Functional explanations in biology explain the structure of the parts and activities of organisms by appeal to the utility of those parts and activities.
An example is Schwenk's (1994) explanation of why snakes have forked tongues. Schwenk argues that the snake's tongue has an important role in following scent trails (of preys and mates). Snakes follow scent trails by comparing the intensities of chemical stimuli at two sides of the body. This allows them to detect the edges of a chemical trail and follow it with minimal deviation. The chemical stimuli are collected by means of the tongue and passed to a pair of chemoreceptors in the snout. In order to compare stimulus intensities at two points, the snakes must be able to sample chemicals at two points simultaneously. This is made possible by the forking. Hence, Schwenk explains the forked character of the tongue by showing that this form is useful to the organism's that have it.
To many people, functional explanations seem a little odd, at least at first sight. Their intuition tells them that explanations should show how the phenomenon to be explained is brought about by the explanatory facts. For example, an explanation of a solar eclipse tells us how an eclipse results from the moon sliding between the sun and the earth as a result of which the light from the sun cannot reach the earth. However, Schwenk's explanation does not tell us how the tongue became forked. Yet, it seems an informative story.
Explanation Without A Cause (1989) addresses the question what functional explanations add to our knowledge (in addition to the facts cited in the explanation). Schwenk describes the form of the tongue, he describes how snakes use their tongue and he tells us how the latter fact explains the first. What does this latter account add to the description of the facts described?
The author (Arno Wouters) argues that the explanatory force of such explanations must be sought in their ability to show how the presence and the character of the items and behaviours to be explained fit into the structure of functional interdependencies that exists between the different parts of an organism, its behaviour and the state of the environment in which it lives. Functional explanations show us (i) how the properties of certain kinds of organisms and the states of the environment in which they live, pose a problem to the life of those organisms, and (ii) how those problems are solved in the organisms in question.