occurs in a system, positive feedback causes further change, in the same direction.
If the loop gain AB is positive, then a condition of positive or regenerative feedback exists. Thus depending on the feedback, state changes can be convergent, or divergent. The result of positive feedback is to augment changes, so that small perturbations may result in big changes.
In the real world, positive feedback loops typically do not cause ever-increasing growth, but are modified by limiting effects of some sort. According to Donella Meadows: "Positive feedback loops are sources of growth, explosion, erosion, and collapse in systems. A system with an unchecked positive loop ultimately will destroy itself. That’s why there are so few of them. Usually a negative loop will kick in sooner or later."
Fields of application
Positive feedback is a mechanism by which an output is enhanced, such as protein levels. However, in order to avoid any fluctuation in the protein level, the mechanism is inhibited stochastically (I). Therefore, when the concentration of the activated protein (A) is past the threshold ([I]), the loop mechanism is activated and the concentration of A increases exponentially if d[A]=k [A] .
A number of examples of positive feedback systems may be found in physiology. One example is the onset of contractions in childbirth, known as the Ferguson reflex. When a contraction occurs, the hormone oxytocin causes a nerve stimulus, which stimulates the hypothalamus to produce more oxytocin, which increases uterine contractions. This results in contractions increasing in amplitude and frequency.
Another example is the process of blood clotting. The loop is initiated when injured tissue releases signal chemicals that activate platelets in the blood. An activated platelet releases chemicals to activate more platelets, causing a rapid cascade and the formation of a blood clot.
Lactation also involves positive feedback in that as the baby suckles on the nipple there is a nerve response into the spinal cord and up into the hypothalamus of the brain. The hypothalamus then stimulates the pituitary gland to produce more prolactin to produce more milk.
A spike in estrogen during the follicular phase of the menstrual cycle causes ovulation.
The generation of nerve signals is another example, in which the membrane of a nerve fibre causes slight leakage of sodium ions through sodium channels. This results in a change in the membrane potential, which in turn causes more opening of channels, and so on. So a slight initial leakage results in an explosion of sodium leakage which creates the nerve action potential.
In excitation–contraction coupling of the heart, an increase in intracellular calcium ions to the cardiac myocyte is detected by ryanodine receptors in the membrane of the sarcoplasmic reticulum. These ryanodine receptors transport calcium out into the cytosol in a positive feedback physiological response.
In most cases, such feedback loops culminate in counter-signals being released that suppress or breaks the loop. Childbirth contractions stop when the baby is out of the mother's body. Chemicals break down the blood clot. Lactation stops when the baby no longer nurses.
Positive feedback is a well studied phenomenon in gene regulation, where it is most often associated with bistability. Positive feedback occurs when a gene activates itself directly or indirectly via a double negative feedback loop. Genetic engineers have constructed and tested simple positive feedback networks in bacteria to demonstrate the concept of bistability. A classic example of positive feedback is the lac operon in Escherichia coli. Positive feedback plays an integral role in cellular differentiation, development, and cancer progression, and therefore, positive feedback in gene regulation can have significant physiological consequences. Random motions in molecular dynamics coupled with positive feedback can trigger interesting effects, such as create population of phenotypically different cells from the same parent cell. This happens because noise can become amplified by positive feedback. Positive feedback can also occur in other forms of cell signaling, such as enzyme kinetics or metabolic pathways.
Positive feedback loops have been used to describe aspects of the dynamics of change in biological evolution. For example, beginning at the macro level, Alfred J. Lotka (1945) argued that the evolution of the species was most essentially a matter of selection that fed back energy flows to capture more and more energy for use by living systems. At the human level, Richard Alexander (1989) proposed that social competition between and within human groups fed back to the selection of intelligence thus constantly producing more and more refined human intelligence. Crespi (2004) discussed several other examples of positive feedback loops in evolution. The analogy of Evolutionary arms races provide further examples of positive feedback in biological systems.
During the Phanerozoic the biodiversity shows a steady but not monotonic increase from near zero to several thousands of genera. It has been shown that changes in biodiversity through the Phanerozoic correlate much better with hyperbolic model (widely used in demography and macrosociology) than with exponential and logistic models (traditionally used in population biology and extensively applied to fossil biodiversity as well). The latter models imply that changes in diversity are guided by a first-order positive feedback (more ancestors, more descendants) and/or a negative feedback arising from resource limitation. Hyperbolic model implies a second-order positive feedback. The hyperbolic pattern of the world population growth has been demonstrated (see below) to arise from a second-order positive feedback between the population size and the rate of technological growth. The hyperbolic character of biodiversity growth can be similarly accounted for by a positive feedback between the diversity and community structure complexity. It has been suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the interference of the hyperbolic trend (produced by the positive feedback) with cyclical and stochastic dynamics.
A cytokine storm, or hypercytokinemia is a potentially fatal immune reaction consisting of a positive feedback loop between cytokines and immune cells, with highly elevated levels of various cytokines. In normal immune function, positive feedback loops can be utilized to enhance the action of B-lymphocytes. When a B-cell binds its antibodies to an antigen and becomes activated, it begins releasing antibodies and secreting a complement protein called C3. Both C3 and a B-cell's antibodies can bind to a pathogen, and when a B-cell has its antibodies bind to a pathogen with C3, it speeds up that B-cell's secretion of more antibodies and more C3, thus creating a positive feedback loop.
In psychology, the body receives a stimulus from the environment
or internally that causes the release of hormones. Release of
hormones then may cause more of those hormones to be released,
causing a positive feedback loop. This cycle is also found in
certain behaviour. For example, "shame loops" occur in people who
blush easily. When they realize that they are blushing, they
become even more embarrassed, which leads to further blushing,
and so on.
Winner (1996) described gifted children as driven by positive feedback loops involving setting their own learning course, this feeding back satisfaction, thus further setting their learning goals to higher levels and so on. Winner termed this positive feedback loop as a "rage to master."
Vandervert (2009) proposed that the child prodigy can be explained in terms of a positive feedback loop between the output of thinking/performing in working memory, which then is fed to the cerebellum where it is streamlined, and then fed back to working memory thus steadily increasing the quantitative and qualitative output of working memory. Vandervert also argued that this working memory/cerebellar positive feedback loop was responsible for language evolution in working memory.
In substance dependence a human seeks the effects of a drug, and the drug supplies an effect. The human thereafter continues to seek the effects from the drug. In time the body acclimates to the dosage of the drug, and finds a new homeostasis. The human then must consume a larger quantity of the drug to feel the effects the subject wants, a drug overdose may occur when seeking this new threshold of drug effect. If an accidental overdose doesn't kill the human, eventually the human body can no longer repair itself from the damage (ex. Kidney failure and Liver failure) and death is the final result to this positive feedback.
According to the theory of reflexivity advanced by George Soros, price changes are driven by a positive feedback process whereby investors' expectations are influenced by price movements so their behaviour acts to reinforce movement in that direction until it becomes unsustainable, whereupon the feedback drives prices in the opposite direction.
Systemic risk is the risk that an amplification or leverage or positive feedback process is built into a system. This is usually unknown, and under certain conditions this process can amplify exponentially and rapidly lead to destructive or chaotic behavior. A Ponzi scheme is a good example of a positive-feedback system: funds from new investors are used to pay out unusually high returns, which in turn attract more new investors, causing rapid growth toward collapse. W. Brian Arthur has also studied and written on positive feedback in the economy (e.g. W. Brian Arthur, 1990). Hyman Minsky proposed a theory that certain credit expansion practices could make a market economy into "a deviation amplifying system" that could suddenly collapse, sometimes called a "Minsky moment". Simple systems that clearly separate the inputs from the outputs are not prone to systemic risk. This risk is more likely as the complexity of the system increases, because it becomes more difficult to see or analyze all the possible combinations of variables in the system even under careful stress testing conditions. The more efficient a complex system is, the more likely it is to be prone to systemic risks, because it takes only a small amount of deviation to disrupt the system. Therefore well-designed complex systems generally have built-in features to avoid this condition, such as a small amount of friction, or resistance, or inertia, or time delay to decouple the outputs from the inputs within the system. These factors amount to an inefficiency, but they are necessary to avoid instabilities. The 2010 Flash Crash incident was blamed on the practice of high-frequency trading (HFT), although whether HFT really increases systemic risk remains controversial.
Human population growth
Agriculture and human population can be considered to be in a positive feedback mode, which means that one drives the other with increasing intensity. It is suggested that this positive feedback system will end sometime with a catastrophe, as modern agriculture is using up all of the easily available phosphate and is resorting to highly efficient monocultures which are more susceptible to systemic risk. Technological innovation and human population can be similarly considered, and this has been offered as an explanation for the apparent hyperbolic growth of the human population in the past, instead of a simpler exponential growth. It is proposed that the growth rate is accelerating because of second-order positive feedback between population and technology. Technological growth increases the carrying capacity of land for people, which leads to more population, and so more potential inventors in further technological growth.
Prejudice, social institutions and poverty Gunnar Myrdal described a vicious circle of increasing inequalities, and poverty, which is known as "circular cumulative causation".
Drought intensifies through positive feedback. A lack of rain decreases soil moisture, which kills plants and/or causes them to release less water through transpiration. Both factors limit evapotranspiration, the process by which water vapor is added to the atmosphere from the surface, and add dry dust to the atmosphere, which absorbs water. Less water vapor means both low dew point temperatures and more efficient daytime heating, decreasing the chances of humidity in the atmosphere leading to cloud formation. Lastly, without clouds, there cannot be rain, and the loop is complete.
The climate system is characterized by strong positive and
negative feedback loops between processes that affect the state
of the atmosphere, ocean, and land.
The Intergovernmental Panel on Climate Change
IPCC's Fourth Assessment Report states that "Anthropogenic warming could lead to some effects that are abrupt or irreversible, depending upon the rate and magnitude of the climate change."
A self-fulfilling prophecy is a social positive feedback loop between beliefs and behaviour: if enough people believe that something is true, their behaviour can make it true, and observations of their behaviour may in turn increase belief. A classic example is a bank run.
Another sociological example of positive feedback is the network effect. When more people are encouraged to join a network this increases the reach of the network therefore the network expands ever more quickly. A viral video is an example of the network effect in which links to a popular video are shared and redistributed, ensuring that more people see the video and then re-publish the links. This is the basis for many social phenomena, including Ponzi schemes and chain letters. In many cases population size is the limiting factor to the feedback effect.
On the Internet
Internet recommendation systems are expected to increase the diversity of what we see and do online. They help us discover new content and websites among myriad choices. Some recommendation systems, however, unintentionally do the opposite. Because some recommendation systems (i.e. certain collaborative filters) recommend products based on past sales or ratings, they cannot usually recommend products with limited historical data. This can create positive feedback: a rich-get-richer effect for popular products. This bias toward popularity can prevent what are otherwise better recommendations for that user's preferences. A Wharton study details this phenomenon along with several ideas that may promote diversity.
If a chemical reaction causes the release of heat, and the reaction itself happens faster at higher temperatures, then there is a high likelihood of positive feedback. If the heat produced is not removed from the reactants fast enough, thermal runaway can occur and very quickly lead to a chemical explosion.