iSoul In the beginning is reality.

What is the theory of evolution?

Philosophers of biology try to clarify what scientists are really doing. One answer to “what is the theory of evolution?” is given here excerpted from “Philosophy of Biology” by Thomas Pradeu (Paris-Sorbonne University).  http://thomaspradeu.com/wp-content/uploads/2011/11/Pradeu_Philo-bio_OUP_Final.pdf

“Philosophy of biology” refers to the critical examination of the conceptual, theoretical and methodological foundations of today’s life sciences. … Important founders include David Hull (1935-2010), Michael Ruse (born in 1940), and Elliott Sober (born in 1948). …

In this presentation, I hope to show the diversity of the problems posed in philosophy of biology by drawing attention to seven of them. …

1. The status of the theory of evolution
The theory of evolution is generally considered to be the foundation to every proposition in biology, as well as the primary, if not unique, biological theory. What then, precisely, does “the theory of evolution” mean?

The aim of the theory of evolution is to explain modifications in species over time; their adaptations and their diversification. Darwin was not the first to put forward an explanation for this phenomenon, nor to speak of species evolution (this idea can be found in Lamarck, in Erasmus Darwin, etc.). Nonetheless, Darwin (1859) advanced two decisive theories: common descent (captured in a species tree), that is, the assertion that today’s organisms are descended from common ancestors; and natural selection, according to which there is a process of variation and then of differential survival and reproduction amongst organisms (the “struggle for existence” leading to the “survival of the fittest”, to use the expression Darwin would eventually borrow from Spencer). So what we call the “theory of evolution” is a set of propositions initially put forward by Darwin and then, between the 1920s and 1950s, solidified around the central ideas of common ancestry and natural selection by those active in the “Modern evolutionary synthesis” (Mayr and Provine, 1980). However, as much in Darwin’s case as in the case of the Modern Synthesis, speaking of the theory of evolution causes problems.

Firstly, can we truly speak of the theory of evolution? According to Mayr (1982), Darwin does not propose one but five theories: evolution as such, common descent, gradualism (the idea that species evolution occurs by means of cumulated minor modifications and not by “leaps”), population speciation (the idea of a continuity between population and species, a population of living creatures which undergoes variation being considered as a “nascent species”), and natural selection. Each of these theories met with a different fate. In particular, common ancestry was very quickly accepted by biologists following the publication of The Origin of the Species, while natural selection was neither well understood nor widely accepted in Darwin’s own lifetime. Even though Darwin held to each of them, taken together they did not constitute a unified theoretical structure (Mayr, 1982). Furthermore, precisely as a result of this plurality of ideas in Darwin, they were on the verge of being abandoned at the turning of the 20th century: following work which had rediscovered Mendel’s “laws” of heredity, a certain tension arose between gradualism and speciation (Bowler 1983; Gayon, 1998). Darwin was in the dark regarding the mechanism behind variation in individuals, contenting himself to simply observe the phenomenon. But, to his eyes, it was clear that the variations were gradual and not saltatory. The first “geneticists” found the mechanism of variation in what they called “mutations” but, according to them, mutations were quite precisely leaps and not gradual modifications: for de Vries, in particular, species appeared suddenly following one of these mutations (Allen 1969). The Darwinian theory of gradualism and natural selection thus found themselves strongly rejected (Bowler 1983; Gayon, 1998). The first step of the Modern Synthesis (corresponding roughly to a period between the 1920s and 1930s) was the unification of genetics and Darwinism, primarily under Fisher’s (1930) influence. Fisher showed that mutations, whose effects are generally limited, are perfectly compatible with Darwinian gradualism and in fact account for the variating mechanism so desperately sought since Darwin’s day. It would, however, be erroneous to think that the Modern Synthesis lead to a unified theory of evolution. The second step of the Modern Synthesis(roughly from the 1930s to the 1950s) involved the aggregation of various disciplines of biology (zoology, botany, systematics, etc.) around a “solid core” of hypotheses (Mayr and Provine, 1980). So the Modern Synthesis came about more as a result of a sociological convergence (the unification of practically all the branches of biology on the basis of principles relative to evolution) than by the formulation of one theory of evolution (Gayon 1998:xiv).

Nevertheless, can we take the common principles all biologists have accepted since the Modern Synthesis and use them to deduce propositions for the “theory of evolution”? This leads us to our second question: can we really speak of a theory of evolution? Concerning Darwin’s own ideas, we should perhaps speak not so much of a veritable theory as of a descriptive generalization which created a paradigm (in the sense of an exemplary model, widely imitated afterward) for understanding species evolution, at least in as far as common descent is concerned (Gayon, 1998). Nevertheless, it has often been emphasized (e.g., Ghiselin 1969; Lewens 2007a; Sober 2011) that Darwin had complied to the canons of theory construction of his time, and in particular to the views of Whewell and Herschel. Concerning the theory of evolution as it has been presented since the Modern Synthesis, philosophers of science have attempted to determine whether or not it constitutes a veritable theory. Many are the philosophers who have doubted its validity as a theory, their primary argument being that biology, since it is a “historical” science, cannot formulate laws, and hence cannot offer theories in a nomological sense (Smart, 1963; see also Beatty 1995). Most of Smart’s arguments are invalid and rely on a false understanding of biology (Ruse, 1973; Hull, 1969, 1977): contrary to his claims, biology deals not with such and such albino mouse but with processes of a much wider scope, like the conditions for the expression of recessive genes, crossing-overs, the notion of geographically isolated populations, etc. – which all are processes about which generalizations are possible. On the other hand, it is undeniable that biological entities are spatiotemporally situated within an evolutionary history: for example, a biological species is a historical entity, the product of an evolutionary history, and not a class of objects open to abstract generalization which disregards spatiotemporal conditions, as is standard in physics. Consequently, formulating laws of biology, that is, general abstract propositions, at the level of historical entities, seems impossible. But for Hull, for instance, biology can formulate laws about entities that are not defined genealogically, in particular at levels of organization higher than particular taxa (Hull 1978: 353-354). From this point of view, the claim that biology could not offer laws at all is misleading (Hull 1976, 1978).It is, however, difficult to assess the scope of the claim that laws cannot be formulated about historical entities: in the context of that debate, doesn’t physics run the risk of isolating itself from the majority of other empirical sciences, all “historical” in the sense we have defined, such as biology, geology, and the social sciences? If physics be the only science capable of formulating laws, should it remain a model for philosophy of science in general? Furthermore, certain branches of physics, like astronomy, also deal with historical entities. If the future reveals all empirical sciences to be “historical”, wouldn’t we have to soften our stipulation that a science must necessarily produce (spatiotemporally unrestricted) laws? Alternatively, we could suggest other, more “relaxed”, conceptions of what a “law” is. Finally, the implicit assertion stating that a science cannot advance any theories once it advances no laws, must be handled with caution, as it depends on one particular vision of theories which, we shall now show, is not only not well suited to biology, but is also not the only vision of scientific theories possible.

In the 1970s, philosophers of biology brought precision to the debate around the problem of the theory of evolution’s being a genuine theory or not by posing the following question: If the theory of evolution is a theory, is this in the “syntactic” or “semantic” sense of the term? According to the syntactic conception, best expressed by Hempel (1965), a theory is a hypothetico-deductive system in which, based on just a few axioms, one must be capable of deducing a large number of propositions. According to the semantic conception, defended in particular by van Fraassen (1972) and Suppe (see in particular Suppe 1977), a theory is a collection of models that must serve as the representation of empirical phenomena. In the semantic conception, to describe a theory is to present a class of models and to specify the manner in which those models reflect the phenomena. It quickly became apparent that the theory of evolution was not a theory in the syntactic sense of the term. Several biologists (M. B. Williams, 1970; Lewontin, 1970) and philosophers (Ruse, 1973) have attempted an axiomatization of the theory of evolution, but this has lead more to uncovering the theory of evolution’s “structural core” than it has to a veritable axiomatization: by means of a method exemplified by Lewontin (1970), they pushed themselves to defining the minimal conditions a population of individuals must meet to be said to evolve by natural selection (for a fresh look at these questions, see Godfrey-Smith, 2007 and 2009). The most enthusiastic advocates for an axiomatization of the theory of evolution finally ended up showing that this effort could only be partial (M. B. Williams, 1981). Several philosophers of biology have defended the view that if the theory of evolution is a “theory,” then it is so in the semantic rather than the syntactic sense of the term (e.g., Thompson 1983; Lloyd, 1988): it can be interpreted as a collection of models that must serve as the representation of empirical phenomena (important discussions of models in recent philosophy of biology, with a different perspective than that of Thompson or Lloyd, include Godfrey-Smith, 2006 and Weisberg 2006). Were a consensus to emerge regarding the semantic conception of scientific theories (but see Ereshefsky 1991), the oft repeated claim that the theory of evolution is not really a theory would have to be just as quickly flatly rejected. The work carried out by philosophers and biologists on the structure of the theory of evolution by natural selection since the end of the 1980s (e.g., Lloyd, 1988; Gould, 2002) sets its goal precisely as the clear definition of these models and the conditions for their testing.

July 2014

Post Navigation