Norms of reaction
Encyclopedia
In ecology and genetics, a norm of reaction describes the pattern of phenotypic
expression of a single genotype
across a range of environments. One use of norms of reaction is in describing how different species—especially related species—respond to varying environments. But differing genotypes within a single species will also often show differing norms of reaction relative to a particular phenotypic trait and environment variable. For every genotype, phenotypic trait, and environmental variable, a different norm of reaction can exist; in other words, an enormous complexity can exist in the interrelationships between genetic and environmental factors in determining traits. The concept was introduced by Richard Woltereck.
Scientifically analyzing norms of reaction in natural populations can be very difficult, simply because natural populations of sexually reproductive organisms usually do not have cleanly separated or superficially identifiable genetic distinctions. However, seed crops produced by humans are often engineered to contain specific genes, and in some cases seed stocks consist of clones
. Accordingly, distinct seed lines present ideal examples of differentiated norms of reaction. In fact, agricultural companies market seeds for use in particular environments based on exactly this.
Suppose the seed line A contains an allele
a, and a seed line B of the same crop species contains an allele b, for the same gene
. With these controlled genetic groups, we might cultivate each variety (genotype) in a range of environments. This range might be either natural or controlled variations in environment. For example, an individual plant might receive either more or less water during its growth cycle, or the average temperature the plants are exposed to might vary across a range.
A simplification of the norm of reaction might state that seed line A is good for "high water conditions" while a seed line B is good for "low water conditions". But the full complexity of the norm of reaction is a function, for each genotype, relating environmental factor to phenotypic trait. By controlling for or measuring actual environments across which monoclonal seeds are cultivated, one can concretely observe norms of reaction. Normal distributions, for example, are common. Of course, the distributions need not be bell-curves.
, altruism
, or schizophrenia
(see Nature versus nurture
).
Popular conception of genotype/phenotype interaction
TRAIT SCALE
<--6----------5----------4----------3----------2----------1----------0-->
^ (Genotype A) ^ ^ (Genotype B) ^
| | | |
Environ <------> Other Environ <------> Other
extreme extreme extreme extreme
The problem with this common simplified image is not that it does not represent a possible norm of reaction. Rather, by reducing the picture from two dimensions to just one, it focuses only on discrete, non-overlapping phenotypic expressions, and hides the more common pattern local minima and maxima in phenotypic expression, with overlapping ranges of phenotypic expression between genotypes.
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
expression of a single genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...
across a range of environments. One use of norms of reaction is in describing how different species—especially related species—respond to varying environments. But differing genotypes within a single species will also often show differing norms of reaction relative to a particular phenotypic trait and environment variable. For every genotype, phenotypic trait, and environmental variable, a different norm of reaction can exist; in other words, an enormous complexity can exist in the interrelationships between genetic and environmental factors in determining traits. The concept was introduced by Richard Woltereck.
A monoclonal example
Cloning
Cloning in biology is the process of producing similar populations of genetically identical individuals that occurs in nature when organisms such as bacteria, insects or plants reproduce asexually. Cloning in biotechnology refers to processes used to create copies of DNA fragments , cells , or...
. Accordingly, distinct seed lines present ideal examples of differentiated norms of reaction. In fact, agricultural companies market seeds for use in particular environments based on exactly this.
Suppose the seed line A contains an allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
a, and a seed line B of the same crop species contains an allele b, for the same gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
. With these controlled genetic groups, we might cultivate each variety (genotype) in a range of environments. This range might be either natural or controlled variations in environment. For example, an individual plant might receive either more or less water during its growth cycle, or the average temperature the plants are exposed to might vary across a range.
A simplification of the norm of reaction might state that seed line A is good for "high water conditions" while a seed line B is good for "low water conditions". But the full complexity of the norm of reaction is a function, for each genotype, relating environmental factor to phenotypic trait. By controlling for or measuring actual environments across which monoclonal seeds are cultivated, one can concretely observe norms of reaction. Normal distributions, for example, are common. Of course, the distributions need not be bell-curves.
Misunderstanding genetic/environmental interactions
Popular non-scientific or lay-scientific audiences frequently misunderstand or simply fail to recognize the existence of norms of reaction. A widespread conception is that each genotype gives a certain range of possible phenotypic expressions. In popular conception, something which is "more genetic" gives a narrower range, while something which is "less genetic (more environmental)" gives a wider range of phenotypic possibilities. This limited conceptual framework is especially prevalent in discussions of human traits such as IQ, Sexual orientationSexual orientation
Sexual orientation describes a pattern of emotional, romantic, or sexual attractions to the opposite sex, the same sex, both, or neither, and the genders that accompany them. By the convention of organized researchers, these attractions are subsumed under heterosexuality, homosexuality,...
, altruism
Altruism
Altruism is a concern for the welfare of others. It is a traditional virtue in many cultures, and a core aspect of various religious traditions, though the concept of 'others' toward whom concern should be directed can vary among cultures and religions. Altruism is the opposite of...
, or schizophrenia
Schizophrenia
Schizophrenia is a mental disorder characterized by a disintegration of thought processes and of emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid or bizarre delusions, or disorganized speech and thinking, and it is accompanied by significant social...
(see Nature versus nurture
Nature versus nurture
The nature versus nurture debate concerns the relative importance of an individual's innate qualities versus personal experiences The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism, or innatism) versus personal experiences...
).
Popular conception of genotype/phenotype interaction
TRAIT SCALE
<--6----------5----------4----------3----------2----------1----------0-->
^ (Genotype A) ^ ^ (Genotype B) ^
| | | |
Environ <------> Other Environ <------> Other
extreme extreme extreme extreme
The problem with this common simplified image is not that it does not represent a possible norm of reaction. Rather, by reducing the picture from two dimensions to just one, it focuses only on discrete, non-overlapping phenotypic expressions, and hides the more common pattern local minima and maxima in phenotypic expression, with overlapping ranges of phenotypic expression between genotypes.
See also
- Canalisation (genetics)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. In other words, it means robustness. The term canalisation was coined by C. H. Waddington...
- Differential SusceptibilityDifferential susceptibility hypothesisAccording to the differential susceptibility hypothesis by Belsky individuals vary in the degree they are affected by experiences or qualities of the environment they are exposed to...
- Wider reaction norm of females
- Genetic determinismGenetic determinismGenetic determinism is the belief that genes determine morphological and behavioral traits and do so with little or no influence from environmental factors....
- Nature versus nurtureNature versus nurtureThe nature versus nurture debate concerns the relative importance of an individual's innate qualities versus personal experiences The nature versus nurture debate concerns the relative importance of an individual's innate qualities ("nature," i.e. nativism, or innatism) versus personal experiences...
- Phenotypic plasticityPhenotypic plasticityPhenotypic plasticity is the ability of an organism to change its phenotype in response to changes in the environment. Such plasticity in some cases expresses as several highly morphologically distinct results; in other cases, a continuous norm of reaction describes the functional interrelationship...