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.