A biological function is the reason some object or process occurred in a system that evolved through a process of natural selection. That reason is typically that it achieves some result, such as that chlorophyll helps to capture the energy of sunlight in photosynthesis, and hence the organism is more likely to survive and reproduce.
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, and in more modern times formed part of the Cuvier–Geoffroy debate.
In evolutionary 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 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 spandrels, side effects of functional systems.
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.
Function is not the same as purpose in the teleological sense. 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.
One of four causes
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.
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- Diagram on The Four Areas of Biology
- Further Diagrams on The Four Areas of Biology (Documents No. 5, 6 and 7 in English)