4 The structure and metaphysics of scientific theories
4.5 A case study: the theory of natural selection
data as the realist holds, or only organizes it as the instrumentalist holds, the theory can do neither without recourse to claims about this realm of unob-servable things, events, processes, properties that an empiricist epistemology makes problematic. But the final epistemic arbiter for science is observation.
And yet, as we shall see below, how observation tests any part of science, theoretical or not, is no easy thing to understand.
lineages whose members have heritable variation in fitness, there will be descent with adaptational modification. Provided that members of these lines of descent reproduce in large enough numbers, then in highly varied environments there will be diversity among the reproducing members of the lines of descent. And if the environments remain stable enough, there will be increasing adaptation from generation to generation, and consequent complexity as well. It is the action of the environment, of course, where
“natural selection” comes in. “Natural selection” is a potentially misleading metaphor which labels the action of the local environment in removing the less fit among members of reproductive lineages competing with one another and with members of other lineages. The environment, on Darwin’s view, does not create adaptations; it doesn’t even actively shape them: it is more like a passive filter of totally independent variations and not at all like an active selector of novel variations it calls forth to choose between.
In order to capture the theory’s generality, we can’t express it as one about giraffes, or mammals, or animals, or even organisms. We need to express it as a claim about reproducing members of any line of (reproduc-tive) descent. So stated, the theory may not be immediately recognized as a claim about the evolution of plant and animal life on Earth. That is because as a general claim about a mechanism of evolution that could obtain any-where in the universe at any time (something needed to make it a scientific law), it can’t mention things that are specific to the Earth. What is more, the lineages of reproducing members on the Earth include much more than the animals and plants we recognize: it will include genes, genomes (sets of genes on the same chromosome, for instance), single-celled asexual organ-isms, families, groups and populations, along with individual organisms – animals and plants. All these items reproduce, show heritable traits and variation in them, and so will participate in distinct evolutionary processes leading to adaptations at different levels of biological organization. Just as having long necks is an adaptation in giraffes whose distribution the theory explains, similarly, being able to survive in boiling water is an adaptation for certain gene sequences, which enables the theory to explain their persis-tence in hot springs all over the world.
Some natural scientists, and some philosophers of science, have argued that being a purely causal theory, which has no room for purpose and tele-ology, Darwin’s theory has of course overturned Kant’s dictum that there will never be a Newton for the blade of grass. If they are correct, Darwin’s mechanism of blind variation and natural selection, along with its twenti-eth-century extensions, which explain heredity and variation in purely phys-ical and chemphys-ical terms, represents a vindication of the scientific program of mechanism that began with Newton.
Notice that the theory of natural selection makes a hypothetical claim: if there is variation in heritable traits and if these variants differ in fitness, then there will be adaptational change. Like the kinetic theory of gases which tells us how gases behave, if they exist, without telling us that there are
gases, Darwin’s general theory does not assert that adaptational evolution obtains. For that conclusion we need initial conditions: the assertion that some things which exist do reproduce, that their offsprings’ traits are inher-ited from their parents, and that these traits are not always exact copies, but do in fact vary from parent to offspring and among offspring. On the Origin of Species, of course, made such assertions about lineages of the many plants and animals Darwin had been studying for 30 years by the time it was published in 1859. Like most other works of biology, it describes a great deal about evolution on this particular planet, along with a general theory about evolu-tion that could be realized by things elsewhere in the universe that look nothing like what we recognize as animals and plants, just so long as they show heritable variations in fitness to their environments.
Another thing to notice about Darwin’s theory is that while evolution by natural selection requires reproduction with heritable variation, it is silent on how reproduction takes place, and tells us nothing about the mechanism of heredity: how traits are transmitted from parent to offspring. It pre-supposes that there is a mechanism of heredity, but it is silent on genetics – the mechanism of hereditary transmission on Earth. And of course, as it is silent on the nature of the hereditary mechanism, it must also be silent on the source of the variations which are continually being manifested from generation to generation and among which the environment “selects” by fil-tering the less fit. Much twentieth-century biology has been devoted to pro-viding the theory of how hereditary variation occurs on Earth. Such a theory is required to apply Darwin’s theory of natural selection in detail to explain the direction and rate of evolution on this planet over the past 3.5 billion years.
Darwin’s theory of natural selection is very general, and very abstract. It doesn’t mention particular biological systems – mammals, animals, eukary-otes – it is silent on how hereditary traits are transmitted, or what the source and rate of variation in these traits is. The general version of Darwinism seems to say so little by itself, that many biologists and philosophers have insisted that these few abstract statements are not the theory. Rather, they treat the set of models that illustrate or realize these principles as the theory, in the way the semantic theory advocates. When we specify differing subject matters for the theory, sexual v. asexual species, plants v. animals, genes v.
individual organisms v. families of individuals, with different mechanisms and rates of variation in hereditary transmission, we generate different models of evolution by natural selection. The generic (not genetic) state-ment of the theory is too abstract and has insufficient content, on this view, to count as the theory of natural selection biologists will recognize. But the wide range of models have enough structure in common to constitute a family of models, just as the semantic theory suggests.
There is another powerful reason to find the semantic view of Darwinian theory attractive. The problem stems from what is perhaps the oldest and at the same time most vexing problem facing the theory of natural selection. It
was a nineteenth-century philosopher, Herbert Spence, who characterized Darwinism as the theory of “the survival of the fittest”, meaning that the fittest will survive to out-reproduce the less fit and by iteration produce evo-lution. And the label “survival of the fittest” has stuck. Indeed, it is not inapt. For it appears that the central claim of the theory can be expressed as follows in the principle of natural selection (PNS):
PNS Given two competing populations, x and y, if x is fitter than y, then in the long run, x will leave more offspring than y.
The trouble arises for the theory when we ask what “fitter than” means. If the PNS is to be a contingent empirical law, then one thing we have to rule out is that differences in fitness are defined as differences in number of off-spring left in the long run. For that would turn the PNS into the explanato-rily uninformative necessary truth that “if x leaves more offspring than y in the long run, then in the long run x will leave more offspring than y”. Logi-cally necessary truths cannot be scientific laws, and cannot explain any con-tingent empirical fact. The PNS could explain differences in offspring numbers on this meaning of fitness only if events (like having more off-spring) can provide their own explanations – something we ruled out in Chapter 2.
We could of course refuse to define fitness. Instead we could just hold, along with realists about theoretical entities, that “fitness” is a theoretical term, like “positive charge” or “atomic mass”. But that seems implausible and unsatisfying. After all, we know that taller giraffes and speedier zebras are fitter without the aid of instruments of indirect observation; we know what fitness is . . . it’s the organism’s ability to solve problems presented to it by the environment: avoiding predators, securing prey, keeping suffi-ciently warm and dry (unless a fish), etc. But why are these the problems which an organism must solve to be fit? How do they combine into overall fitness? How do we compare organisms for fitness when their abilities to solve any one of these problems differ? The most reasonable answers to these questions appear that (a) the problems the environment presents organisms with are ones whose solution increases the organism’s chances to survive and reproduce; (b) we can combine the degree to which an organism solves these various problems by measuring the organism’s number of offspring; and (c) two organisms are equally fit, no matter how differently they deal with environmental problems, provided they have the same number of offspring.
The only thing wrong with these answers is that they show how almost inevitable the temptation is to define “fitness” in terms of reproduction, thus turning the PNS itself into a definition.
The proponent of the semantic approach to theories has little difficulty with this outcome. The semantic theory can accept that the PNS is a defini-tion; theories are sets made up of definitions like the PNS and claims about the different things in the world that satisfy this definition. The variety of
things, even on the Earth, let alone on other worlds in other galaxies, that can realize or instantiate an evolutionary process, whether it be genes, organ-isms, groups and cultures, seems to cry out for a semantic approach to Dar-winism. The theory’s silence on the detailed mechanisms that provide the heredity and the variations in hereditary traits required for evolution here on Earth – nucleic acids and mutations in them – are presumably mechanisms quite different from what we can anticipate finding elsewhere in the uni-verse. This is yet another reason to treat Darwinian theory as a set of models that can be realized in many different ways by many different systems.
Yet a problem remains for the semantic approach, to the theory of natural selection. On the semantic approach a scientific theory is really more than the set of the models that take its name. It’s that set along with the assertion that things in the world realize, satisfy, instantiate, exemplify these defini-tions sufficiently well to enable us to predict their behavior (observable or unobservable) to some degree of accuracy. Without this further assertion, a scientific theory is no different from a piece of pure set-theory. So, even the exponent of the semantic theory must recognize that asserting a theory is to make a substantive claim about the world, in particular, it is to say that the same causal process is at work making all these different phenomena satisfy the same definition. Thus, in the end, like the axiomatic account, the seman-tic approach is committed to the truth of some general claims which them-selves cry out for explanation. It is not really enough then to identify a set of models that share a structure in common and are applicable to a diversity of empirical phenomena, and not explain why they do so. Unless we find our-selves at the end of inquiry when no further explanations of the fundamental laws of nature can be given, there will have to be some underlying mechan-ism or process which is shared among all the different things that realize the same set-theoretical definition, an underlying mechanism which explains why the predictions we can make employing the model are confirmed. Thus, the semantic view of theories has all the same intellectual obligations to explain why theories are true or approximately true or at least moving suc-cessively closer to the truth than the axiomatic account does. That is, it is also committed to the truth of some substantive general laws about the way things are in the world, laws about natural selection among them. So, in the end it will have to face the problems raised by the role “fitness” plays as the key explanatory variable in Darwinian theory.
Summary
The axiomatic account of scientific theories explains how the theoretical laws of a theory work together to provide an explanation of a large number of empirical or observable regularities by treating theories as deductively organized systems, in which the assumptions are hypotheses confirmed by the observations that confirm the generalization derived from them. This conception of laws as hypotheses tested by the consequences deduced from
them is known as “hypothetico-deductivism”, a well-established account of how theories and experience are brought together.
Theories often explain by identifying the underlying unobserved processes or mechanisms that bring about the observable phenomena which test the theories. Reductionism labels a long-standing view about the relationship of scientific theories to one another. According to reductionism, as a science deepens its understanding of the world, narrower, less accurate and more special theories are revealed to be special cases of or explainable by deriva-tion from broader, more complete, more accurate and more general theories.
Derivation requires the logical deduction of the axioms of the narrower theory from the broader theory, and often the correction of the narrower theory before the deduction is effected. Reductionists seek to explain the progress of science over the period since the Newtonian revolution by appeal to these inter-theoretical relations. The reduction of scientific theories over centuries, which seems to preserve their successes while explaining their fail-ures (through correction), is easy to understand from the axiomatic perspect-ive on the structure of scientific theories.
However, the hypothetico-deductivism of the axiomatic account of theo-ries, and indeed the general epistemological perspective of science as based on observation and experiment, faces grave difficulty when it attempts to explain the indispensability of terms in theories that identify theoretical, unobservable entities, like cellular nuclei, genes, molecules, atoms and quarks. For on the one hand, there is no direct evidence for the existence of the theoretical entities these terms name, and, on the other hand, theory cannot discharge its explanatory function without them. Some theoretical entities, such as gravity, are truly troublesome and at the same time, we need to exclude from science mysterious and occult forces and things for which no empirical evidence can be provided. The notion that meaningful words must eventually have their meanings given by experience is an attractive one. Yet finding a way for theoretical language to pass this test while excluding the terms of uncontrolled speculation as meaningless is a challenge that an account of scientific theories must face.
The puzzle, that hypothesizing theoretical entities is indispensable to explanation and unregulated by experience, is sometimes solved by denying that scientific theories seek to describe the underlying realities that system-atize and explain observational generalizations. This view, known as instru-mentalism, or antirealism, treats theory as a heuristic device, a calculating instrument for predictions alone. By contrast, realism (the view that we should treat scientific theory as a set of literally true or false descriptions of unobservable phenomena), insists that only the conclusion that theory is approximately true can explain its long-term predictive success. Instrumen-talists controvert this explanation.
The axiomatic approach to theories has difficulty accommodating the role of models in science. Instrumentalism does not, and as models become more central to the character of scientific theorizing, problems for the axiomatic
approach and for realism mount. The issue here ultimately turns on whether science shows a pattern of explanatory and predictive successes which can only be explained by realism and the existence of theories that organize and explain the success of the models scientists develop.
Darwin’s theory of natural selection provides a useful “test bed” for apply-ing and assessapply-ing the adequacy of some of the competapply-ing conceptions of scientific theory articulated in this chapter.
Study questions
1 Deductive or axiomatic systems do not seem to provide an illuminating account of how the components of a theory “work together”. After all, any two laws can figure as the axioms of some theory or other made up on the spur of the moment. Can you offer a more precise notion of how the laws in a theory “work together”?
2 Is “constructive empiricism” really a viable middle course between instrumentalism and realism?
3 Evaluate the following argument for realism: “As technology progresses, yesterday’s theoretical entities become today’s observable ones. Nowa-days we can detect cells, genes and molecules. In the future we will be able to observe photons, quarks, etc. This will vindicate realism.”
4 What makes the semantic approach, with its emphasis on models, more amenable to instrumentalism than to realism?
5 Does instrumentalism owe us an explanation of the success of science? If so, what is it? If not, why not?
6 Can the causal mechanism of variation and selection which Darwin uncovered be applied to explain the purposive character of phenomena beyond those of interest strictly to biologists, such as anatomy? For example, can it be employed to explain human behaviors and human social intuitions as the results of variation and environmental selection, and not the conscious choice of individuals or groups of them?
Suggested reading
The history of philosophical analysis of scientific theorizing is reported in F. Suppes, The Structure of Scientific Theories. The axiomatic approach was perhaps first fully articulated in R. Braithwaite, Scientific Explanation.
Perhaps the most influential and extensive account of theories, and of science in general to emerge from the period of logical empiricism is E. Nagel, The Structure of Science, first published in 1961. This magisterial work is worthy of careful study on all topics in the philosophy of science. Its account of the nature of theories, its development of examples, and its identification of philosophical issues remains unrivaled. Nagel’s discussion of the structure of theories, of reductionism and the realism/antirealism issue set the agenda for the next several decades. Two extracts from this work are to be found in
Balashov and Rosenberg, Philosophy of Science: Contemporary Readings, “Experi-mental Laws and Theory” discusses the relationship between theories and the generalizations they explain, and “The Cognitive Status of Theories”
illustrates the post-positivist treatment of the realism/instrumentalism debate.
The view of scientific progress reflected in Nagel’s notion of reduction is examined in W. Newton-Smith, The Rationality of Science. M. Spector, Con-cepts of Reduction in Physical Science, and A. Rosenberg, The Structure of Biologi-cal Science, expound and examine the relations among theories in these two compartments of natural science. But many papers have been written and continue to appear on this issue especially in the journals, Philosophy of Science and The British Journal for Philosophy of Science. P. Feyerabend’s vigorous attack on the complacent picture of progress as reduction, “Explanation, Reduction, and Empiricism”, reprinted in Balashov and Rosenberg, has been very influential, especially when harnessed together with some interpreta-tions of Thomas Kuhn’s views, as we shall see in Chapter 6. Kitcher, “Theo-ries, Theorists, and Theoretical Change”, offers a sophisticated discussion of theoretical continuity through replacement, with particular reference to the case of phlogiston and oxygen. This paper, too, is reprinted in Balashov and Rosenberg, and treats matters also taken up again in Chapter 6.
Hempel’s paper, “The Theoretician’s Dilemma”, in Aspects of Scientific Explanation, expresses the problem of reconciling the indispensability of theoretical entities for explanation with the empiricist demand that the terms naming those entities be observationally meaningful. Other papers in Aspects, including “Empiricist Criteria of Significance: Problems and Changes”, reflect these problems. Among the earliest and most vigorous post-positivist arguments for realism is J.J.C. Smart, Between Science and Philosophy. The debate between realists and antirealists or instrumentalists to which Hempel’s problem gives rise is well treated in J. Leplin (ed.), Scientific Realism, which includes papers defending realism by R. Boyd and E. McMullin, a development of the “pessimistic induction” from the history of science to the denial of realism by L. Laudan, a statement of van Fraassen’s
“constructive empiricism”, and a plague on both realism and antirealism pronounced by Arthur Fine, “The Natural Ontological Attitude”. Van Fraassen’s views are more fully worked out in The Scientific Image. J. Leplin, A Novel Argument for Scientific Realism is a more recent defense of realism against van Fraassen and others. P. Churchland and C.A. Hooker (eds), Images of Science: Essays on Realism and Empiricism, is a collection of essays discussing
“constructive empiricism”. Laudan’s arguments against realism are power-fully developed in “A Confutation of Convergent Realism”, reprinted in Balashov and Rosenberg. This anthology also includes an illuminating discus-sion of van Fraassen’s views and realism by Gutting, “ Scientific Realism v.
Constructive Empiricism: A Dialogue”, and a historically informed defense of realism, Ernest McMullin, “A Case for Scientific Realism”.
The semantic view of theories is elaborated by F. Suppes in The Structure