Adaptation and Evolutionary Theory Robert N. Brandon There is virtually universal disagreement among students of evolution as to the meaning of adaptation. (Lewontin, 1957) Much of past and current disagreement on adaptation centers about the , definition of the concept and its application to particular examples: these argu- ments would lessen greatly ifprecise definitions for adaptations were available. (Bock and von Wahlert, 1965) The development of a predictive theory [of evolution] depends on being able to specify when a population is in better or worse evolutionary state. For this purpose an objective definition of adaptedness is necessary. (Slobodkin, 1968) The conception of adaptation was not introduced adequate definition of relative adaptedness. As we into biology in 1859, Rather what Darwin did will see such analysis cannot be divorced from an was to offer a radically new type of explanation analysis of the structure of evolutionary theory. of adaptations and in so doing he altered the The other major aim of this paper is to expose conception. As the above quotes indicate we have this structure, to show how it differs from the not in the last century sufficiently delimited this standard philosophical models of scientific the- conception and it is important to do so. ories, and to defend this differentiating feature In this paper we will analyse and, I hope, (and hence to show the inadequacy of certain clarify one aspect of the conception of adaptation. views about the structure of scientific theories One of the aims of this paper is a theoretically which purport to be con~plete). Reprinted from Robert Brandon, Studies in the History and Philosophy of Science, Vol. 9, Adaptation and Evolutionary Theory, pp. 9, 181-206. Copyright 1978, with permission from Elsevier. I owe a debt of gratitude to all those who read earlier versions of this paper and helped me improve it. Where possible I have tried to footnote contributions. Here I want to give special thanks to Ernst Mayr and Paul Ziff whose comments and criticisms have had pervasive effects on the evolution of this paper. 104 ROBERT N. BRANDON A note on defining is needed. Definitions are occurrence in nature is controversial. One could often thought to be of two kinds, descriptive and speak of an abstract theory of evolution which stipulative. (See, for example, I-Iempel (1966), covers natural selection, group selection and even chapter 7.) Descriptive definitions simply describe the selection of tin cans in junk yards. But most the meaning of terms already in use; stipulative of the interesting problems don't arise at this level definitions assign, by stipulation, special mean- of generality. In this paper we will be primarily ing to a term (either newly coined or previously concerned with natural selection, i.e. with intra- existing). According to this view the project of specific intraenvironmental selection. Thus we will defining a term is either purely descriptive or be concerned with the adaptedness of individual purely stipulative. This view is mistaken. The organisms, not with the adaptedness of populations. project at hand calls for neither pure linguistic Let me illustrate the confusion that results analysis nor pure stipulation; it is much more com- from the failure to relate adaptedness to the plex. Briefly, we examine the conceptual network proper level of selection. One of the more prom- of evolutionary biology. We find that according inent definitions of relative adaptedness is due to evolutionary theory there is a biological prop- to Thoday.' Basically it says: a is better adapted erty, adaptedness, which some organisms have than b if and only if a is more likely than b to more of than others. Those having more of it, have offspring surviving 10' (or some other large or those better adapted, tend to leave more off- number) years from now. Either the long-range spring. And this is the mechanism of evolution. probability of offspring corresponds to the short- The project calls for conceptual analysis but range probability of offspring or it does not. such analysis is sterile unless it is coupled with (Corresponds means: a's long-range probability an examination of the biological property which of offspring is greater than b's long-range prob- is the object of the conception. Any definition ability of offspring if and only if a's short- which fails to fit the conceptual network must range probability of offspring is greater than b's be rejected, as must any which fails to apply to short-range probability of offspring.) If it does the property. The project calls for an element correspond then we should stick to the more of stipulation but our stipulatory freedom is easily measurable short-range probability. If not, constrained both by theoretical and conceptual then since natural selection is not foresighted, i.e. requirements and, one hopes, by the real world. it operates only on the differential adaptedness A note on the restricted scope of this paper is of present organisms to present environments, the also needed. Biologists talk about the adaptecl- long-range probability of offspring is irrelevant to ness of individual organisms and of populations. natural selection. Selection occurs at the level of individuals and, Why has Thoday's definition been so favorably presumably, at higher levels. That is, there is received? Because the long-range probability intrapopulational selection and interpop~~lational of descendants is important to selection at or selection. It is vital that we keep these levels above the species level. For instance, one plaus- separate and that we see the relation between ible explanation of the predominance of sexual selection and adaptation.' Selection at the level of reproduction over asexual modes of reproduction individual organisms has as its cause differences is that the long-range chances of survival are in individual adaptedness and its effect is adap- greater for populations having sex (see Maynard tions for individual organisms. We will follow Smith, 1975, pp. 185ff). But if one is interested standard practice in calling selection at this level in selection at the population level then the natural selection. Any benefit to the population relevant notion of adaptedness would be that from natural selection is purely fortuitous. One which applies to populations. Until recently even must distinguish between a group of adapted biologists have failed to distinguish intra- and organisms and an adapted group of organisms. inter-populational selection. Thoday's definition, For instance, a herd of fleet gazelles is not neces- not being selection relative, lends itself to this sarily a fleet herd of gazelles. Similarly group confusion. To keep matters as clear as possible we selection will have as its cause differences in group will only be concerned with natural selection and adaptedness and as its effect group adaptations. with that notion of adaptation which properly The theory of group selection is quite clear; its relates to it. ADAPTATION AND EVOLUTIONARY THEORY 105 1. The Role of the Concept of Relative that there is variation, (I), and that some of the Adaptedness in Evolutionary Theory traits which vary are heritable, (2), it follows that the variation within a species tends to be preserved. The following three statements are crucial coin- (Of course this tendency can be counterbalanced ponents of the Darwinian (or neo-Darwinian) by other factors.) theory of evol~tion:~ When (3) holds, when there are differences in reproductive rates, it follows from (1) and (2) 1. Variation: There is (significant) variation in that the variation status quo is disrupted, that is, morphological, physiological and behavioral that there are changes in the patterns of variation traits among members of a species. within the species. For our purposes we can count 2. Heredity: Some traits are heritable so that such changes as evolution. (For a fuller explica- individuals resemble their relations more tion of the concept of evolution see Brandon, than they resemble unrelated individuals and, 1978.) Thus when (1)-(3) hold evolution occurs. in particular, offspring resemble their parents. We have seen that (1) is in a sense trivial and 3. Differential Fitness: Different variants (or requires no explanation. We have also seen that different types of organisms) leave different (2) is non-trivial and is to be explained by mod- numbers of offspring in immediate or re- ern theories of genetics, but that this explanation mote generations. is not essential to Darwinian theory. In contrast, the distinguishing feature of a Darwinian theory When the conditions described above are of evolution is its explanation of (3).4 The focus satisfied organic evolution occurs. A thorough of this paper is the conception used for such examination into the history of our awareness explanations. of these conditions would be interesting and The distinguishing feature of a Darwinian worthwhile but will not be attempted here (see theory of evolution is explaining evolutionary Mayr, 1977). Suffice it to say that in Darwin's time change by a theory of natural selection. Of course, each was a non-trivial statement. In what follows that is not the only possible sort of explanation we will examine them predominantly from our of evolution. 111 his own time Darwin convinced own point of view. the majority of the scientific comn~unitythat Ignoring the parenthetical 'significant' (1) evolution has and does occur but hardly anyone could not help but be true. The uniqueness bought his natural-selection-explanation of it. of complex material systems is now taken for (For an excellent source book on the reception granted; and so we expect variation among of Darwin's theory see Hull, 1973.) The alternat- individuals of a species. Their similarity needs ives of Darwin's day, e.g. divine intervention and explaining not their variation.