Fisher's geometric model (FGM) is an

evolution Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...

ary model of the

effect size In statistics, an effect size is a value measuring the strength of the relationship between two variables in a population, or a sample-based estimate of that quantity. It can refer to the value of a statistic calculated from a sample of data, the ...

s and effect on fitness of spontaneous

mutations In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mitos ...

proposed by

Ronald Fisher Sir Ronald Aylmer Fisher (17 February 1890 – 29 July 1962) was a British polymath who was active as a mathematician, statistician, biologist, geneticist, and academic. For his work in statistics, he has been described as "a genius who ...

to explain the distribution of effects of mutations that could contribute to adaptative evolution.

Conceptualization

Sometimes referred to as the Fisher–Orr model, Fisher's model addresses the problem of adaptation (and, to some extent,

complexity Complexity characterises the behaviour of a system or model whose components interact in multiple ways and follow local rules, leading to nonlinearity, randomness, collective dynamics, hierarchy, and emergence. The term is generally used to ch ...

), and continues to be a point of reference in contemporary research on the genetic and evolutionary consequences of

pleiotropy Pleiotropy (from Greek , 'more', and , 'way') occurs when one gene influences two or more seemingly unrelated phenotypic traits. Such a gene that exhibits multiple phenotypic expression is called a pleiotropic gene. Mutation in a pleiotropic ...

. The model has two forms, a geometric formalism, and a microscope analogy. A microscope which has many knobs to adjust the lenses to obtain a sharp image has little chance of obtaining an optimally functioning image by randomly turning the knobs. The chances of a clear image are not so bad if the number of knobs is low, but the chances will decrease dramatically if the number of adjustable

parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...

s (knobs) is larger than two or three. Fisher introduced a geometric metaphor, which eventually became known as Fisher's geometric model. In his model, Fisher argues that the functioning of the microscope is analogous to the fitness of an organism. The performance of the microscope depends on the state of various knobs that can be manipulated, corresponding to distances and orientations of various lenses, whereas the fitness of an organism depends on the state of various

phenotypic In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological pro ...

character such as body size and

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for foo ...

length and depth. The increase in the fitness of an organism by random changes is then analogous to the attempt to improve the performance of a microscope through randomly changing the positions of the knobs on the microscope. The analogy between the microscope and an evolving organism can be formalized by representing the phenotype of an organism as a point in a high-dimensional data space, where the dimensions of that space correspond to the traits of the organism. The more independent dimensions of variation the phenotype has, the more difficult is improvement resulting from random changes. If there are many different ways to change a phenotype it becomes very unlikely that a random change affects the right combination of traits in the right way to improve fitness. Fisher noted that the smaller the effect, the higher the chance that a change is beneficial. At one extreme, changes with infinitesimally small effect have a 50% chance of improving fitness. This argument led to the widely held position that evolution proceeds by small mutations. Furthermore, Orr discovered that both the fixation probability of a beneficial mutation and the fitness gain that is conferred by the fixation of the beneficial mutation decrease with organismal complexity. Thus, the predicted rate of adaptation decreases quickly with the rise in organismal complexity, a theoretical finding known as the ‘cost of complexity’.

References

{{Reflist Genetics Evolutionary biology