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In
evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
, function is the reason some object or process occurred in a system that
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
through
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
. That reason is typically that it achieves some result, such as that
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
helps to capture the energy of sunlight in
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
. Hence, the organism that contains it is more likely to survive and reproduce, in other words the function increases the organism's fitness. A characteristic that assists in evolution is called an
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
; other characteristics may be non-functional
spandrels A spandrel is a roughly triangular space, usually found in pairs, between the top of an arch and a rectangular frame; between the tops of two adjacent arches or one of the four spaces between a circle within a square. They are frequently fill ...
, though these in turn may later be co-opted by evolution to serve new functions. In
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...
, function has been defined in many ways. In
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
, it is simply what an organ, tissue, cell or molecule does. In the philosophy of biology, talk of function inevitably suggests some kind of
teleological Teleology (from and )Partridge, Eric. 1977''Origins: A Short Etymological Dictionary of Modern English'' London: Routledge, p. 4187. or finalityDubray, Charles. 2020 912Teleology" In ''The Catholic Encyclopedia'' 14. New York: Robert Appleton ...
purpose, even though natural selection operates without any goal for the future. All the same, biologists often use teleological language as a shorthand for function. In contemporary philosophy of biology, there are three major accounts of function in the biological world: theories of causal role, selected effect, and goal contribution.


In pre-evolutionary biology

In
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
, a function is an activity or process carried out by a system in an
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...
, such as
sensation Sensation (psychology) refers to the processing of the senses by the sensory system. Sensation or sensations may also refer to: In arts and entertainment In literature * Sensation (fiction), a fiction writing mode * Sensation novel, a Briti ...
or
locomotion Locomotion means the act or ability of something to transport or move itself from place to place. Locomotion may refer to: Motion * Motion (physics) * Robot locomotion, of man-made devices By environment * Aquatic locomotion * Flight * Locomo ...
in an animal. This concept of function as opposed to
form Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens. Form also refers to: *Form (document), a document (printed or electronic) with spaces in which to write or enter data * ...
(respectively Aristotle's ''ergon'' and ''morphê'') was central in biological explanations in
classical antiquity Classical antiquity (also the classical era, classical period or classical age) is the period of cultural history between the 8th century BC and the 5th century AD centred on the Mediterranean Sea, comprising the interlocking civilizations of ...
. In more modern times it formed part of the 1830
Cuvier–Geoffroy debate The Cuvier–Geoffroy debate of 1830 was a scientific debate between the two French naturalists Georges Cuvier and Étienne Geoffroy Saint-Hilaire.Racine, Valerie, "Essay: The Cuvier-Geoffroy Debate". Embryo Project Encyclopedia (2013-10-07). http:/ ...
, where Cuvier argued that an animal's structure was driven by its functional needs, while Geoffroy proposed that each animal's structure was modified from a common plan.


In evolutionary biology

Function can be defined in a variety of ways, including as adaptation, as contributing to evolutionary fitness, in animal behaviour, and, as discussed below, also as some kind of causal role or goal in the philosophy of biology.


Adaptation

A functional characteristic is known in
evolutionary biology Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life ...
as an
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the po ...
, and the research strategy for investigating whether a character is adaptive is known as
adaptationism Adaptationism (also known as functionalism) is the Darwinian view that many physical and psychological traits of organisms are evolved adaptations. Pan-adaptationism is the strong form of this, deriving from the early 20th century modern synthesi ...
. Although assuming that a
character Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...
is functional may be helpful in research, some characteristics of organisms are non-functional, formed as accidental
spandrels A spandrel is a roughly triangular space, usually found in pairs, between the top of an arch and a rectangular frame; between the tops of two adjacent arches or one of the four spaces between a circle within a square. They are frequently fill ...
, side effects of neighbouring functional systems.


Natural selection

From the point of view of
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
, biological functions exist to contribute to fitness, increasing the chance that an organism will survive to reproduce. For example, the function of
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...
in a plant is to capture the energy of sunlight for
photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
, which contributes to evolutionary success.


In ethology

The
ethologist Ethology is the scientific study of animal behaviour, usually with a focus on behaviour under natural conditions, and viewing behaviour as an evolutionarily adaptive trait. Behaviourism as a term also describes the scientific and objectiv ...
Niko Tinbergen named four questions, based on
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
's
Four Causes The four causes or four explanations are, in Aristotelian thought, four fundamental types of answer to the question "why?", in analysis of change or movement in nature: the material, the formal, the efficient, and the final. Aristotle wrote th ...
, that a biologist could ask to help explain a
behaviour Behavior (American English) or behaviour (British English) is the range of actions and mannerisms made by individuals, organisms, systems or artificial entities in some environment. These systems can include other systems or organisms as wel ...
, though they have been generalised to a wider scope. 1) Mechanism: What mechanisms cause the animal to behave as it does? 2)
Ontogeny Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the s ...
: What developmental mechanisms in the animal's embryology (and its youth, if it learns) created the structures that cause the behaviour? 3) Function/adaptation: What is the evolutionary function of the behaviour? 4) Evolution: What is the
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological s ...
of the behaviour, or in other words, when did it first appear in the evolutionary history of the animal? The questions are interdependent, so that, for example, adaptive function is constrained by embryonic development.


In philosophy of biology

Function is not the same as purpose in the
teleological Teleology (from and )Partridge, Eric. 1977''Origins: A Short Etymological Dictionary of Modern English'' London: Routledge, p. 4187. or finalityDubray, Charles. 2020 912Teleology" In ''The Catholic Encyclopedia'' 14. New York: Robert Appleton ...
sense, that is, possessing conscious mental intention to achieve a goal. In the philosophy of biology,
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
is a blind process which has no 'goal' for the future. For example, a tree does not grow flowers for any purpose, but does so simply because it has evolved to do so. To say 'a tree grows
flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechanis ...
s to attract
pollinator A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains. Insects are the m ...
s' would be incorrect if the 'to' implies purpose. A function describes what something ''does'', not what its 'purpose' is. However, teleological language is often used by biologists as a shorthand way of describing function, even though its applicability is disputed. In contemporary philosophy of biology, there are three major accounts of function in the biological world: theories of causal role, selected effect, and goal contribution.


Causal role

Causal role theories of biological function trace their origin back to a 1975 paper by Robert Cummins. Cummins defines the functional role of a component of a system to be the causal effect that the component has on the larger containing system. For example, the heart has the actual causal role of pumping blood in the circulatory system; therefore, the function of the heart is to pump blood. This account has been objected to on the grounds that it is too loose a notion of function. For example, the heart also has the causal effect of producing a sound, but we would not consider producing sound to be the function of the heart.


Selected effect

Selected effect theories of biological functions hold that the function of a biological trait is the function that the trait was selected for, as argued by Ruth Millikan. For example, the function of the heart is pumping blood, for that is the action for which the heart was selected for by evolution. In other words, pumping blood is ''the reason'' that the heart has evolved. This account has been criticized for being too restrictive a notion of function. It is not always clear which behavior has contributed to the selection of a trait, as biological traits can have functions, even if they have not been selected for. Beneficial
mutation 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, m ...
s are initially not selected for, but they do have functions.


Goal contribution

Goal contribution theories seek to carve a middle ground between causal role and selected effect theories, as with Boorse (1977). Boorse defines the function of a biological trait to be the statistically typical causal contribution of that trait to survival and reproduction. So for example, zebra stripes were sometimes said to work by confusing predators. This role of zebra stripes would contribute to the survival and reproduction of zebras, and that is why confusing predators would be said to be the function of zebra stripes. Under this account, whether or not a particular causal role of a trait is its function depends on whether that causal role contributes to the survival and reproduction of that organism.


See also

* Preadaptation


References

{{DEFAULTSORT:Function (Biology) Biology terminology