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After the Synthesis

The Modern Synthesis settled a framework for evolutionary biology by the late 1940s: evolution as change in gene frequencies, driven primarily by natural selection acting on random genetic variation. The framework has proven durable. It has also been extended, challenged, and supplemented from multiple directions since. The developments below are not a single movement — they come from different sub-disciplines, address different questions, and do not share a common programme. What they share is that each has stretched or contested the boundaries of the classical Synthesis.

Neutral theory

Motoo Kimura’s neutral theory of molecular evolution (1968) proposed that the majority of evolutionary changes at the molecular level — substitutions of one nucleotide for another in DNA — are selectively neutral. They are not favoured or disfavoured by natural selection; they spread through populations by genetic drift alone. The theory did not deny the importance of natural selection for adaptive evolution at the organismal level, but it argued that selection is not the primary cause of molecular variation. Most molecular differences between species, on this account, are noise rather than signal.

The neutral theory was initially controversial but is now part of the standard framework. It reshaped how variation is understood: not all genetic difference is adaptive, and the background rate of neutral change provides a molecular clock useful for estimating divergence times between species. The nearly neutral theory, developed by Tomoko Ohta, extended Kimura’s framework to include mutations of very slight effect, where drift and selection interact.

Kin selection and inclusive fitness

W. D. Hamilton’s theory of inclusive fitness (1964) addressed a problem Darwin had noted but not resolved: the existence of altruistic behaviour, particularly in social insects where sterile workers devote their lives to the reproductive success of the queen. Hamilton’s rule formalised the conditions under which natural selection can favour a gene for altruism: the cost to the altruist must be outweighed by the benefit to relatives, weighted by their degree of relatedness — rB > C, where r is relatedness, B is benefit to the recipient, and C is cost to the actor.

Inclusive fitness theory offered an account of the extreme sociality of hymenoptera (ants, bees, wasps), where workers are more closely related to their sisters than they would be to their own offspring, making sterile cooperation a genetic strategy. It also provided a framework for understanding cooperation and conflict in social species more broadly. Robert Trivers’ work on reciprocal altruism (1971) and parent-offspring conflict (1974) extended the analysis to cases where relatedness alone does not explain cooperation.

Gene-centric selection

George C. Williams’s Adaptation and Natural Selection (1966) mounted a rigorous critique of group selection — the idea that natural selection can act on groups of organisms, favouring groups whose members cooperate. Williams argued that most apparent cases of group-level adaptation could be explained more parsimoniously by individual- or gene-level selection, and that genuine group selection requires restrictive conditions rarely met in nature.

Richard Dawkins’ The Selfish Gene (1976) popularised and extended Williams’ argument, framing organisms as “survival machines” built by genes for gene replication. The gene-centric view was influential both within biology and in public understanding of evolution. It also provoked sustained criticism. Richard Lewontin and Stephen Jay Gould’s “Spandrels of San Marco” (1979) argued that the adaptationist programme — the assumption that every trait is an adaptation shaped by selection — systematically overestimates the power of natural selection and neglects structural, developmental, and historical constraints on form. The Spandrels critique was directed at the broader adaptationist stance, not only at Dawkins, but the gene-centric framework became one of its primary targets. Lewontin’s broader work on the organism-environment dialectic (The Dialectical Biologist, 1985, with Levins) argued that organisms and their environments are mutually constitutive — a position that anticipated niche construction theory by nearly two decades.

Sociobiology

E. O. Wilson’s Sociobiology: The New Synthesis (1975) drew together Hamilton’s inclusive fitness, Trivers’ reciprocal altruism, and the gene-centric perspective into a systematic account of social behaviour across the animal kingdom. The book’s final chapter extended the framework to human societies — proposing that aggression, sex roles, and other social patterns have evolutionary origins amenable to biological analysis.

The response was immediate and polarising. Lewontin, Gould, and other members of the Sociobiology Study Group at Harvard published a public critique in The New York Review of Books (1975), arguing that Wilson’s extension to humans amounted to genetic determinism and carried political implications — echoes of Social Darwinism and biological justifications for inequality. Wilson denied the charges; the controversy became one of the most visible public disputes in twentieth-century biology. The substance of the debate — how far evolutionary analysis of behaviour can be extended to humans, and what the political stakes are of doing so — has continued through evolutionary psychology and remains contested.

Within biology, however, the non-human chapters of Sociobiology were less controversial and broadly influential. The systematic application of inclusive fitness and game theory to animal social behaviour became a mainstream research programme — behavioural ecology — that has continued to develop since.

Multilevel selection

Williams’ 1966 critique largely suppressed group selection as a respectable concept in evolutionary biology for two decades. Its revival came through David Sloan Wilson and Elliott Sober, whose Unto Others: The Evolution and Psychology of Unselfish Behavior (1998) argued that selection can and does operate at multiple levels simultaneously — on genes, organisms, and groups — and that the question is not which level is “real” but how the relative strengths of selection at different levels interact in particular cases.

Multilevel selection theory reframed group selection not as an alternative to individual selection but as a component of a multi-level process. The claim is that traits favouring group success can spread even when they are costly to individuals, provided the between-group selection is strong enough to override within-group disadvantage. The framework has been applied to the evolution of eusociality, the major evolutionary transitions (Maynard Smith and Szathmáry, 1995), and human cooperation.

The relationship between multilevel selection and inclusive fitness remains debated. Some theorists (Steven A. Frank, Andy Gardner) argue the two frameworks are mathematically equivalent — different accounting methods for the same underlying processes. Others maintain they are genuinely different in what they foreground and how they direct empirical research. E. O. Wilson’s late-career shift to group selection (The Social Conquest of Earth, 2012), explicitly abandoning the inclusive fitness framework he had helped popularise, intensified the dispute without resolving it.

Punctuated equilibrium

Niles Eldredge and Stephen Jay Gould proposed punctuated equilibrium in 1972, based on a pattern in the fossil record: most species, once they appear, show little morphological change over most of their history (stasis), with significant change concentrated in geologically brief episodes associated with speciation events. Gould’s phrase — “stasis is data” — captured the argument that the absence of change in the fossil record is not a failure of preservation but a real biological phenomenon requiring explanation.

Punctuated equilibrium was initially read as a challenge to Darwinian gradualism. The ensuing debate was heated and sometimes confused — critics accused Gould and Eldredge of denying natural selection, which they did not; proponents sometimes overstated the pattern’s universality. Many in the field now read punctuated equilibrium as describing a real pattern in many lineages without overturning the Synthetic framework: speciation events may be associated with rapid selection in small isolated populations, consistent with (though emphasising different aspects of) the Synthesis’s own mechanisms. Others, following Gould’s later work in The Structure of Evolutionary Theory (2002), see it as requiring a more substantial rethinking of macroevolutionary theory.

Evo-devo

Evolutionary developmental biology — evo-devo — emerged as a distinct field in the 1980s and 1990s, driven by the discovery that the genes controlling body-plan development are deeply conserved across the animal kingdom. The Hox genes — regulatory genes that specify body-segment identity — are found in organisms as different as fruit flies and vertebrates, arranged in the same chromosomal order and performing analogous functions. Sean B. Carroll’s Endless Forms Most Beautiful (2005) brought the field to a wider audience.

The deep homology of developmental genes suggested that evolutionary innovation works not mainly by inventing new genes but by altering when, where, and how much existing genes are expressed — changes in regulation rather than structure. This shifted attention from the gene as unit of selection to the developmental process as a site of evolutionary possibility and constraint. John Gerhart and Marc Kirschner’s work on “facilitated variation” (2005) argued that the organisation of development itself makes certain kinds of variation more likely than others — a structured, non-random channel through which mutation produces phenotypic change.

Alongside evo-devo, Lynn Margulis’ work on symbiosis and the endosymbiotic origin of eukaryotic cells challenged the Synthesis from a different direction — arguing that cooperation and merger between organisms, not only competition and gradual modification, are major drivers of evolutionary innovation. Margulis’ programme was initially marginal but is now part of the standard picture of early cellular evolution.

Niche construction

John Odling-Smee, Kevin Laland, and Marcus Feldman formalised niche construction theory in Niche Construction: The Neglected Process in Evolution (2003). The core claim is that organisms do not merely adapt to environments — they modify their environments and thereby alter the selection pressures acting on themselves and their descendants. Beavers build dams; earthworms transform soil chemistry; humans reshape entire ecosystems. These modifications persist across generations, creating a form of ecological inheritance alongside genetic inheritance.

Niche construction does not deny the importance of natural selection. It argues that selection pressures are not independent variables acting on passive organisms, but are themselves partly products of the organisms’ own activities — a feedback loop the classical Synthesis framework does not foreground.

Epigenetics

Epigenetic inheritance — the transmission of information across generations through mechanisms other than DNA sequence (methylation patterns, histone modifications, cytoplasmic factors) — has raised the question of whether inheritance is exclusively genetic. In some cases, environmental effects on gene expression can be transmitted from parent to offspring without any change to the DNA sequence itself.

The evolutionary significance of epigenetic inheritance is debated. That epigenetic mechanisms exist and affect development is not in question. Whether they persist across enough generations to play a role in evolutionary change — rather than being reset each generation — is the contested point. Some researchers see epigenetic inheritance as evidence that the Synthesis’s gene-centred framework is incomplete; others argue the phenomena are accommodated within the existing framework as environmental effects on gene expression.

The Extended Evolutionary Synthesis debate

The developments above — evo-devo, niche construction, epigenetic inheritance, along with developmental plasticity and cultural evolution — have been gathered under the banner of the Extended Evolutionary Synthesis (EES) by Massimo Pigliucci, Gerd Müller, Kevin Laland, and colleagues. Their argument, set out in Evolution: The Extended Synthesis (2010) and in a 2014 Nature exchange (“Does evolutionary theory need a rethink?”), is that the Modern Synthesis, while not wrong, is inadequate — that it foregrounds some processes (gene-frequency change under selection) while backgrounding others (development, niche construction, non-genetic inheritance) that are equally important.

The response from defenders of the standard framework — Jerry Coyne, Brian Charlesworth, and colleagues — is that the Synthesis was never as narrow as the EES proponents claim, and that the new developments are extensions within the existing framework, not challenges requiring a new one. The 2014 Nature exchange (Laland et al., “Yes, urgently”; Wray et al., “No, all is well”) is a useful entry point to both positions.

The status of the EES is genuinely unresolved. It is not a fringe position — its proponents include mainstream evolutionary biologists — but neither is it the consensus. Whether the classical Synthesis needs extending or merely updating is a live question in the contemporary field.

Sources

Kimura, M. (1983). The Neutral Theory of Molecular Evolution. Cambridge University Press.
Hamilton, W. D. (1964). The genetical evolution of social behaviour (I and II). Journal of Theoretical Biology, 7(1), 1–52.
Williams, G. C. (1966). Adaptation and Natural Selection. Princeton University Press.
Gould, S. J., & Lewontin, R. C. (1979). The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme. Proceedings of the Royal Society of London B, 205(1161), 581–598.
Levins, R., & Lewontin, R. C. (1985). The Dialectical Biologist. Harvard University Press.
Dawkins, R. (1976). The Selfish Gene. Oxford University Press.
Wilson, E. O. (1975). Sociobiology: The New Synthesis. Harvard University Press.
Sober, E., & Wilson, D. S. (1998). Unto Others: The Evolution and Psychology of Unselfish Behavior. Harvard University Press.
Eldredge, N., & Gould, S. J. (1972). Punctuated equilibria: An alternative to phyletic gradualism. In T. J. M. Schopf (Ed.), Models in Paleobiology (pp. 82–115). Freeman, Cooper.
Carroll, S. B. (2005). Endless Forms Most Beautiful: The New Science of Evo Devo. W. W. Norton.
Gerhart, J., & Kirschner, M. (2005). The Plausibility of Life: Resolving Darwin’s Dilemma. Yale University Press.
Odling-Smee, F. J., Laland, K. N., & Feldman, M. W. (2003). Niche Construction: The Neglected Process in Evolution. Princeton University Press.
Pigliucci, M., & Müller, G. B. (Eds.). (2010). Evolution: The Extended Synthesis. MIT Press.
Laland, K. N., et al. (2014). Does evolutionary theory need a rethink? Nature, 514, 161–164.
Gould, S. J. (2002). The Structure of Evolutionary Theory. Harvard University Press.