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)