The many conceptual and empirical advances in evolutionary biology during the second half of the twentieth century that I have briefly sketched in part I of this essay naturally led to a broader theoretical turmoil. More and more people felt like the Modern Synthesis (MS) was increasingly becoming too restrictive a view of evolution to keep playing the role of biology’s “standard model.” This group included Carl Schlichting and myself, Mary Jane West-Eberhard (2003), Eva Jablonka, and others. But arguably none made a more concerted, if partial, effort than Stephen Jay Gould in his magnum opus, The Structure of Evolutionary Theory, published in 2002.

The Structure is comprised of two parts, one tracing the history of evolutionary ideas, both pre-and post-Darwin, and the second one presenting Gould’s view of contemporary theoretical debates within the field. While the constructive part of the book focuses too much on paleontology and multilevel selection, Gould correctly identified three conceptual pillars of Darwinism that got imported wholesale into the Modern Synthesis:

1. Agency: the locus of action of natural selection. For Darwin, this was the individual organism, while within the MS the focus expanded to the gene, thus leading to an overall increase of agency. Gould advocated further expansion, to include multiple levels of selection, from the gene to the individual to kin groups to species. This suggestion is perfectly in line with that of other authors advocating an Extended Evolutionary Synthesis (EES).

2. Efficacy: the causal power of natural selection relative to other evolutionary mechanisms. According to Darwin, natural selection is the chief mechanism of evolutionary change, and certainly the only one capable of producing adaptation. The MS formally described—by means of population genetic theory—four additional mechanisms: mutation, recombination, migration, and genetic drift. Gould adds a positive role for developmental constraints to the picture, and advocates of the EES further expand on this theme, including concepts such as those of evolvability (i.e., change over time of evolutionary mechanisms themselves), facilitated variation (from developmental biology), and niche construction (from ecology), among others.

3. Scope: the degree to which natural selection can be extrapolated from micro-to macro-evolutionary outcomes. As we have seen last time, this has been controversial early on, with the MS settling for the same basic picture proposed by Darwin: so-called macro-evolutionary processes are simply micro-evolutionary ones writ large. Gould, of course, questions this, on the basis of the already discussed theory of punctuated equilibria. Proponents of the EES also doubt the received view, suggesting that species selection and group-level ecological characteristics may partially, though not entirely, decouple micro-from macro-evolution.

If Gould’s general take is right, then, evolutionary theory has changed over time and the process can best be tracked conceptually by keeping tabs on changes in the agency, efficacy, and scope of natural selection within the theory. This, incidentally, makes natural selection the fundamental idea in biological evolution, and rightly so. No other concept, not even that of common descent, has had such a complex and convoluted history within the field. Moreover, what the EES is attempting to do can also be understood within Gould’s framework.

Now, as we have seen so far, the latter part of the twentieth century and the beginning of the twenty-first century have seen a renewed debate about the status of contemporary evolutionary theory, with a number of calls for an expansion of the Modern Synthesis into an Extended Evolutionary Synthesis. But what does the latter look like, at the current state of the discussion?

I provided an early sketch of it in a paper published in Evolution back in 2007 (available to Socratic level subscribers from my archives), and an updated and expanded version of that sketch has been put out by Laland and collaborators in 2015. My early analysis began by noting that philosopher Karl Popper famously interpreted the MS as a theory of genes, lacking a comparable theory of forms (i.e., phenotypes). The field got started, however, as a theory of forms in Darwin’s days, with genetics taking on a fundamental role only after the rediscovery of Mendel’s work at the turn of the twentieth century. Consequently, I suggested, a major goal that an EES aims for is an improvement and unification of our theories of genes and of forms. This, seems to me, may best be achieved through an organic grafting of novel concepts onto the foundational structure of the MS, particularly evolvability, phenotypic plasticity (i.e., the ability of a single genotype to produce different phenotypes in response to environmental variation), epigenetic inheritance, complexity theory (from mathematics), and the theory of evolution in highly dimensional adaptive landscapes (from population genetics).

Laland et al.’s paper from 2015 is the most focused and systematic attempt to articulate the EES, explicitly aiming at clearing away inconsistencies in previous works. They begin with a comparison of core assumptions of the MS versus the EES. To give you an idea of what they are getting at, here are the entries for inheritance:

Genetic inheritance (MS): Genes constitute the only general inheritance system. Acquired characters are not inherited.

Inclusive inheritance (EES): Inheritance extends beyond genes to encompass (transgenerational) epigenetic inheritance, physiological inheritance, ecological inheritance, social (behavioural) transmission and cultural inheritance. Acquired characters can play evolutionary roles by biasing phenotypic variants subject to selection, modifying environments and contributing to heritability.

They then run through a series of alternative interpretations of important evolutionary phenomena according to the two frameworks. For instance, in the case of developmental plasticity:

MS: conceptualized as a genetically specified feature of individuals that can evolve under selection and drift. Focus is on the conditions that promote adaptive evolution of plastic versus non-plastic phenotypes. The primary evolutionary role of plasticity is to adjust phenotypes adaptively to variable environments. Plastic responses regarded as pre-filtered by past selection.

EES: considers reducing plasticity to a genetic feature to be explanatorily insufficient. Retains an interest in adaptive evolution of plasticity, but also focuses on how plasticity contributes to the origin of functional variation under genetic or environmental change, and how the mechanisms of plasticity limit or enhance evolvability, and initiate evolutionary responses. Many plastic responses viewed as reliant on open-ended (e.g., exploratory) developmental processes, and hence capable of introducing phenotypic novelty.

Moreover, Laland et al. provide readers with a comparison of different predictions originating from the competing frameworks. For instance, in the case of the relationship between genetic and phenotypic change:

MS: genetic change causes, and logically precedes, phenotypic change, in adaptive evolution.

EES: phenotypic accommodation (a non-genetic process) can precede, rather than follow, genetic change, in adaptive evolution.

Laland et al. also present a graphical outline of the structure of the Extended Evolutionary Synthesis, as they see it . It is instructive to comment on a number of features of their model. Phenotypic evolution—the target of explanation of the entire framework, just as it was for Darwin—is assumed to be affected by three classes of processes: those that generate novel variation, those that bias selection, and those that modify the frequency of heritable variation.

Beginning with the first class, these processes include classical ones like mutation, recombination, gene expression, and developmental regulatory processes. But also EES-specific ones like environmental induction (of developmental processes), niche construction, phenotypic accommodation, and facilitated variation. The second class (processes that bias selection) include only EES-related entries: developmental bias and niche construction, while the third class (processes that affect heritable variation) are all classical (mutation pressure, selection, drift, and gene flow) but are in turn affected by the previous class.

The resulting picture is one of complete and, seems to me, highly coherent, meshing of the MS and the EES perspectives, where the latter adds to but does not really replace any of the previously recognized mechanisms. Which brings me to the next question I wish to address concerning the most recent developments of the now more than 150-year-old Darwinian tradition: is the proposed shift from the MS to the EES akin to a Kunhian paradigm shift?

One of the most controversial aspects of the discussion surrounding the MS versus EES debate is the extent to which the new framework is claimed to be distinct from the old one. At one extreme, there are scientists who simply reject the idea that the EES presents much that is new, claiming that whatever new concepts are being advanced were in fact already part of the MS, either implicitly or explicitly. At the opposite extreme, some supporters of the EES have been making statements to the effect that the new framework somehow amounts to a rejection of fundamental aspects of Darwinism, akin to what philosopher Thomas Kuhn famously termed a “paradigm shift” within the discipline, thus aligning themselves with a tradition that can be fairly characterized as anti-Darwinian. My own position has always been that the truth lies somewhere in the middle (in this case!): the EES is significantly different from the MS, and yet the change does not reflect any kind of scientific revolution within modern biology, but rather more of the same process that has led us from the original Darwinism to neo-Darwinism to the MS itself.

Kuhn famously argued—on the basis, crucially, of examples drawn exclusively from physics—that science goes through an alternation of two phases: during “normal” or “puzzle solving” science, practitioners are focused on addressing specific issues from within a given theoretical framework and set of methods (the “paradigm”), which itself is not the target of empirical testing or conceptual revision. From time to time, however, a sufficient number of “anomalies,” or unresolved puzzles, accumulate and precipitate a crisis within the field. At that point scientists look for a new paradigm, better suited to take into account the insofar unresolved issues. If they find it, the new framework is quickly adopted and deployed in turn to guide a new phase of normal science.

Kuhn suggested a number of approaches to tell whether a paradigm shift has occurred (or, in our case, is in the process of occurring). These include five criteria for theory comparison, as well as three classes of potential incommensurability between theories. Let’s begin by examining the five criteria: (1) accuracy, (2) consistency (internal and with other theories), (3) explanatory scope, (4) simplicity, and (5) fruitfulness of the accompanying research program. Here is how the MS and EES compare, in my mind, according to the Kuhnian criteria:

Accuracy, MS: building on the original Darwinism, it has produced quantitative accounts of the change over time of the genetic makeup of natural populations.

Accuracy, EES: incorporates the same methods and results of both the original Darwinism and the MS, adding the explanation of developmental and other self organizing biological phenomena.

Consistency, MS: as internally consistent as any major scientific theory, features explicit external links to genetics, molecular biology, and ecology.

Consistency, EES: same degree of internal and external consistency as the MS, with the addition of external links to developmental biology, genomics, and complexity theory, among others.

Scope, MS: new facts about the biological world that are explained have been consistently uncovered for the past several decades.

Scope, EES: further expands the scope of the MS by explicitly including questions about the origin of evolutionary novelties, the generation of biological form, and the problem of genotype–phenotype mapping.

Simplicity, MS: uses a limited number of mechanisms (natural selection, genetic drift, mutation, migration, assortative mating) to account for evolutionary change over time.

Simplicity, EES: makes use of all the mechanisms of the MS, adding a number of others such as epigenetic inheritance, evolvability, facilitated (i.e., self-emergent) variation, etc.

Fruitfulness, MS: has a history of more than 70 years of vigorous research programs, building on the previous fruits of the original Darwinism.

Fruitfulness, EES: builds on the ongoing research program of the MS but has also already led to empirical (e.g., emergent properties of gene networks and of cell assemblages) and conceptual (e.g., evolvability, phenotypic plasticity) discoveries, though of course it is very much a work in progress as of the moment of this writing.

Even this brief survey ought to make it clear that the MS => EES is not a paradigm shift, but rather an organic expansion. Then there is the second test proposed by Kuhn to consider, a test in a sense more stringent, that of incommensurability. If two theories are incommensurable in even one of the three classes, a good argument can be made that a paradigm shift is occurring. The classes in question are methodological, observational, and semantic.

Methodological incommensurability refers to the notion that different paradigms lead scientists to pick different “puzzles” as objects of research, as well as to the idea that scientists then develop distinct approaches to the solution of those puzzles. The EES takes on board the same puzzles, and the same set of approaches, of the MS, but it also adds new puzzles (such as the appearance of so-called evolutionary novelties, like eyes, feathers, spines, and so forth), which were largely untouched, or dealt with only superficially, by the MS. It further adds new approaches, like interpretations of evolutionary changes in terms of niche construction, developmental plasticity, or epigenetic inheritance.

Observational incommensurability is tightly linked to the idea that observations are theory dependent: what is considered a “fact” within one theoretical context may not be such in a different theoretical context. For instance, in pre-relativity physics there was a (supposed) fact of the matter that some kind of substance, referred to as ether, had to be present in space in order for light to travel through it. After the famous Michelson–Morley experiment demonstrating that there was no such thing as ether, the relevant fact became the constancy of the speed of light and therefore the relativity of frames of reference. Nothing like that seems to be happening in evolutionary biology at the moment: the very same facts that have been catalogued and explained by the MS enter into the empirical corpus of the EES, to be further expanded with new facts that come to the forefront because of the additional conceptual advancements.

Semantic incommensurability has to do with shifts in the meaning of terms used by scientists, one of Kuhn’s examples being that of “mass,” which is a conserved, static quantity in Newtonian mechanics, but becomes interchangeable with energy within the framework of Einstein’s relativity. Again, I do not discern any analogous shift in the terminology used by proponents of the MS versus EES. Key biological concepts, such as species, genes, phenotypes, niche, and so forth, retain similar and perfectly commensurable meanings, even though our understanding of their referents becomes increasingly sharp.

It seems, therefore, that Darwinism after the Modern Synthesis has proceeded along similar lines to those followed by Darwinism before the MS: a continuous expansion of both empirical knowledge and conceptual understanding, an expansion that is likely to continue for the remainder of the current century and beyond.

This discussion is in part an opportunity to call for a bit of house cleaning, so to speak, on the part of evolutionary biologists and philosophers of science. For instance, it is truly astounding that in France the Modern Synthesis, and in particular population genetics, was not included in standardized university curricula, or addressed within main research programs until the 1970s. Against the Darwinian picture that was developing abroad, French life scientists supported various forms of Lamarckism throughout the twentieth century, and some of that attitude still lingers. There is no good scientific reason for that, and it is hard not to pin such an attitude on sheer nationalism and the cultural worship of Lamarck. Needless to say, that sort of thing has no place in a mature science. The French are not the only culprits here, and the fact that there are “German,” “Russian,” and other “traditions” within evolutionary biology is more than a little bizarre.

It’s also somewhat surprising that behavioral biologists are still clinging to simplistic notions from sociobiology and evolutionary biology, which have long since been debunked. It’s not the basic idea that behaviors, and especially human behaviors, evolve by natural selection and other means that is problematic. The problem, rather, lies with some of the specific claims made, and methods used, by evolutionary psychologists.

It is also both surprising and problematic that some researchers are still pursuing non-“mechanistic” or non-“physicalist” research programs, whatever that means. Indeed, a major point of the EES is to help bring the focus back on the organism and even the ecosystem, and yet—as I just argued above—this does not require a wholly alternative synthesis at all.

Over time, Darwinism has advanced its own agenda by incorporating a variety of themes proposed by its critics, including “saltationism” (punctuated equilibrium) and “Lamarckism” (epigenetic inheritance, phenotypic plasticity, and niche construction). This is fine, so long as we keep in mind that the terms within scare quotes above are to be understood in a modern, radically updated sense, and not along the lines of what biologists were thinking decades or even centuries ago. It’s this inherent flexibility of Darwinism that has allowed people with views as divergent as Stephen Jay Gould and Richard Dawkins to (rightly) claim the Darwinian mantle.

This ability to incorporate critical ideas is neither just a rhetorical move nor somehow indicative of serious problems inherent in the Darwinian approach. In the end, the various Darwinian traditions in evolutionary biology are best understood as a wide ranging family of conceptual and research approaches, always in dialectic dialogue with each other, always in a constructive tension that transcends the agendas and (sometimes strong) personalities of the many individual scientists that recognize themselves as intellectual descendants of Charles Darwin. More than a century and a half later, evolutionary theory keeps evolving.

Scientific theories are always provisional accounts of how the world works, intrinsically incomplete, and expected to be replaced by better accounts as science progresses. The theory of evolution, colloquially referred to as “Darwinism,” is, of course, no exception. It began in 1858 with joint papers presented to the Linnaean Society by Charles Darwin and Alfred Russell Wallace and was formalized shortly thereafter in On the Origin of Species. The original theory featured two conceptual pillars: the idea of common descent (which was accepted by a number of scholars even before Darwin), and that of natural selection as the chief mechanism of evolution, and the only one capable of generating adaptation.

The first bit of tinkering took place shortly thereafter, when Wallace himself, together with August Weismann, proposed to drop any reference to Lamarckian theories of heredity because of the newly proposed notion of the separation between sexual and somatic cellular lines, thus generating what is properly known as neo-Darwinism. After undergoing a temporary crisis, as a result of increasing skepticism from paleontologists and developmental biologists, we enter two phases of the so-called Modern Synthesis, the biological equivalent of the Standard Model in physics: the first phase consisted in the reconciliation between Mendelism (i.e., genetics) and Darwinism (i.e., the theory of natural selection), leading to the birth of population genetics; the second phase consisted in an expansion of the theory to include fields like natural history, population biology, paleontology, and botany.

What happened to “Darwinism” after 1950? The Modern Synthesis (MS) reigned as the dominant paradigm in the field, rather unchallenged until the late 1980s and early 1990s. At which point a number of authors, coming from a variety of disciplines, began to question not so much the foundations but the accepted structure of the MS. By the very late twentieth-century and early twenty-first-century, calls to replace the MS with an Extended Evolutionary Synthesis (EES) had begun to grow loud, and to be countered by equally loud voices raised in defense of the MS. How did this happen, and what does it mean for the current status and future of evolutionary theory? To understand this we need to step back for a moment and take a broad view of conceptual developments in the biological sciences during the second half of the twentieth century.

The second half of the twentieth century has been an incredibly exciting time for biology, a period that has put the discipline on the map at least at the same level of interest as physics, the alleged queen of sciences, and arguably even more so. Let me remind you of some of the major developments that have made this possible, because they all—directly or indirectly—eventually fed into the current discussion over the MS versus the EES as dominant conceptual frameworks in evolutionary biology.

A major breakthrough in one of the foundational fields of the Modern Synthesis, population genetics, came with the invention of a technique called gel electrophoresis, which for the first time made it possible to directly assess protein and gene frequencies in large samples drawn from natural populations. While research on electrophoresis began as early as the 1930s, it was the breakthrough work of Richard Lewontin and John Hubby in 1966 that set population genetics on fire. The unexpected discovery was, as the authors put it, that “there is a considerable amount of genic variation segregating in all of the populations studied …[it is not] clear what balance of forces is responsible for the genetic variation observed, but [it is] clear the kind and amount of variation at the genic level that we need to explain.” This new problem posed by a much larger degree of genetic variation than expected in natural populations eventually led to a revolution in population genetics, and also directly to the origination of the impactful neutral theory of molecular evolution first proposed in 1968 by Motoo Kimura.

The neutral theory was a landmark conceptual development because for the first time since Darwin it challenged the primacy of natural selection as an agent of evolutionary change. To be sure, Kimura and colleagues didn’t think that phenotypic evolution (i.e., the evolution of complex traits, like eyes, hearts, etc.) occurred in a largely neutral fashion, but if it turned out that much of what goes on at the molecular level is independent of selective processes, then the obvious question is how is it possible that largely neutral molecular variation can give rise to non-neutral phenotypic outcomes. Eventually, the debate about the neutral theory—which raged on intensely for a number of years—was settled with a sensible and empirically consistent compromise: a lot of molecular variation is “near-neutral,” which means that the role of stochastic processes such as genetic drift at the molecular level is significantly higher than might have been expected on the basis of a face-value reading of the tenets of the Modern Synthesis.

What could possibly connect the near-neutral molecular level with the obviously functional and therefore likely selected phenotypic level? The obvious answer was: development. The only problem was that developmental biology had famously been left out of the Modern Synthesis. It looked like something was seriously amiss with modern evolutionary theory.

Things began to change as an offshoot of yet another revolution in biology: the rapid advances made in molecular biology after the discovery of the structure of DNA in 1953. While molecular biology kept accelerating its pace independently of organismal biology for several decades—until their confluence in the era of evolutionary genomics—in the late 1970s the existence of homeotic genes regulating embryonic patterns of development in Drosophila was discovered. It soon turned out that this and similar classes of regulatory genes are both widespread and evolutionarily conserved (i.e., they don’t change much over time), so that they are one of the major keys to the understanding of the complex interplay among genotype, development, and phenotype.

This new approach eventually flourished into a new field, known as evolutionary developmental biology, or evo-devo for short, and one of its major contributions so far has been a marked shift of emphasis in the study of morphology and development, from the sort of classical population genetic studies focused on structural genes to an emphasis on regulatory genes and their potential to help us build a credible theory of the origin of evolutionary novelties (i.e., new structures like wings or flower). As Prud’homme and colleagues put it in 2007:

Because most animals share a conserved repertoire of body-building and -patterning genes, morphological diversity appears to evolve primarily through changes in the deployment of these genes during development. … Morphological evolution relies predominantly on changes in the architecture of gene regulatory networks and in particular on functional changes within [individual] regulatory elements. … Regulatory evolution: (i) uses available genetic components in the form of preexisting and active transcription factors and regulatory elements to generate novelty; (ii) minimizes the penalty to overall fitness by introducing discrete changes in gene expression; and (iii) allows interactions to arise among any transcription factor and [regulatory genes].

The picture that emerges from this and many other studies is not incompatible with the simple mathematical models that were incorporated into the Modern Synthesis, but it does present us with a much more complex and nuanced understanding of genetic, developmental, and phenotypic evolution, so much so that it is little wonder that people have been increasingly referring to the current, very much in flux, version of evolutionary theory as the Extended Synthesis.

I have already mentioned the molecular biology revolution initiated in the 1950s, which eventually led to the genomic revolution. Both these radical developments initially affected evolutionary biology only indirectly, by providing increasingly powerful new analytical tools, such as gel electrophoresis, and later on gene sequencing. But inevitably genomics itself became an evolutionary science, once technical developments made it possible to sequence entire genomes more quickly and cheaply, and molecular biologists fully internalized, as Theodosius Dobzhansky famously put it, that nothing in biology makes sense except in the light of evolution. The structure and function, as well as the sheer diversity, of genomes are themselves not understandable if not through evolutionary lenses, so that genomics and evolutionary biology currently represent a rare example of synergism between scientific disciplines: the first provides tools for the latter to advance, while the second one allows for a theoretical understanding of the data that the first one accumulates at such a heady pace.

While of course other disciplines within biology have made progress during the second part of the twentieth century—ecology, for instance—the next bit of this panoramic view I wish to briefly comment on concerns yet another area of inquiry that had played only a secondary role during the Modern Synthesis: paleontology. The field had always been a thorn in the side of Darwinism, since many paleontologists early on had rejected the Darwinian insight, proposing instead the idea that macro-evolutionary change was qualitatively distinct from the sort of micro-evolution that Darwin famously modeled on the basis of plant and animal breeding (and of course, notoriously, creationists have always made a big deal of the distinction between micro- and macro-evolution, often without understanding it). Indeed, it was this very rejection, together with the apparent incompatibility of Mendelism and Darwinism, that led to the above mentioned period of “eclipse” of the Darwinian theory at the turn of the twentieth century.

Paleontology’s early alternative to Darwinism took the shape of orthogenetic theory (according to organisms change in the same direction over millions of years), which in turn was essentially a scaled-up version of Lamarckism, since it postulated an inner vital force responsible for long-term evolutionary trends, which many paleontologists saw as otherwise inexplicable within the Darwinian framework. It was George Gaylor Simpson’s magistral role within the Modern Synthesis that cleared away any remnants of orthogenesis from paleontology, doing for that field what Fisher, Haldane and Sewall Wright had done for Mendelian genetics: he convincingly argued that the sort of so-called “micro”-evolutionary processes accounted for by Darwinism could be extrapolated to geological timescales, thus yielding the appearance of macro-evolutionary changes of a qualitatively different nature. In reality, Simpson argued, the second is simply a scaled up version of the former.

Simpson, however, was arguably too successful, essentially making paleontology a second-rate handmaiden to population genetics while overlooking the potential for its original contributions—theoretical as well as empirical—to the overall structure of evolutionary theory. Eventually, Simpson’s “conservatism,” so to speak, led to a backlash: Niles Eldredge and Stephen Jay Gould, the enfants terribles of modern paleontology, published in 1972 a landmark paper proposing the theory of punctuated equilibria, according to which evolution, when seen at the macroscopic scale, works by fits and starts: long periods of stasis during which not much appears to be happening in a given lineage, interrupted by sudden “bursts” of phenotypic change. The theory was immediately misunderstood by many population geneticists, who thought that Eldredge and Gould were attempting to revive an old notion known as “hopeful monsters,” i.e., of instantaneous evolutionary change resulting from genome-wide restructuring.

To be fair, at some point Gould’s own anti-establishment rhetoric, and the fact that creationists often mentioned him in their support, contributed to the confusion. But in fact, the sort of punctuations that Eldredge and Gould saw in the fossil record takes place over tens of thousands of generations, thus leaving plenty of time for standard Darwinian processes to do their work. As they pointed out later on in the debate, the real novel issue is that of prolonged stasis, over millions of years, not the allegedly (but not really) “instantaneous” change. A major class of explanation proposed especially by Gould for this observed stasis had to do with developmental processes and constraints, which nicely connects the new paleontology with the emerging field of evo-devo mentioned above, making both of them into pillars of the ensuing Extended Synthesis in evolutionary biology.

(next time: the Stephen Jay Gould conceptual revolution and the birth of the Extended Evolutionary Synthesis)

Last time we have seen how evolutionary theory has evolved over the past century and a half, why so many contemporary biologists are calling for what they refer to as the Extended Evolutionary Synthesis (see here and here), and how Darwin, building on David Hume, definitely rebutted the intelligent design argument advanced by William Paley. All as part of a discussion of a paper I published back in 2012, entitled “Biology’s last paradigm shift. The transition from natural theology to Darwinism.” (full text here) In this second part we are going to look at whether the transition between natural theology and Darwinism constituted a paradigm shift, according to criteria laid out by philosopher of science Thomas Kuhn. As I mentioned last time, in the paper I also apply the same analysis to what happened after Darwinism, to more and more recent incarnations of evolutionary theory, but will not discuss that section here.

According to Kuhn, change in science is comprised of two distinct and alternating phases: during “normal science” scientists use the dominant theoretical and methodological tools within a field of inquiry to solve “puzzles”, i.e. problems arising within a particular theory. However, from time to time the number of such problems that cannot be resolved within the adopted framework (“anomalies”) becomes large enough to trigger a crisis, which is then resolved if a new “paradigm” is arrived at to replace the old framework and provide new guidance for further normal-puzzle solving science.

Typically, one of the problems with the Kuhnian approach is that Kuhn did not define exactly what he meant by paradigm, which means that it is not entirely clear what may constitute a paradigm shift. For the purposes of my argument, I will use the commonly accepted interpretation of paradigms as encompassing the “disciplinary matrix,” which means not just the dominant theory or theories within a given field, but also the accompanying methodologies, training strategies for the next generation of scientists, and – no less important – the pertinent metaphysical and epistemological assumptions.

Kuhn suggested five criteria for comparing competing paradigms and for theory choice: 1) Accuracy; 2) Consistency, both internal and with other theories; 3) Scope, in terms of how widely the explanatory reach of a theory extends; 4) Simplicity; and 5) Fruitfulness, in terms of further research. Roughly speaking, then, the comparison between the two paradigms of natural theology and Darwinism is striking. Let’s go through it criterion by criterion.

Accuracy

Natural theology: all explanations are ad hoc, since God’s will is inscrutable.

Darwinism: it can explain some surprising facts about the biological world, like the complexities of the flower structure in some orchid species, or the intricacies of the life cycles of some parasites.

Consistency

Natural theology: internally inconsistent with the idea of an all-powerful, all good God (the problem of natural evil).

Darwinism: as internally consistent as any major scientific theory; external links to other sciences, particularly Darwin’s prediction that the age of the earth had to be greater than what commonly thought by geologists and physicists of the time (turns out, he was right).

Scope

Natural theology: allegedly all-encompassing, but supernatural “explanations” are epistemologically empty. That is, to say “God did it” sounds like an explanation, but it really doesn’t explain anything.

Darwinism: new facts about the biological world that are explained by the theory have been consistently uncovered for more than one and a half centuries.

Simplicity

Natural theology: deceptively simple, if one neglects the obvious question of the origin and makeup of the Creator.

Darwinism: in its original form invokes a small number of mechanisms to explain biological history and complexity; more recent versions invoke more mechanisms, but still a relatively limited number.

Fruitfulness

Natural theology: did not lead to any research program or discovery.

Darwinism: has maintained a vigorous research program for more than one and a half centuries.
According to the above summary, then, the Darwinian paradigm is definitely preferable to Paley’s natural theology – not surprisingly. More interestingly for our purposes here, these are all clear signs of a paradigm shift, the only one ever occurred in evolutionary biology, I argue in the rest of the original paper.

Kuhn’s theory of paradigm shifts famously included another controversial notion: incommensurability, the idea that crucial concepts within a given paradigm are simply not comparable to what superficially appear to be equivalent concepts within another paradigm. Kuhn identified three distinct types of incommensurability: methodological, observational and semantic.

Methodological incommensurability refers to the notion that different paradigms lead scientists to pick different “puzzles” as objects of research, as well as to the idea that scientists then develop distinct approaches to the solution of those puzzles. Obviously, natural theology and Darwinism are methodologically incommensurable: while they both rely on observation and comparative analyses, their goals are entirely different. For Paley, the focus is on the intricate complexity of living organisms, constantly interpreted as an obvious indication of the will and omnipotence of the Creator. Darwin, instead, pays particular attention to precisely those biological phenomena that are troubling to the notion of intelligent design, as in this famous passage:

“I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidæ with the express intention of their feeding within the living bodies of Caterpillars.” (letter collected by Francis Darwin 1887).

More broadly, the sort of “puzzles,” to use Kuhn’s terminology, that Darwinists began to pay attention to concern the historical relationships between different species of organisms (something that is defined out of existence within the natural theological paradigm, since species are specially created), as well as the kind of ecological settings that bring about different adaptations (again, a problem ruled out within natural theology, where adaptations are the direct result of an intelligent act).

Observational incommensurability is tightly linked to the idea that observations are theory-dependent: what is considered a “fact” within one theoretical context may not be such in a different theoretical context. This is perhaps one of the most controversial of Kuhn’s notions, famously illustrated with images from Gestalt psychology, where the same pattern of lines on paper can be interpreted in dramatically different fashions (e.g., a vase or two faces, an old or a young woman, a rabbit or a duck, etc.).

The problem, of course, is that if we take the Gestalt metaphor seriously, we are led to the position that there is no true or even better way to interpret the data, which in turn leads to the constructivist temptation: any theory is just as good as any other, and there really is no way to measure progress in science. Kuhn strongly disavowed such an extreme interpretation of his ideas, and the notion of theory-dependence of observations is now commonly accepted in philosophy of science and embedded in textbook treatments of the subject.

Be that as it may, it is hard to imagine examples of observational incommensurability between natural theology and Darwinism, in part no doubt because no sophisticated way of gathering data was accessible – beyond direct observation and rudimentary experiments – to proponents of the two paradigms.

Finally we get to semantic incommensurability. This has to do with shifts in the meaning of terms used by scientists, one of Kuhn’s examples being the concept of “mass,” which is a conserved, static quantity in Newtonian mechanics, but becomes interchangeable with energy within the framework of Einstein’s relativity.

For the purposes of our discussion, one could make the argument that a similar situation holds for the shifting concept of species between natural theology and Darwinism. Both paradigms do refer to “species,” but the meaning of the term is entirely different. For Paley, species were fixed entities set in place by the action of the Creator – in that sense not far from Newton’s own conception of the physical world, and particularly of the laws governing it. For Darwin, however, species are ever changing entities with no sharp boundaries, which are altered by evolutionary processes in a continuous, gradualistic fashion.

All in all, then, it appears that whether we use the first set of Kuhnian criteria or the various notions of incommensurability, there are very strong reasons to conclude that the shift between natural theology and Darwinism was, in fact, a paradigm shift. It was also, in a very important sense, a shift from a proto-scientific to a scientific view of biology: Darwin and Wallace abandoned any reference to supernatural forces, thus literally establishing a whole new field of science, which keeps, ahem, evolving even today.