Ecology, evolution and explanatory patterns in biology

J. theor. Biol. (1972) 36, 593-616

Ecology, Evolution and Explanatory Patterns in Biology w. J.

VAN

DJZR

STEEN

Department of Biology, Free University, Amsterdam, The Netherknds (Received 13 December 1971, and in revised form 17 March 1972) In biology, several misunderstandings prevail with respect to the nature of scientific explanation. Confusion arises especially when alternative explanatory patterns concerning the same subject matter are evaluated in regard to their relative merits. As a consequence, the relations between different branches of biology (particularly “ecology” and “evolutionary biology”) are obscured. A sophisticated terminology is needed to analyze the situation. For this purpose, useful concepts may be borrowed from philosophy of science. The present paper introduces such a terminology and, subsequently, provides an introductory

analysis of conflicts in present-day biology. The

discussion is rather complicated. Therefore, the reader is advised first to glance through the conclusions in section 6 to get a general idea.

1. Introductioll There is currently a renewed interest in patterns of explanation in biology. The subject is one of considerable weight, but it is also extremely controversial. This is partly due to the fact that the interrelated, but quite different problems involved may result in Gordian knots rather than sensible discussions. The following topics are often intermingled in recent arguments: (i) the relations between different concepts of explanation in philosophy and in science; (ii) the status of specisc logical types of explanation (historical, functional) in biology (in this paper, the expression “type of explanation” is also used informally and in a wider sense); (iii) the status of various modes of explanation bearing on different levels of organization (cell, individual, population, etc.); and (iv) the evaluation of alternative explanations (which may involve different types or modes) concerning the same subject matter. The purpose of this paper is to disentangle prevailing Gordian knots. The topics (i)-(iii) are dealt with first to prepare the grounds for a discussion of the main subject, the relations between “ecological” and “evolutionary” explanations. Part of the exposition is elementary, because some readers may not be acquainted with recent developments in the philosophy of science. The 593

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terminology used is largely taken over from Hempel (1965) and Stegmiiller (1969). The views presented in section 2 are to a large extent derived from the authoritative studies by these authors. The relevant conceptions concerning explanation have been developed on the basis of an article by Hempel & Oppenheim (1948, reprinted in Hempel, 1965).

2. The Concept of Explanation (A)

EXPLANATORY

ARGUMENTS:

MODELS

IN THE PHILOSOPHY

OF SCIENCE

An explanation answers a why-question, but not every answer to any why-question counts as a scientific explanation. According to a widely accepted interpretation in the philosophy of science, scientific explanations can be reconstructed as arguments. The conclusion of an explanatory argument (the explanandum) is a statement about the “explained” event. The premisses (explanans) should contain at least one general law, beside so-called statements about initial conditions (initial conditions for short), which concern events bearing on the event to be explained. When the explanation relates to an open system, statements about boundary conditions (boundary conditions for short) must be specified in addition. These conditions relate, for example, to the absence of external influences that could disturb the system under study. In deductive-nomological explanations (deductive explanations for short) the explanandum follows with certainty (deductively) from the explanans, i.e. the explanandum is true provided the statements in the explanans are true. The explanans contains at least one universal law, which is taken to be valid under the relevant boundary conditions. In probabilistic explanations, on the other hand, the explanans confers a certain degree of probability on the explanandum. More accurately, a relation of inductive probability then obtains between the statements in the explanans and the explanandum. The explanans contains at least one probabilistic law, which specifies some statistical probability. The two concepts of probability must be distinguished carefully (see, e.g. Carnap, 1966). Two potential probabilistic explanations may be conflicting because they confer a different probability on a similar explanandum even though the premisses are acceptable in either case. This shows that a decision procedure is needed to evaluate the relevance of premisses in probabilistic explanations. A similar problem does not exist with respect to deductive explanations. In the notation of symbolic logic, examples of the most simple forms of deductive and probabilistic explanations may be rendered as follows (reference to boundary conditions is omitted for convenience).

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Deductive explanation:

W(Px = Qx) Pa

Qa

universal law explanans initial condition > explanandum

in words:

for all X, x is P implies x is Q a is P a is Q Probabilistic explanation: p(G, F) = 0.8 Fa

___-

[O-S]

Ga

probabilistic law explanans initial condition > explanandum

in words (loosely): the statistical probability

of an F being a G is O-8 a is F therefore: the inductive probability of the statement “a is G” is 0% There is a second class of explanations, viz. those concerning the derivation of laws. Such explanations need not contain initial conditions in the premisses, but they must contain laws. The above distinctions show that there are at least four categories of explanation: deductive and probabilistic explanation, both of laws and of events. Beside explanation, there are other types of systematization such as prediction and retrodiction. The number of types is actually very large, i.e. the classification of the relevant arguments into explanations, predictions and retrodictions is extremely coarse (Stegmtiller, 1969). A discussion of the subject is beyond the scope of this paper. Explanations may be split or supplemented such that a chain of arguments results. Such a chain is called a genetic explanation (Stegmtiller, 1969). The following, very simple examples are instructive. b)(px = Qx) step 1 Pa (x) (Px 3 Rx) Pa Ra

splitted into step 2

Qa (4 (Qx = Rx) Qa Ra

(Qa in step 2 is explained by step 1)

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If step 2 is taken as the original explanation, we have a case of supplementation (step 2 is supplemented with step 1). A second example of supplementation runs as follows. b>U’x = Qx) Jx>(Qx = R-4 (x)(Px 3 Rx) (x) (Px 3 Rx) Pa supplemented with and/or Ra (x) (Sx 3 Px) Sa Pa

The reader will have no difficulty in constructing other examples. The present examples show that a given event or law may be open to several alternative, acceptable explanations.

The process of supplementation can be continued as far as we like, or as far as available data and theories permit. However, an indefinite continuation cannot be realized. Ultimately, certain premisses must be taken for granted (some as axioms, others as unexplained statements of fact). Therefore, explanations cannot be complete in the sense of closed. Another type of completeness is likewise impossible: there are no total explanations covering each aspect of an explanandum event. Only those aspects of events which can be expressed with the linguistic apparatus of prevailing theories, are open to explanation. The expression “explanation of events (phenomena, things)” is indeed misleading. “Explanation of facts” is a less confusing expression, if facts are conceived as aspects of concrete entities (van der Steen & Jager, 1971). The term “explanation of events” is none the less used below, in agreement with common usage. Parenthetically, some reasoning on the above lines shows that “facts” are to some extent theory-loaded (Feyerabend, 1963). (B)

DEVIATIONS

FROM

THE

ABOVE

CATEGORIES

OF EXPLANATION

The models for scientific explanation presented above are idealizations. The following deviating forms of (deductive or probabilistic) scientific explanation can be distinguished. Inaccurate explanations do not permit an assessment of logical validity, because one or several of the statements involved contain ambiguities. In rudimentary explanations the explanans is incomplete. Some premiss may be omitted simply because it is unknown, or because it is self-evident, or because the specialist involved is not acquainted with it. Partial explanations cover only some aspect of the explanandum

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(i.e. the explanans only permits the derivation of a statement which is weaker than the proposed explanandum). In explanation sketches one or several of the premisses are outlined informally because they are not known with sufficient accuracy. Pseudo-explanations are not genuine explanations in any sense because the premisses are not acceptable. The boundaries between the above forms of explanation are not sharp. Explanations conforming to the ideal pattern discussed in section (A) will be called strict deductive and strict probabilistic explanations. The above deviating forms will be referred to collectively as informal explanations. (C)

LAWS

OF NATURE

AND

METHODOLOGICAL

PRINCIPLES

It is difficult to elaborate criteria which good explanations must satisfy. The evaluation of explanations involves among other things an appraisal of the laws they contain. Some remarks about laws are therefore in order. There is no sharp boundary between laws and hypotheses. In the remaining part of this paper, the term “law” is used in a broad sense, and the relevant principles are held to be applicable both to laws and to hypotheses. They hold also for theories, which may be regarded as coherent sets of laws. (Theories, beside laws, naturally can be used in explanations.) A law must be related to the empirical level, and the relevant empirical domain must be delineated. This implies that some form of testability must be available (possibility of indirect or direct cotimation, or falsitication). According to a second methodological principle, the logical (mathematical) form of a law must not be unduly complicated (principle of parsimony). The laws in two competing explanations may differ with respect to degrees of testability and parsimony. It is possible that one of the relevant laws is to be preferred because of its degree of testability, whereas the other one is in a better position with respect to parsimony. The situation is even more complicated because there is a third major methodological principle (often overlooked), viz. the principZe of coherence. It says that a general statement should only qualify as a law ifit is systemically related to other laws. The value of a law is increased according as it is better entrenched in a theory. Different laws may have different “degrees of coherence”. (D)

MODEL

EXPLANATIONS

Acceptable explanations containing premisses which are obviously or supposedly false, are called model explanations. They are often needed when situations calling for explanation are so complex, that considerable simplifications are justified.

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There is no sharp boundary between model explanations and “ordinary” (strict or informal) explanations, because almost any law of nature is only approximately valid. This is an obvious consequence of using the principle of parsimony in the construction of laws and theories. The term “model” is here used to denote theories which involve obvious simplifications. (It is well-known that the term has many other meanings.) (E)

SOURCES

OF CONFUSION

The circularity falIacy

In many scientific explanations, knowledge concerning the explanans statements partly derives from the truth of the explanandum being known. There is a wide-spread temptation among philosophers and scientists, to argue that such explanations are inadequate because they involve circular reasoning (in a wide sense of this term). This argument may be called the circularity fallacy. (The term “fallacy” is presently also used in a wide sense.) The situation is not so simple as the argument suggests. The following line of reasoning illustrates this. A majority of explanations put forward by biologists are informal because (among other things) boundary conditions are not specified. The relevant conditions are essential, since biology is concerned with open systems. They are often unknown due to the complexity of living systems. In most cases, it is reasonable to stipulate that some set of boundary conditions was apparently satisfied because the event to be explained did actually happen. There are several other types of acceptable “circular” arguments which relate to scientific explanations. Confusion may arise in discussions if the logical structure of the arguments is not sufficiently cleared up. By way of an example, the controversy between numerical and evolutionary taxonomists is partly the result of such a confusion. Numerical taxonomists sometimes claim that phenetic similarities cannot be explained with reference to phylogenetic relationships since any classification is necessarily based on the study of phenetic properties. The thesis that the relevant evolutionary explanations are not acceptable because they are circular, is an oversimplification (for details, see Hull, 1967). The formalization

fallacy

In many papers, there is an implicit argument concerning explanation in biology which may be reconstructed as follows. “Strict explanations are better than informal explanations; in biology, establishing strict explanations is presently more difficult than in physics due to the current lack of formal-

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ization; therefore, biology is a juvenile science (as compared with physics) that may become adult according as formalization proceeds.” I propose to call this argument the formalization fallacy. It is misleading mainly because the first premiss is incomplete. The following expansion is more to the point. “Strict explanations are better than informal explanations, other things being equal.” It is indeed possible that informal explanations must be preferred above available strict explanations with respect to some subject matter, if they satisfy some methodological principle (e.g. testability or coherence of the relevant laws) to a higher degree. There is no reason to assume without more ado, that strict explanations are better than related informal ones with respect to any methodological principle. The thesis that this situation will obtain in the future is equally dogmatic. A second argument unmasking the formalization fallacy runs as follows. Formalization (like any change in theory) is apt to induce changes in subject matter. The sets of facts eventually covered by the emerging theories are usually of a highly specific nature. Some reflection on explanation in physics clearly illustrates this. Physical theory yields powerful strict explanations of highly specific events brought about in the laboratory. A strict explanation of physical ,events occurring “in the field”, on the other hand, is seldom possible. The situations in the field are complex, and it is most difficult to assess the initial conditions needed in the relevant explanations. (Naturally, physical theory may easily provide informal explanations of field events.) The important point is that physicists are less interested in field situations than biologists. The relative merits of formalization in different branches of science appear to depend on the importance attached to the explanation of field data. The above argument should not be used to disparage formalization in biology. Formalization may greatly facilitate even the elaboration of informal explanations relating to field data (cf. developments in mathematical population genetics). The argument only shows, first, that the import of formalization and strict explanation must not be exaggerated and, second, that establishing strict explanations is not the sole purpose of formalization. The fallacy of misplaced preference Another misleading argument which is important in the present context runs as follows. “It is impossible that two alternative explanations concerning the same event are equally acceptable; therefore, whenever such alternatives are propounded, at most one of them eventually should be accepted.” I shall call this argument the fallacy of misplacedpreference. It is objectionable on several grounds. First, the relevant alternatives may concern different aspects of an event, i.e. the explanations may relate to the same

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event but not to the same fact. In this case, it is intuitively obvious that they could be acceptable to the same extent. Second, the discussion of genetic explanations [see section 2(A)] showed that explanations concerning the samefict need not be incompatible. It is an obvious possibility that different explanatory patterns dealing with similar facts should coexist because they are useful in different contexts. If the above line of reasoning is correct, a possible explanation must never be rejected exclusively on the ground that some competing argument is acceptable. It is plausible that the following, naive picture of the nature of science underlies the fallacy of misplaced preference. “The development of science is towards an ultimate, coherent and unchangeable theory of reality.” Several arguments count against this vision. First, any scientific theory whatsoever can only explain a limited (not: finite) set of facts, which is determined by the linguistic apparatus of the theory. The notion of a theory explaining “everything” or “reality as a whole” has no clear cognitive content, since the latter terms are notoriously vague. Second, the history of science points in quite another direction (Kuhn, 1962). This is often overlooked due to the continuous rewriting of history in scientific texts. Ambiguities Differences of opinion concerning the nature and the acceptability of particular explanations may simply result from the fact that the term “explanation” is used with different meanings. A few examples will illustrate this. Scientists might argue that the basic models for explanation propounded by philosophers [see section 2(A)] do not look like the explanations actually put forward in scientific papers. However, they should bear in mind that these models are only meant to cover logical reconstructions of explanations. No one will deny that such reconstructions are useful in evaluating explanations. Philosophers of science sometimes suggest that only strict (deductive or probabilistic) explanations are genuine. A scientist might object that many informal explanations in science constitute valuable achievements. This is merely a verbal disagreement. It is unimportant whether, for example, some valuable argument is called an “explanation” (in a scientist’s terminology) or an “explanation sketch” (in a philosopher’s terminology), provided that the respective linguistic conventions are known. Verbal disagreements of the above type also occur in discussions among scientists, when different investigators use different concepts of explanation. Scientists occasionally use the term “explanation” even for certain types of description, discovery (especially of evidence concerning “missing links”),

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theory construction, etc. This terminology should be avoided, because it unnecessarily adds to existing confusion. Disagreement concerning the value of a particular explanation may also result from ambiguities in explanans or explanandum statements. Consider the following, more concrete example. Theories of evolution are supposed to explain the origin of species. This can be interpreted in at least two ways. The theories could explain either general “facts” (more accurately: laws) concerning speciation or the origin of particular species. Disagreement about whether some theory does explain speciation might be purely verbal, if expressions such as “origin of species” are indeed used in different ways. It is possible that seemingly competing explanations actually deal with different events or laws. (It is also possible that different facts concerning the same event are involved; cf. the above discussion of the fallacy of misplaced preference.)

3. Some Remarks on Historical

and Functional Explanation

Many biologists maintain that biology should remain an autonomous science, and that it is not reducible to physics. One argument in favour of this thesis, with which the present section is concerned, is based upon the alleged specific logical status of certain explanatory patterns in biology. Section 4 deals with arguments relating to levels of organization and reduction. (A)

HISTORICAL

EXPLANATION

Several authors (e.g. Simpson, 1964) argue that some historical events (or aspects thereof) are nonrecurrent and therefore unique. This would be obvious especially in regard to evolutionary phenomena. It is also maintained that the causation of nonrecurrent events cannot be subsumed under general laws precisely because they are unique. Therefore, the explanation of such events would not conform to any of the models discussed above, since the premisses cannot contain any law. Certain aspects of evolution (and, more generally, history) would be covered by specific historical explanations that should take the form of, for example, reconstructions or coherent narratives (Goudge, 1967). The above line of reasoning contains a logical error besides some linguistic confusion. The logical error resides in the implicit assumption that there are recurrent and non-recurrent events (see, for example, Watson, 1966). Actually, any event may be regarded as nonrecurrent and unique. To say that certain events are similar is to maintain that one or several general names

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apply to them. General names only refer to aspects of events. It depends on the aspects we are interested in (i.e. on the language used in the relevant context), whether we should regard distinct phenomena as “similar”. Two events may be similar in all relevant aspects only with respect to some theory. If some historical events were so unique that relating them to laws would be logically impossible, a description would be impossible by the same token. Any description involves the use of general names denoting aspects shared with other events, and any such aspects may be subject to lawful relations. Therefore, the argument in favour of specific historical explanations is self-defeating. The argument also involves linguistic confusion. Reconstructions and descriptions concerning past evolutionary events, obtained by using the techniques and theories of several disciplines, are very illuminating. This leads some investigators to use the term “explanation” for “description”. (Notice that descriptions need not relate to observables.) It does not follow that a specific type of explanation is involved, but only that there are specific terminologies. Even a more correct use of the term “explanation” in relation to evolution could result in confusion. With respect to many evolutionary phenomena, some form of explanation is currently possible beside description. The synthetic theory of evolution (see especially Mayr, 1963) is a valuable source of informal explanations. Mathematically formulated theories covering various branches of population biology, yield interesting (equally informal) model explanations (Levins, 1968). The same could hold for Williams’ (1970) axiomatic version of the (more restricted) Darwinian theory. It is presently unclear in how far the respective approaches relate to the same classes of facts, events, or laws. In addition, different notions of explanation could underly these approaches. Therefore, it is feasible that controversies about the explanatory power of different theories partly reflect linguistic confusion [see section 2(~): ambiguities]. The fact that disagreements about the value of the synthetic theory are long-lasting, strongly points in this direction. (B)

FUNCTIONAL

EXPLANATION

Many why-questions in biology can be answered superficially by the ascription of a function to properties or behaviour patterns of organisms. The following examples suffice to convey the idea. Question: “Why do vertebrates have kidneys ?’ Answer : “Because they perform the indispensable function of (play a role in) excretion.” Question : “Why do insectivorous species of bird migrate from temperate to (sub)tropical regions in winter ?’ Answer : “Because migration is essential for survival ; resources in temperate areas are insufficient in winter.”

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It is most difficult to define the concept of function and to translate statements referring to functions (functional statements) into a non-functional language (Beckner, 1968, 1969). As a consequence, it is equally difficult to decide, (i) whether a logical reconstruction of functional statements answering why-questions will show that they should count as genuine (“functional”) explanations; and (ii) whether such explanations have a specific logical status. The remaining part of this section contains informal arguments which strongly suggest that differences of opinion about “functional explanation” result from linguistic confusion, and that the term “functional explanation” is inappropriate. Many philosophers of science have proposed translation schemes for functional statements (for references see van der Steen, 1971). The following statement is generally used as a paradigm: “the function of the heartbeat in vertebrates is to circulate the blood”. Thorough analyses suggest that hardly any translation of this statement could provide a basis for explaining (in some reasonable sense of the term) the presence of a beating heart in some vertebrate. At best, very weak informal explanations are possible. An obvious reaction on the part of biologists could run as follows. “Philosophers have misdirected their efforts. No biologist would be tempted to explain the existence of beating hearts in organisms in this way.” However, the question cannot be settled in such an easy way. Consider this counter-example. A biologist discovers some unknown organ in a species of invertebrate, the function of which is initially unknown. Subsequent work shows that the organ produces an essential hormone. Many investigators would claim that the presence of the organ is somehow “explained” in this way, because its function has been detected. From a logical point of view, the example is comparable with that concerning the heartbeat. The fact that evaluations by biologists are different in the two cases relates to the pragmatic context: the term “explanation” is used especially when new functions are discovered. This situation lends itself to the following interpretation. The discovery of new functions is illuminating because they fit the pattern of established biological theories. According to most theories of evolution, properties of organisms generally have adaptive value due to the action of natural selection. Assigning a function to some item comes close to saying that it has adaptive value. Therefore, the discovery of functions may help to indirectly confirm some theory of evolution. (Similar remarks could be made with respect to other theories.) In addition, when a biologist ascribes a function to some property, he may suggest implicity that an informal “evolutionary” explanation is possible concerning the existence (origin) of the property. (The functional ascription itself is then not meant as an explanation.) T.B.

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Apparently, “functional explanations” in papers by biologists are very compact arguments. They are not so problematic as some philosophers suggest, provided that they are unpacked (reconstructed) in an adequate way. Much less confusion would obtain if the term “explanation” were omitted in this context. Arguments of the type discussed are not really meant as explanations in a strict sense. The present analysis does not entail that “functional statements” cannot be used in any “ordinary” explanation. The reverse is true (cf. van der Steen, 1971). (C)

CONCLUDING

REMARKS

The upshot of the present section is that there are no “historical” or “functional” explanations with a specific logical status. Naturally, evolutionary and “functional” approaches in biology do relate to specific empirical data. It is useful to retain the term “historical explanation” for (“ordinary”) explanations which involve long periods of time. The term ‘functional description” should be used for the above cases of functional “explanation”.

4. Explanation and Levels of Organization Theories in biology may deal with quite different levels of organization (e.g. cells, individuals, populations). There is much disagreement about the relative importance of theories relating to different levels. A few remarks on the reduction of theories are needed to evaluate this situation. (A)

THE REDUCTION

OF THEORIES

The laws in a fully-fledged theory are systemically interrelated in a complex way. The logical derivation of some law from other ones counts as an explanation [cf. section 2(~)]. Different theories, taken as a whole, also may show such relationships. When a theory TI “explains” a theory T,, T2 is said to be reducible to TI. TI is the reducer theory, and T2 the reduced theory. A reduction of T, to TI implies: (i) that the terms (concepts) of T2 are defined with the help of terms in TI ; and (ii) that the laws of T2 are deducible from those of TI (Nagel, 1961). I propose to use the terms “strict reduction”, “informal reduction”, “incomplete reduction”, etc., with meanings corresponding to those of “strict explanation”, etc. Established theories, as such, seldom come to stand in a reductive relationship. Theories do change continuously with respect to the nature of laws and the meaning of concepts. Reducing some theory normally involves changing

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it. (Strictly speaking, it is replaced by a theory which shows some degree of resemblance to it.) It is not fruitful to discuss the nature of reduction in more detail, since the subject is rather controversial [see Schaffner (1967) for a technical account, and van der Steen (1970) for an elementary survey]. (B)

REDUCTIONISM

AND

ORGANICISM

According to the so-called reductionism, biology is, or will be, reducible to physics and chemistry (for a modern defense of the thesis, see Schaffner, 1969). Orgunicism (organismic biology) holds that such a reduction is in principle impossible (for reference, see the critical account by Beckner, 1968). A main argument put forward by organicists relates to emergent properties. Entities on any level of organization would be characterized by such, specific properties which do not occur on lower levels. Specific laws, therefore, would be essential in the explanation of events at any particular level. In another terminology, properties of (laws relating to) wholes could not be fully explicable in terms of properties of (laws relating to) parts. This argument contains a logical mistake: knowledge of wholes can be taken into account in defining concepts concerning their parts (see, for example, Hempel & Oppenheim, 1948, reprinted in Hempel, 1965; Nagel, 1961; van der Steen, 1970). The antithesis between organicism and reductionism is a linguistic one, or partly so. Naturally, the question of whether reducing biology to physics and chemistry is actually (not only in principle, i.e. logically) possible, must be left to future research. The outcome will partly depend on the nature of future changes in subject matter. In the present paper, the term “reductionism” will be used more broadly and loosely, to denote any “theory” which overstresses the import of some level of organization in the explanation of events or laws at higher levels. The converse attitude may be called antireductionism. (C)

RBDUCTION

AND

GENETIC

EXPLANATION

The reduction of some theory T, to a theory Tl does not entail that the value of explanations in terms of T2 is diminished. The reverse is rather true, since the degree of coherence of T,-laws [cf. section 2(c)] is increased. The reduction does imply that the laws in the premisses of T,-explanations are in turn explained by T,-laws. The resulting arguments are genetic explanations [cf. section 2(A)]. In many situations it is desirable that reduced theories, and explanations based on them, maintain a status of their own. Thus it is possible that initial conditions can be formulated much more easily in the language of a reduced theory than in those of the corresponding reducer theory.

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The above remarks are particularly important in the case of informal reductions. (Strict reduction is seldom possible in biology due to the complexity of living systems.) They may lead to a greater theoretical coherence in science without improving otherwise on existing explanations of empirical data. (D)

BIOLOGICAL

THEORIES

AND

REDUCTIONISM

: SOME

ILLUSTRATIONS

Genes and natural selection

Many biologists maintain that the theory of natural selection must be couched in terms of gene frequencies. This view is not objectionable, but certain versions of the thesis are unduly reductionistic. Williams’ (1966) book provides an example. He argues as follows (p. 22-26). Effective selection is possible only if the selected entity has a high degree of permanence and a low rate of endogenous change, relative to the degree of bias (differences in selection coefficients). Both genotypes and phenotypes, as opposed to genes, are extremely temporary manifestations. Therefore, natural selection should primarily act on genes. Only the selection of genes can produce cumulative change. As it stands, the argument is misleading for several reasons. Particular genes are temporary manifestations to the same extent as phenotypes or genotypes. The selection theory is concerned with kinds of genes, but selection does not “operate” on kinds, but on particular entities. (This is how the concept of “to operate” should be used.) Confusion arises because the term “selection” is ambiguous: it has different meanings related with theoretical and empirical contexts, respectively. Some reflection suffices to see: (i) that phenotypic traits, like genes, may persist over many generations; and (ii) that it makes sense to say that selection operates on particular phenotypes (individuals). Williams rightly argues that it is profitable to formulate theories in terms of genes. (Actually, the relevant theories are about something like kinds of kinds of genes; the level of abstraction is much higher than one would suspect at a first glance, and the relations with the empirical level are extremely indirect.) Such theories have a high degree of coherence, and their structure is relatively simple (principle of parsimony). On the other hand, their degree of testability and their explanatory power are definitely not spectacular. It is frequently useful to explain particular selection processes primarily with reference to the phenotypic level. Selection of phenotypes by predators could be taken as an example. The fact that laws relating to the phenotypic level are seldom formulated explicitly must not mislead us into thinking that they can be dispensed with in explaining events at this level.

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Laws relating to the phenotypic level may be explained by (reduced to) a theory about “genie selection”. Generally, such explanations will be informal: it is difficult to establish strict explanations, since the interactions among genes and the relations between genes and phenotypic traits are highly complex. The situation is like that sketched at the end of section 4(c). The conclusion that theories concerning the phenotypic level lose their import as soon as some form of reduction is achieved, is a gratuitous form of reductionism. The problem of entities above the level of individual organisms Some biologists (e.g. Ehrlich & Holm, 1962) are suspicious of current concepts which are supposed to denote entities above the level of individual organisms, and properties of such entities (cf. “population”, “community”, “niche”, “climax”, etc.). They hold that individual organisms are the basic units of “population biology” (this term is presently used in a very broad sense). This “individualistic” view leads to statements such as: “species do not compete, at best individuals can be said to compete”, and “the so-called unity of a community only consists in overlapping ranges of tolerance of individuals.” The reduction&tic overtones are obvious. The “cybernetic system view” of entities above the Ievel of individuals (e.g. Margalef, 1968) is in sharp contrast with the above conception [see also Baker (1966) for comments on the controversy]. The individualistic approach certainly has some merits. Many of the relevant “population concepts” are admittedly vague. They may lead one wrongly to postulate the existence of certain entities when there is no clear operational meaning. On the other hand, the implicit contention that any theory in population biology must relate to individuals since they are the basic units, is misdirected. Almost all areas covered by field studies in population biology contain kinds of organisms in staggering amounts, and the relations between them are most complex. General directives are needed in choosing subjects for investigation and methods of research. No biologist will claim that beginning a field study should look like grabbing in a lucky dip. General theories, referring to entities above the level of individual organisms, play a major role in providing directives. Such theories are indispensable even if they have obvious shortcomings. They are used implicitly even in research by numerical taxonomists. Concepts for “entities” above the level of individuals and their properties are badly needed if we are to convey general ideas about complex situations in the field, even if these entities are not regarded as natural kinds in any

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sense. It is partly a linguistic issue whether the properties must be conceived as specific (emergent) or not [cf. section ‘I(B)]. If the individualistic approach were carried through to its utmost consequences, it would destroy itself because the subject matter of population biology would vanish. Attempts at reducing theories about higher levels of “organization” to an individualistic theory are worthwhile, but they presuppose that some higher level approach, including a specific linguistic apparatus, is available. Though it is interesting to quibble about what species, communities, etc., are not, only some new conceptualization and theory construction will lead to theoretical advancement. In the meantime, some classic terminology should be maintained for lack of a better one. 5. Alternative

Explanations

in Ecology and Evoh~tionary Biology

There are many persistent disagreements in ecology and evolutionary biology concerning the scope and the usefulness of various theories and explanatory patterns. An important example is the controversy about whether the evolutionary approach is indispensable in ecology. Explanations in ecology that do not refer to evolutionary history will be called “ecological explanations” for convenience. Historical explanations concerning evolutionary phenomena are referred to as “evolutionary explanations”.

In most cases, the problems discussed in the previous sections are important in decisions about the value of particular explanatory alternatives. It is understandable that they are not touched upon in most discussions. An analysis along the above lines requires a great effort which generally is out of proportion with respect to the profits. Some such approach is indispensable, however, when differences of opinion are not resolved in a more pragmatic way. The latter situation prevails in present-day ecology and related domains of biology. Lewontin’s (1969) study of alternative explanations in biology is a valuable starting point. The approach offered below is partly an expansion of his arguments, but it also adds new materials. The discussion will proceed by way of analyzing examples. First, some remarks are made about Lewontin’s views to get the terminology straight. His illuminating examples are not reproduced; the interested reader should consult the original article. (A)

LEWONTIN’S

VIEWS

Lewontin (1969) shows that disagreement about the value of alternative explanations may reflect a conflict of interest. The terminology used may wrongly suggest that the explanations are about the same laws or events. In

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the present terminology, the alternatives involved could relate either to different events, or to different aspects (facts) concerning the same event. There are also examples of incompatible explanations dealing with the same laws or facts. One should add to this observation that different real alternatives could be equally acceptable (cf. section 2). Quite frequently, the best we can hope in explaining some event in biology (an “outcome”) is to show that it is possible with respect to some set of premisses. That is, the premisses only serve then to exclude part of the alternative outcomes. Lewontin calls the relevant explanations exclusive, and he contrasts them with complete explanations. In the present terminology, the terms to be used are “informal explanations” and “strict explanations”. Instead of “probabilistic” versus “deductive” explanation, Lewontin uses another terminology : “sufficient” versus “exact” explanation. It is obvious that his terminology deviates in several respects from current linguistic practice in the philosophy of science. It is imperative that some generally accepted convention comes into usage if discussions about explanation in biology are to be devoid of confusion. Lewontin particularly emphasizes the differences between two wellknown, opposing schools in ecology representing the “equilibrium approach” and the “historical approach”, respectively. He shows that there is no real conflict. Equilibrium explanations and historical explanations are not incompatible, since they explain different things and use different data. Combining both views to establish more encompassing explanations is di5cult. (B)

ADAPTATION

OF CLUTCH

SIZE IN GREAT

TITS

The British ornithologist Lack (see Birch & Ehrlich, 1967) has studied the ecology of great tits in oak and pine forests in Great Britain. Contrary to expectation, the clutch size in oak forests did not clearly exceed that in pine forests. As resources are more scarce in pine forests, the relevant populations would be expected to change their clutch size accordingly (such adaptations are known to occur in birds). This would decrease the mortality due to starvation. Lack favours the following explanation. The pine forests were planted by man in comparatively recent times. As yet, the birds did not have sufficient time to adapt themselves to the new conditions since the action of natural selection is rather slow. They are still adapted to conditions in the past. Birch and Ehrlich propose an alternative explanation by stating that the various populations interbreed, and that gene flow counteracts the effect of natural selection. The marginal pine forest populations would be adapted to conditions elsewhere, and not to conditions in the past. According to Birch and Ehrlich, explanations in ecology should refer to prevailing environmental conditions. Only if no such explanation can be

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proffered, hypotheses about past events may be invoked, since such hypotheses are not testable (falsifiable). In short, they prefer ecological explanations above evolutionary explanations. Ecological explanations would generally suffice even with respect to populations in man-made environments : Lack would underrate the tempo of natural selection. The arguments deal with the informal explanation of a probabilistic law (concerning some statistical distribution with respect to clutch size). Both alternatives involve an amount of circular reasoning, since the inferences concerning adaptation are partly based on the data to be explained. However, this does not invalidate the arguments [cf. section 2(~)]. The two explanations have a similar explanandum, and they are clearly incompatible. After some modifications in the premisses, they could combine all the same to yield a more complex explanation. (Both a slow rate of natural selection and gene 0ow could play a role.) Two seemingly incompatible, informal explanations may supplement each other when one of them emphasizes data left out in the premisses of the other one. It is difficult to evaluate the relationships between

the two arguments in more detail, because both are explanation sketches: the precise form of the premisses is unknown. In other words, it is maintained that data of a certain kind should constitute an explanation, but a strict explanation in terms of these data is not offered. It is important to notice that the argument of Birch and Ehrlich, and not only that of Lack, rests on a hypothesis concerning the tempo of natural selection. It presupposes that certain effects of selection, in the present case, are such that they can be left out of consideration. So the authors sketch a law concerning the tempo of selection processes in general, and they assume that this law holds in a particular case. Therefore, Birch and Ehrlich wrongly argue that their premisses are in a better position with respect to testability than those of Lack. In either argument, assumptions are made relating to selection in the past. This undermines the thesis that ecology should refrain from evolutionary approaches as much as possible (see also the remaining examples). It is obvious that explanatory arguments in scientific texts are usually not written out exhaustively, and that their logical nature is seldom expounded. Doing so would indeed result in papers with an excessive length. The above example shows that this has also disadvantages, since logical confusion is easily introduced in this way. (C)

EXPLAINING

THE ABSENCE

OF A SPECIES

IN SOME

AREA

Suppose, some species is not found on a geographically isolated island, although it is present on a neighbouring continent or on other islands of the same archipelago.

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The following explanatory accounts are possible. (They are greatly simplified, and the exact logical structure of the arguments is not given.) (i) Data concerning the ecology of the species and the physical conditions on the island show that the species could not live there. That is, certain necessary conditions for the occurrence of the species are not satisfied. (ii) On the island, the species ’ “niche” is already Med. Even if members of the species would repeatedly reach the island, they would be outcompeted. (There is something like a resistance of natural communities against invaders.) (iii) Data concerning the geographic properties of the island and the dispersal power of the species show that it is improbable that the species should ever reach the island. (iv) Historical data concerning the “evolution” of the species and its distribution in the past show (or suggest) that it is improbable that the species would as yet have reached the island. It is possible that more than one of the above types of explanation are acceptable with respect to the same data [e.g. (i), (ii), and (iii)]. The reasons for this are easy to understand. If any of the conditions, necessary for a species to occur in some locality, is not satisfied, this may provide an explanation of its absence. If several conditions are not satisfied, referring to any one of them may yield an explanation. As shown in section 2(~), the coexistence of different acceptable explanations for the same fact may relate to the availability of genetic explanations. The present example shows that, in addition, there are other cases in which really alternative explanations coexist as acceptable arguments. The laws in the respective explanations (sketched above) will be embedded in quite different theories. Thus, explanation (type) (i) could relate to an approach such as that of Andrewartha and Birch (1954), which is rather “individualistic” [see section 4(~)]. The laws in explanation (ii) could derive from some theory in “community ecology”. This shows that acceptable explanations concerning the same “fact” may relate to different levels of organization. It is probable that (ii), after expansion, proves to be a highly informal explanation, whereas (i) could be a strict explanation [cf. section 2(~)]. This does not in itself constitute a sufficient reason to prefer (i) above (ii), since the context of interest should be taken into account. A comparison of(i) and (iii) suggests that a deductive (i) and a probabilistic (iii) explanation concerning the same “fact” may coexist. The deductive argument must not be preferred without mere ado, again, because the context must be regarded. The example also suggest that evolutionary and ecological explanations relating to the same data might both be acceptable [compare (iv) with (i)]. One could object as follows to the above approach. The respective “explanations” relate to different “factors” determining the distribution

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and abundance of species. No explanation is valuable when it is taken in isolation. Some comprehensive theory should be constructed which deals with all the factors, and any explanation should be couched in terms of it. The present example is misleading because it stresses only a situation in which part of the relevant information happens to be redundant. The objection may be countered by the following argument. It is highly probable (in view of, for example, the complexity of field situations) that any comprehensive theory would have the features sketched below: (i) it can only have a strong coherence when relatively great simplifications are made (use of models); (ii) its degree of testability is relatively low; and (iii) its explanatory power with respect to general laws is greater than that with respect to more specialized laws, whereas a strict explanation of particular events or situations in terms of the theory is mostly impossible. The function of the theory would be primarily to make connections between different domains of knowledge and to lay a basis for posing problems in a new way. It would not be meant primarily to explain data of a relatively specific kind. Naturally, it is fruitful to combine the different approaches involved in the above explanations, whenever this is possible. A few illustrations will clarify this. If attempts to establish explanations of type (i) and (ii) do not meet with success, this may strengthen an argument of type (iii) or (iv). If (i) and (iii) hold in a particular case, their combination yields a more powerful explanation. The main purpose of the logical points made above is to show that rash decisions concerning the usefulness of particular types of explanation are undesirable. The complexity of the problems does not permit general conclusions. The next example will further emphasize this point. (D)

EXPLAINING

THE PRESENCE

OF A SPECIES

IN SOME

AREA

The logical features of explanations concerning the presence (distribution, abundance) of a species in some area differ widely from those relating to the absence of a species (see previous example). The following argument elucidates this. (The terminology used involves simplifications. Thus, it is not quite appropriate in the case of probabilistic explanations.) There is a large number of “necessary conditions“ which must be satisfied for a species to be able to live in some area. A strict explanation of a species’ presence should refer to all the essential conditions. In practice, this cannot be achieved. Explanations usually refer to a restricted set of conditions, depending on the interest of the investigator and the area of biology he is working in (i.e. the explanations are mostly informal). The remaining necessary conditions are often unknown, and they are often accounted for by the stipulation that boundary conditions are apparently satisfied. [The

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division of a set of conditions into necessary and boundary conditions in any particular case is somewhat arbitrary, and depends on how the relevant explanations are construed; cf. van der Steen & Jager (1971)]. As shown in the previous example, the situation is quite different in explanations relating to the absence of a species. In principle, a strict explanation is possible when it is shown that one of the necessary conditions involved is not satisfied. Knowledge concerning other conditions may be informative, but it need not be indispensable in the explanations. Arguments given below show that there are still more differences between explanations referring to the presence and the absence, respectively, of species in some area. Suppose, areographic analysis and experimentation combine to show that the distribution and abundance of some species is closely correlated with the values of certain environmental variables, e.g. temperature and humidity. Such data could suffice to informally explain the presence of the species in regions where it occurs. The explanation would explicitly or implicitly refer to some theory (e.g. a physiological one) and, besides, it would take for granted that certain boundary conditions (e.g. those concerning competitors, parasites and predators) are satisfied. [This involves an element of circularity that does not invalidate the explanation cf. section 2(E)]. It is possible to expand the explanation in two ways. First, it may be supplemented with other premisses relating to the above conditions. This is a development in the direction of “totality” [cf. section 2(A)]. Second, any one of the premisses could be explained in its turn. This is a development in the direction of “closedness”, i.e. the resulting chain of explanations would constitute a genetic explanation [cf. section 2(~)]. There are many possible ways to arrive at genetic explanations, which partly depend on the interest of the investigator. One, but only one, of the possibilities is to supplement the original data through some evolutionary approach. The original explanation concerning the presence of a species in some area, presupposes by way of a premiss that the species was previously present in the relevant area. In fact, the explanation only shows that the species can continue to maintain itself. Some informal evolutionary explanation (relating to “evolution” or “origin” in a strict sense, or to geographic data concerning, for example, area expansion) could be invoked to account for the relevant premiss. It is interesting that, in this case, an ecological and an evolutionary (or other type of historical) explanation combine to yield a genetic explanation. In explanations concerning the absence of a species in an area (see previous example), the two types of explanation could coexist without constituting a chain (a genetic explanation). In the latter case they are apt to supplement each other with respect to “totality” rather than “closedness” (see above).

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The reader should bear in mind that absolute “totality” or “closedness” is impossible [cf. section 2(~)]. Still, there is something like a degree of totality and a degree of closedness. This sketchy reconstruction leads to a very important conclusion. The relations between different approaches in biology may greatly differ according to the particular subjects of investigation. It is undesirable to make general statements about the relations between different branches of biology. In particular, the question of whether “ecology” should include an evolutionary approach is misdirected.

The explanation under discussion (concerning a species’ “presence”) could be supplemented in many other ways (e.g. by a more fundamental physiological approach). It is arbitrary, where the process of supplementation is stopped. Contrary to what the formulations of some biologists (e.g. Mayr, 1961, 1969) could suggest, there is no such thing as a set of ultimate causes which may be contrasted withproximate.causes. This terminology is misleading because the distinction is a relative one, contrary to what the term “ultimate” suggests. Mayr suggests that “evolutionary biology” is concerned with “ultimate causes”, and “functional biology” with proximate causes. This is equally misleading: a physiological approach of distribution and abundance, likewise, could be said to provide “ultimate” causes. The present analysis shows that a more sophisticated linguistic machinery is needed to cope with the situation in present-day biology.

6. Conclusions

The structure of the present paper is complicated due to the intricacy of the respective problems and their relations. The following summarizing theses serve to clarify which points are essential. (1) In biology, there is an enormous confusion relating to explanation. The evaluation of alternative explanatory approaches to the same subject matter requires logical tools which have not sufficiently penetrated the discussions among biologists. (2) Some more sophisticated linguistic apparatus is needed to remedy this situation. I propose to borrow part of the terminology from philosophy of science, as sketched in this paper. Even though all the prevailing terminologies are controversial, one of them must be adopted to prevent further confusion. It is obvious that any terminology currently used by biologists, is insufficient. (3) When one of several competing explanations is favoured by an investigator, his decision may depend partly on an overstress of some methodological principle [section 2(c)].

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(4) “Circular” arguments of a certain type are an essential element in scientific reasoning [section 2(~)]. (5) The view that formalization should always lead to increasing possibilities for strict explanations, is misleading. It is unwise to overstress the import of formalization in biology [section 2(~)]. (6) The acceptability of some explanation does not provide sufficient grounds for canceling an alternative explanation. Different acceptable arguments concerning the same data may coexist in various ways [see, for example, section Z(E)]. (7) Many heated arguments about the import of specific explanatory patterns are due to linguistic confusion [see, for example, section Z(E)]. (8) The thesis that there are specific logical types of explanation in biology is debatable. Especially the concept of functional explanation (concerning a special type of “explanation” referring to biological functions) is seldom used in a coherent way [section 3(s)]. (9) Current “individualistic” approaches in population biology are selfdefeating, if carried through to their logical consequences [section 4(o)]. (10) It is not even profitable to argue about the relations between “ecology” and “evolutionary biology” in a general way. The same holds with respect to other domains of biology. The relations involved are not the same in each particular case (section 5, several places). (11) Two seemingly incompatible explanations may supplement each other after some reconstruction [section 5(~)]. (12) Different explanations may supplement each other in quite different ways [section 5(~) 1. REFERENCES H. G. & BIRCH, L. C. (1954). lie Distribution and Abundance of Animals. Chicago: University of Chicago Press. BAKER, H. G. (1966). BioScience 16, 35. BECKNFR, M. (1968). i’7ze BiologicaZ Way of Thought. 2nd Ed. Berkeley and Los Angeles: University of California Press. BECKNER, M. (1969). J. Hist. Biol. 2, 151. BIRCH, L. C. & EHRLICH, P. R. (1967). Nature, Lmd. 214,349. CARNAP, R. (1966). Philosophical Foundations ofphysics. New York, London: Basic Books. EHRLICH, P. R. & HOLM, R. W. (1962). Science, N. Y. 137,652. FEYERAEIEND, P. K. (1963). In Philosophy of Science. (B. Baumrin, ed.) Vol. II. New York: Interscience. GOUDGE, T. A. (1967). The Ascent of Life, a Philosophical Study of the Theory of Evolution. Toronto: University of Toronto Press. HEMPEL, C. G. (1965). Aspects of Scientific Explanation. New York: The Free Press; London: Collier-Macmillan. HULL, D. L. (1967). Evolution, Lancaster, Pa. 21, 174. KUHN, T. S. (1962). The Structure of Scientific Revolutions. Chicago: University of Chicago press. ANDREWARTHA,

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