Four notions of biological function

Abstract

I argue that there are at least four different ways in which the term ‘function’ is used in connection with the study of living organisms, namely: (1) function as (mere) activity, (2) function as biological role, (3) function as biological advantage, and (4) function as selected effect. Notion (1) refers to what an item does by itself; (2) refers to the contribution of an item or activity to a complex activity or capacity of an organism; (3) refers to the value for the organism of an item having a certain character rather than another; (4) refers to the way in which a trait acquired and has maintained its current share in the population. The recognition of a separate notion of function as biological advantage solves the problem of the indeterminate reference situation that has been raised against a counterfactual analysis of function, and emphasizes the importance of counterfactual comparison in the explanatory practice of organismal biology. This reveals a neglected problem in the philosophy of biology, namely that of accounting for the insights provided by counterfactual comparison.

Introduction

The part of biology that investigates the life of individual organisms (the way they are built, the way they work, and the way they behave) is commonly called ‘functional biology’. As the name suggests, the study of function is central to this part of biological inquiry. In this paper, I present a classification of different uses of the term ‘function’ in biology. My ultimate aim is to understand the practice of explanation in functional biology (see Wouters, 1999, Wouters, 2003). A study of what biologists call ‘function’ seems an appropriate first step towards fulfilling this aim.

Theories of function abound in contemporary philosophy of science. The most recent survey dates from 1995. It lists nine different approaches (Allen & Bekoff, 1995, p. 19). The past seven years have added even more richness (for example Walsh, 1996, Buller, 1998, Craver, 2001). Roughly speaking, one may distinguish three main approaches to the notion of function: the etiological approach (Neander, 1980, Neander, 1983, Neander, 1991a, Neander, 1991b; Millikan, 1984, Millikan, 1989b, Mitchell, 1989, Brandon, 1990),¹ which defines functions in terms of past natural selection; the life chances approach (Canfield, 1964, Canfield, 1965, Ruse, 1971, Wimsatt, 1972, Bigelow and Pargetter, 1987, Horan, 1989), which defines functions as effects that enhance the life chances of their bearers; and the systemic approach (Cummins, 1975, Cummins, 1983, Craver, 2001), which defines functions as contributions to a complex capacity or activity.²

A decade ago, different theories of function were usually seen as rival accounts of the unique meaning that the notion of function was assumed to have. However, in the course of the 1990s awareness grew that there are several senses of function involved in the study of biology. Nowadays many philosophers acknowledge the existence of two main notions of function: an evolutionary notion that serves to explain trait presence and a non-evolutionary one that serves to explain complex activities or capacities (see for example Millikan, 1989a, Millikan, 1993, Godfrey-Smith, 1993, Griffiths, 1993, Kitcher, 1993, Amundson and Lauder, 1994, Walsh, 1996). Major topics in the contemporary debate about biological function include the question of whether there is unity underlying different kinds of function (see for example Millikan, 1989b, Godfrey-Smith, 1993, Kitcher, 1993), the question of whether the evolutionary notion of function includes both past and present selection or past selection only (Walsh, 1996), and the question of whether this evolutionary notion should appeal to past selection or to past contribution to fitness (Buller, 1998).

In this paper I argue that there are four different kinds of things called ‘function’ by at least one important biologist, namely (1) the activities of the items of organisms (function as activity), (2) the roles of the items and activities of organisms in a complex activity or capacity (function as biological role), (3) the advantages for the organism in its struggle for life of having certain traits rather than others (function as biological advantage), and (4) the effects for which certain traits were selected in the past (function as selected effect). I will characterize these different kinds of functions, discuss their differences, and illustrate them by means of examples from biological textbooks and research papers. The distinctions are rooted in differences in explanatory roles.

The distinction between function as activity and function as biological role has its origins in an important paper by the animal morphologists Walter Bock and Gerd von Wahlert (Bock & von Wahlert, 1965). I improve on their account. My main addition, however, to the existing views on different kinds of function in biology is the notion of function as biological advantage, as distinct from function as biological role and function as selected effect. As I will show by means of examples, biological advantages are determined by counterfactual comparison.

The recognition of a separate notion of biological advantage is important because it reveals a major problem with regard to explanations in functional biology, namely the appeal to advantages (or, more generally, to counterfactually determined biological value) in functional explanations. Current accounts of functional explanation fail to address this problem.