Is the "Modern Synthesis" effectively dead?

The "Modern Synthesis," or modern evolutionary synthesis, refers to a framework of evolutionary theory developed and promoted by prominent biologists in the 1940s. The term comes from the subtitle of a 1942 book by Julian Huxley. The central theme was the integration of "classic" evolution with population genetics.

Although the original version was fairly broad, the later versions of the "Modern Synthesis" were much less so. The so-called "hardening" of the Modern Synthesis has been documented by many historians; notably, Stephen Jay Gould. By the time of the Darwin Centennial (1959) most biologists thought of the "Modern Synthesis" as a form of Darwinism + population genetics where natural selection was pretty much the only game in town.

This led Gould to proclaim, in 1980, that the Modern Synthesis is "effectively dead." It's worth examining what he meant when he said that so here's an explanation from his 2002 book, The Structure of Evolutionary Theory, published just before he died of cancer.

Given the furor provoked, I would probably tone down—but not change in content—the quotation that has come to haunt me in continual miscitation and misunderstanding by critics: "I have been reluctant to admit it—since beguiling is often forever—but if Mayr's characterization of the synthetic theory is accurate, then that theory, as a general proposition, is effectively dead, despite its persistence as textbook orthodoxy" (Gould, 1980). (I guess I should have written the blander and more conventional "due for a major reassessment" or "now subject to critical scrutiny and revision," rather than "effectively dead." But, as the great Persian poet said, "the moving finger writes, and having writ ..." and neither my evident piety nor obvious wit can call back the line—nor would tears serve as a good emulsifier for washing out anything I ever wrote!)

Yes, the rhetoric was too strong (if only because I should have anticipated the emotional reaction that would then preclude careful reading of what I actually said). But I will defend the content of the quotation as just and accurate. First of all, I do not claim that the synthetic theory of evolution is wrong, or headed for complete oblivion on the ashheap of history; rather, I contend that the synthesis can no longer assert full sufficiency to explain evolution at all scales (remember that my paper was published in a paleobiological journal dedicated to the studies of macroevolution). Two statements in the quotation should make this limitation clear. First of all, I advanced this opinion only with respect to a particular, but (I thought) quite authoritative, definition of the synthesis: "if Mayr's characterization of the synthetic theory is accurate." Moreover, I had quoted Mayr's definition just two paragraphs earlier. The definition begins Mayr's chapter on "species and transspecific evolution" from his 1963 classic—the definition that paleobiologists would accept as most applicable to their concerns. Mayr wrote (as I explicitly quoted): "The proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species."

Second, I talked about the theory being dead "as a general proposition," not dead period. In the full context of my commentary on Mayr's definition, and my qualification about death as a full generality, what is wrong with my statement? I did not proclaim the death of Darwinism, or even of the strictest form of the Modern Synthesis. I stated, for an audience interested in macroevolutionary theory, that Mayr's definition (not the extreme statement of a marginal figure, but an explicit characterization by the world's greatest expert in his most famous book)—with two restrictive claims for (1) "all evolution" due to natural selection of small genetic changes, and (2) transspecific evolution as "nothing but" the extrapolation of microevolutionary events.—must be firmly rejected if macroevolutionary theory merits any independent status, or features any phenomenology requiring causal explanation in its own domain. If we embrace Mayr's definition, then the synthesis is "effectively dead" "as a general proposition"—that is, as a theory capable of providing a full and exclusive explanation of macroevolutionary phenomena. Wouldn't most evolutionary biologists agree with my statement today?

If Mayr was correctly describing his own theory (he was), then Gould is correct when he says that the "Modern Synthesis" is effectively dead as a comprehensive theory of evolution (he is).

We also know now that Neutral Theory (Nearly-Neutral Theory) and random genetic drift needed to be incorporated into evolutionary theory and the "Modern Synthesis" did not accommodate such changes. There are lots of other things that evolutionary theory of the 1940s didn't cover. Here's what Douglas Futuyma says in his Evolution textbook (2009 p.11).

Since the evolutionary synthesis, a great deal of research has elaborated and tested its basic principles, and these principles have withstood the tests. But progress in both evolutionary studies in other fields of biology has required some modifications and many extensions of the basic principles of the evolutionary synthesis, and has spurred additional theory to account for biological phenomena that were unknown in the 1940s. Since Watson and crick established the structure of DNA in 1953, advances in genetics and molecular biology, and in molecular and information technology, have revolutionized the study of evolution. Molecular biology has provided tools for studying a vast number of evolutionary topics such as mutation, genetic variation, species differences, development, and the phylogenetic history of life, in greater depth and detail than ever before, a new mathematical theory has been developed to provide evolutionary interpretations of newly discovered molecular and genetic phenomenon.

As molecular and computational technology has become more sophisticated and available, new fields of evolutionary study have developed. Among these fields is molecular evolution (analysis of the process and history of change in genes), in which the Neutral Theory of Molecular Evolution has been particularly important. This hypothesis, developed especially by Mootoo Kimura (1924-1994), holds that most of the evolution of DNA sequences occurs by genetic drift rather than by natural selection, but it provides a foundation for detecting effects of natural selection on DNA sequences. Because entire genomes can now be sequenced, molecular evolutionary studies have expanded into evolutionary genomics, which is concerned with variation and evolution in multiple genes or even entire genomes. Evolutionary developmental biology is an exciting field devoted to understanding how the evolution of developmental processes underlies the evolution of morphological features at all levels, from cells to organisms.

The advances in these fields, though, are complemented by vigorous research, discoveries, and new ideas about longstanding topics in evolutionary biology, such as the evolution of adaptations and of new species. Since the mid-1960s, evolutionary theory has expanded into areas such as ecology, animal behavior, and reproductive biology, and detailed theories that explain the evolution of particular kinds of characteristics such as lifespan, ecological distribution, and social behavior were pioneered by the evolutionary theoreticians William Hamilton and John Maynard Smith in England and George Williams in the United States. The study of macroevolution has been renewed by provocative interpretations of the fossil record and by new methods for studying phylogenetic relationships. Research in evolutionary biology is progressing more rapidly than ever before.

You may not agree with all the extensions and modifications that Futuyma lists (I don't) but there's no question about the conclusions. The 1940s version of evolutionary theory ("Modern Synthesis") is no longer sufficient to explain what we know in the 21st century. For many biologists, the most important extensions to evolutionary theory took place in the 1970s with the incorporation of Neutral Theory (actually Nearly-Neutral Theory) and recognition of the importance of random genetic drift, especially in molecular evolution. Most people also realized that there was more to macroevolution than just lots of microevolution.

It does not seem reasonable to refer to modern evolutionary theory as "Darwinism," "neo-Darwinism," or the "Modern Synthesis," since all of these terms refer to a version of evolutionary theory that has been modified to such an extent that the old-fashioned terminology is just confusing.

The "Modern Synthesis" is dead. Long live modern evolutionary theory.

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Gould, S.J. (1980) Is a new and general theory of evolution emerging? Paleobiology 6:119-130.