A function is part of an answer to a question about why some object or process occurred in a system that evolved through a process of selection

Selection

In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...

. Thus, function refers forward from the object or process, along some chain of causation

Causation

Causation may refer to:* Causation , a key component to establish liability in both criminal and civil law* Causation in English law defines the requirement for liability in negligence...

, to the goal or success. Compare this to the mechanism

Mechanism (biology)

In biology --and in science in general-- a mechanism is a complex object or, more generally, a process that produces a regular phenomenon. For example, natural selection is one of the mechanisms of biological evolution, other being genetic drift, biased mutation, and gene flow; competition,...

 of the object or process, which looks backward along some chain of causation, explaining how the feature occurred.

A functional characteristic is known as an adaptation

Adaptation

An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

, and the research strategy for investigating whether a character is adaptive is known as adaptationism

Adaptationism

Adaptationism is a set of methods in the evolutionary sciences for distinguishing the products of adaptation from traits that arise through other processes. It is employed in fields such as ethology and evolutionary psychology that are concerned with identifying adaptations...

. Although an assumption that a character is functional may be fruitful as a research method, some characteristics of organisms are non-functional, and may simply be emergent phenomena arising as a side effect of functional systems (see Spandrel (biology)

Spandrel (biology)

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.-Origin of Term:...

). They may also have a reduced function or lost function entirely over time due to changing conditions, in which case they are said to be vestigial.

The correlation of form (also referred to as structure) and function is one of the central themes in biology

Biology

Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

. However, knowing the structure of something does not necessarily reveal its function. To illustrate the point, an enzymologist discovering an unknown enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 would be highly unlikely to identify its function with only structural

Protein structure

Proteins are an important class of biological macromolecules present in all organisms. Proteins are polymers of amino acids. Classified by their physical size, proteins are nanoparticles . Each protein polymer – also known as a polypeptide – consists of a sequence formed from 20 possible L-α-amino...

 information. It is also possible for different structures to carry out the same function. Teeth and the gizzard

Gizzard

The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

 of a bird highlight this point: both grind up food, serving the same function in terms of digestion

Digestion

Digestion is the mechanical and chemical breakdown of food into smaller components that are more easily absorbed into a blood stream, for instance. Digestion is a form of catabolism: a breakdown of large food molecules to smaller ones....

.

As another example, the function of chlorophyll

Chlorophyll

Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

 in a plant is to capture the energy of sunlight for photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, which contributes to growth and reproduction, and reproduction contributes to evolutionary success. Taking a broader view, all functions have the same end in that they contribute to fitness

Fitness (biology)

Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

, though they are each manifested through various proximate

Proximate causation

In philosophy a proximate cause is an event which is closest to, or immediately responsible for causing, some observed result. This exists in contrast to a higher-level ultimate cause which is usually thought of as the "real" reason something occurred.* Example: Why did the ship sink?** Proximate...

 mechanisms.

Function is not the same as purpose

Purpose

Purpose is a word meaning an object to be reached; a target; an aim; a goal; a result that is desired; an intention. Purpose's basic concept is an individual's voluntary behavior activity awareness...

 in the teleological

Teleology

A teleology is any philosophical account which holds that final causes exist in nature, meaning that design and purpose analogous to that found in human actions are inherent also in the rest of nature. The word comes from the Greek τέλος, telos; root: τελε-, "end, purpose...

 sense. 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 terminology is often used by biologists as a sort of 'short hand' way of describing function, even though they know it is technically incorrect. Laypersons may not understand this distinction, however.

As applied to ecosystem

Ecosystem

An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....

s, the word 'function' is has quite a different meaning, and is basically a synonym for processes. Ecosystems do not reproduce and thus cannot be said to have any function in the above sense.

The word may also for non-biological entities, e.g. the ozone layer might be said to have the 'function' of blocking ultraviolet radiation, though this use of the word also does not necessarily imply any teleological cause.