In biology, function has been defined in many ways. In physiology, it is simply what an organ, tissue, cell or molecule does. In evolutionary biology, it is the reason some object or process occurred in a system that evolved through natural selection. That reason is typically that it achieves some result, such as that chlorophyll helps to capture the energy of sunlight in photosynthesis. 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; other characteristics may be non-functional spandrels, though these in turn may later be co-opted by evolution to serve new functions.
Biology is the natural science that studies life and living organisms, including their physical structure, chemical processes, molecular interactions, physiological mechanisms, development and evolution. Despite the complexity of the science, there are certain unifying concepts that consolidate it into a single, coherent field. Biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the creation and extinction of species. Living organisms are open systems that survive by transforming energy and decreasing their local entropy to maintain a stable and vital condition defined as homeostasis.
Physiology is the scientific study of the functions and mechanisms which work within a living system.
Evolutionary biology is the subfield of biology that studies the evolutionary processes that produced the diversity of life on Earth, starting from a single common ancestor. These processes include natural selection, common descent, and speciation.
In the philosophy of biology, talk of function inevitably suggests some kind of teleological 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.
The philosophy of biology is a subfield of philosophy of science, which deals with epistemological, metaphysical, and ethical issues in the biological and biomedical sciences. Although philosophers of science and philosophers generally have long been interested in biology, philosophy of biology only emerged as an independent field of philosophy in the 1960s and 1970s. Philosophers of science then began paying increasing attention to biology, from the rise of Neodarwinism in the 1930s and 1940s to the discovery of the structure of DNA in 1953 to more recent advances in genetic engineering. Other key ideas include the reduction of all life processes to biochemical reactions, and the incorporation of psychology into a broader neuroscience.
Teleology or finality is a reason or explanation for something as a function of its end, purpose, or goal. It is derived from two Greek words: telos and logos. A purpose that is imposed by a human use, such as that of a fork, is called extrinsic. Natural teleology, common in classical philosophy but controversial today, contends that natural entities also have intrinsic purposes, irrespective of human use or opinion. For instance, Aristotle claimed that an acorn's intrinsic telos is to become a fully grown oak tree.
Teleology in biology is the use of the language of goal-directedness in accounts of evolutionary adaptation, which some biologists and philosophers of science find problematic. The term teleonomy has also been proposed. Before Darwin, organisms were seen as existing because God had designed and created them; their features such as eyes were taken by natural theology to have been made to enable them to carry out their functions, such as seeing. Evolutionary biologists often use similar teleological formulations that invoke purpose, but these imply natural selection rather than actual goals, whether conscious or not. Dissenting biologists and religious thinkers held that evolution itself was somehow goal-directed (orthogenesis), and in vitalist versions, driven by a purposeful life force. Since such views are now discredited, with evolution working by natural selection acting on inherited variation, the use of teleology in biology has attracted criticism, and attempts have been made to teach students to avoid teleological language.
In physiology, a function is an activity or process carried out by a system in an organism, such as sensation or locomotion in an animal.This concept of function as opposed to form (respectively Aristotle's ergon and morphê ) was central in biological explanations in classical antiquity. In more modern times it formed part of the 1830 Cuvier–Geoffroy debate, 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 biology, an organism is any individual entity that propagates the properties of life. It is a synonym for "life form".
A sense is a physiological capacity of organisms that provides data for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology, and philosophy of perception. The nervous system has a specific sensory nervous system, and a sense organ, or sensor, dedicated to each sense.
Animal locomotion, in ethology, is any of a variety of methods that animals use to move from one place to another. Some modes of locomotion are (initially) self-propelled, e.g., running, swimming, jumping, flying, hopping, soaring and gliding. There are also many animal species that depend on their environment for transportation, a type of mobility called passive locomotion, e.g., sailing, kiting (spiders), rolling or riding other animals (phoresis).
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.
A functional characteristic is known in evolutionary biology as an adaptation, and the research strategy for investigating whether a character is adaptive is known as adaptationism. Although assuming that a character is functional may be helpful in research, some characteristics of organisms are non-functional, formed as accidental spandrels, side effects of neighbouring functional systems.
In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a phenotypic trait or adaptive trait, with a functional role in each individual organism, that is maintained and has evolved through natural selection.
Adaptationism 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 synthesis, that all traits are adaptations, a view now shared by few biologists. Adaptationists perform research to try to distinguish adaptations from byproducts or random variation. George Williams' Adaptation and Natural Selection (1966) was highly influential in its development, defining some of the heuristics used to identify adaptations.
In evolutionary biology, a spandrel is a phenotypic characteristic that is a byproduct of the evolution of some other characteristic, rather than a direct product of adaptive selection. That is, it is a trait that is not particularly advantageous to have, though it is retained because it is not particularly harmful to have.
From the point of view of natural selection, biological functions exist to contribute to fitness, increasing the chance that an organism will survive to reproduce.For example, the function of chlorophyll in a plant is to capture the energy of sunlight for photosynthesis, which contributes to evolutionary success.
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. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which in his view is intentional, whereas natural selection is not.
Fitness is the quantitative representation of natural and sexual selection within evolutionary biology. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by individuals of the specified genotype or phenotype. The fitness of a genotype is manifested through its phenotype, which is also affected by the developmental environment. The fitness of a given phenotype can also be different in different selective environments.
Reproduction is the biological process by which new individual organisms – "offspring" – are produced from their "parents". Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. There are two forms of reproduction: asexual and sexual.
The ethologist Niko Tinbergen named four questions, based on Aristotle's Four Causes,that a biologist could ask to help explain a behaviour, though they have been generalised to a wider scope. 1) Mechanism: What mechanisms cause the animal to behave as it does? 2) Ontogeny: 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 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.
Function is not the same as purpose in the teleological sense, that is, possessing conscious mental intention to achieve a goal. In the philosophy of biology, evolution 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 flowers to attract pollinators' 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 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 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 mutations are initially not selected for, but they do have functions.
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.
Ontogeny is the origination and development of an organism, usually from the time of fertilization of the egg to the organism's mature form—although the term can be used to refer to the study of the entirety of an organism's lifespan.
Sociobiology is a field of biology that aims to examine and explain social behavior in terms of evolution. It draws from disciplines including ethology, anthropology, evolution, zoology, archaeology, and population genetics. Within the study of human societies, sociobiology is closely allied to Darwinian anthropology, human behavioral ecology and evolutionary psychology.
Vestigiality is the retention during the process of sexual reproduction of genetically determined structures or attributes that have lost some or all of their ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous features in related species. The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful, but if the lack of the feature provides no advantage, and its presence provides no disadvantage, the feature may not be phased out by natural selection and persist across species.
Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and behaviour. Bird feathers are a classic example: initially they may have evolved for temperature regulation, but later were adapted for flight. Interest in exaptation relates to both the process and products of evolution: the process that creates complex traits and the products that may be imperfectly developed. Exaptation was proposed by Stephen Jay Gould and Elisabeth Vrba as a replacement for what they considered to be a teleologically loaded term 'pre-adaptation'.
Teleonomy is the quality of apparent purposefulness and goal-directedness of structures and functions in living organisms brought about by the exercise, augmentation, and, improvement of reasoning. The term derives from two Greek words, τέλος telos and νόμος nomos ("law"), and means "end-directed". Teleonomy is sometimes contrasted with teleology, where the latter is understood as a purposeful goal-directedness brought about through human or divine intention. Teleonomy is thought to derive from evolutionary history, adaptation for reproductive success, and/or the operation of a program. Teleonomy is related to programmatic or computational aspects of purpose.
Cultural selection theory is the study of cultural change modelled on theories of evolutionary biology. Cultural selection theory has so far never been a separate discipline. However it has been proposed that human culture exhibits key Darwinian evolutionary properties, and "the structure of a science of cultural evolution should share fundamental features with the structure of the science of biological evolution". In addition to Darwin's work the term historically covers a diverse range of theories from both the sciences and the humanities including those of Lamark, politics and economics e.g. Bagehot, anthropology e.g. Edward B. Tylor, literature e.g. Ferdinand Brunetière, evolutionary ethics e.g. Leslie Stephen, sociology e.g. Albert Keller, anthropology e.g. Bronislaw Malinowski, Biosciences e.g. Alex Mesoudi, geography e.g. Richard Ormrod, sociobiology and biodiversity e.g. E.O. Wilson, computer programming e.g. Richard Brodie, and other fields e.g. Neoevolutionism, and Evolutionary archaeology.
The "four causes" are elements of an influential principle in Aristotelian thought whereby explanations of change or movement are classified into four fundamental types of answer to the question "why?". Aristotle wrote that "we do not have knowledge of a thing until we have grasped its why, that is to say, its cause." While there are cases where identifying a "cause" is difficult, or in which "causes" might merge, Aristotle held that his four "causes" provided an analytical scheme of general applicability.
Human nature is a bundle of characteristics, including ways of thinking, feeling, and acting, which humans are said to have naturally. The term is often regarded as capturing what it is to be human, or the essence of humanity. The term is controversial because it is disputed whether or not such an essence exists. Arguments about human nature have been a mainstay of philosophy for centuries and the concept continues to provoke lively philosophical debate. The concept also continues to play a role in science, with neuroscientists, psychologists and social scientists sometimes claiming that their results have yielded insight into human nature. Human nature is traditionally contrasted with characteristics that vary among humans, such as characteristics associated with specific cultures. Debates about human nature are related to, although not the same as, debates about the comparative importance of genes and environment in development.
Biological or process structuralism is a school of biological thought that objects to an exclusively Darwinian or adaptationist explanation of natural selection such as is described in the 20th century's modern synthesis. It proposes instead that evolution is guided differently, basically by more or less physical forces which shape the development of an animal's body, and sometimes implies that these forces supersede selection altogether.
Tinbergen's four questions, named after Nikolaas Tinbergen and based on Aristotle's four causes, are complementary categories of explanations for behaviour. These are also commonly referred to as levels of analysis. It suggests that an integrative understanding of behaviour must include both a proximate and ultimate (functional) analysis of behaviour, as well as an understanding of both phylogenetic/developmental history and the operation of current mechanisms.
Edward Stuart Russell OBE FLS was a Scottish biologist and philosopher of biology.
The following outline is provided as an overview of and topical guide to evolution:
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Photosynthesis has been instrumental in the success of life on Earth