Full Idea
The main alternative to the dispositional theory of biological functions (which confer a survival-enhancing propensity) is the etiological theory (effects are functions if they play a role in the causal history of that very component).
Gist of Idea
Biological functions are explained by disposition, or by causal role
Source
Bert Leuridan (Can Mechanisms Replace Laws of Nature? [2010], §3)
Book Reference
-: 'Philosophy of Science' [-], p.8
A Reaction
[Bigelow/Pargetter 1987 for the first, Mitchell 2003 for the second] The second one sounds a bit circular, but on the whole a I prefer causal explanations to dispositional explanations.