The version of evolutionary theory taught when I took Evolution at New Mexico State University in 1973 was called the Modern Synthesis. The Modern Synthesis is still a foundation of evolutionary biology, although it is being expanded with the Extended Evolutionary Synthesis (Under Construction). This page presents an overview; for the people behind it in full — and for the Russian school that reached the idea first — see The Architects of the Modern Synthesis (Under Construction).

The Modern Synthesis merged Darwin's natural selection with Mendel's genetics

Darwin hypothesized a mechanism for evolution, natural selection, in 1859 but he did not understand inheritance. Natural selection is based on the observation that individuals within a population of a given species have different traits. He hypothesized that those individuals with traits better suited to their environment produced more offspring. However he also thought that organisms produced gemmules from throughout their bodies reflecting those traits — inherited "gemmules" producing a blend of the parents' traits — which he named Pangenesis. That actually worked against his theory of natural selection, because a blending inheritance would halve any new advantageous variation in each generation until it faded away. This quantitative mismatch reflected Darwin's lack of mathematical training or understanding, which he admitted-- "I have deeply regretted that I did not proceed far enough at least to understand something of the great leading principles of mathematics, for men thus endowed seem to have an extra sense."¹ A correct analysis of inheritance had in fact been published 7 years after Origin of Species in 1866 by Gregor Mendel, whose experiments showed that inheritance is particulate, carried by discrete factors that pass intact rather than blending. Yet when Mendel's work was rediscovered in 1900, the geneticists who embraced it decided his discrete units contradicted Darwin's gradual variation, and for three decades Mendelism and Darwinism were treated as rival camps.²

Beginning in the 1920s and largely complete by the early 1950s, the Modern Synthesis showed that natural selection depended on Mendelian genetics. The term itself was coined by Julian Huxley, as the title of his 1942 book Evolution: The Modern Synthesis. Huxley was not one of the mathematicians or field naturalists who built the technical core of the synthesis; rather he brought the new evolutionary biology into a single volume useful to scientists and comprehensible to the lay public. Huxley's backgroun made him well suited for that project — he was the first Director-General of UNESCO and a grandson of Thomas Henry Huxley, "Darwin's bulldog." His full profile, including his own unfortunate lifelong commitment to eugenics, is on The Architects page.³

The people, in brief

Four people did much of the work that became the Modern Synthesis, although many more contributed. Each is profiled in full on The Architects page; here they are in short.

R. A. Fisher

1890–1962 · mathematics

Proved that Mendelian genes are exactly what natural selection needs, founding population genetics with Haldane and Wright.

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Theodosius Dobzhansky

1900–1975 · genetics in nature

Showed that mathematicians' models could be applied to wild fruit-fly populations. The bridge between abstract theory and real organisms. Gives this site its name.

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Ernst Mayr

1904–2005 · species

Gave biology its working definition of a species and its account of how new species arise through isolation.

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George Gaylord Simpson

1902–1984 · the fossil record

Showed the fossil record fit the synthesis — and noticed that rates of evolution are not constant but variable.

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What is the synthesis?

In brief: heritable variation arises from random mutation and recombination, undirected with respect to what the organism needs; that variation passes on through Mendelian genes; natural selection and genetic drift change gene frequencies in populations over generations, so that populations, not individuals, evolve; and large differences accumulate from the gradual buildup of many small genetic changes.

Selection is not the only force at work in that account. Sewall Wright, the third of the great population-genetics mathematicians alongside Fisher and Haldane, showed that in small populations gene frequencies can drift by chance alone, independent of any advantage the gene confers — a coin that comes up heads several times in a row, not because it is a better coin. Wright pictured evolution as movement across an "adaptive landscape" of fitness peaks and valleys, with drift able to carry a small population across a valley that selection alone could never cross.⁴ Drift matters most in small populations and for mutations that are nearly neutral — neither helpful nor harmful enough to be subject to selection.

The synthesis held that large-scale evolution of new species, macroevolution, is simply small-scale evolution extended over time — that the differences between species and major groups need no mechanism beyond what we see within populations. It also generally treated the mutation rate, the rate at which new variation is supplied, as a roughly constant background quantity. That assumption results in the straight line in the diagram above.

Genes are made of DNA

The Modern Synthesis was built on an abstract gene concept — a unit of heredity defined by how it behaved in a cross. But what was the mechansim? Nobody who built the synthesis in the 1920s, 1930s, and 1940s knew what a gene actually was. That changed in 1953, just after the synthesis was essentially complete, when James Watson and Francis Crick, working from X-ray diffraction images made by Rosalind Franklin, worked out the double-helix structure of Deoxyribonucleic Acid (DNA).⁵ The gene, an abstraction that Fisher and Wright had described only mathematically, was now a physical molecule with a defined structure, one that could be read, copied, damaged, and repaired. DNA consists of strings of "nucleotides" that form the double helix ladder structure. There are 4 nucleotides with slightly different molecular structures. Sets of three form "codons" that code for the 20 different amino acids that make up proteins. I won't go into the mechanism by which those sequences are used to create proteins here.

That discovery combined with the synthesis to promote a more DNA-centered, gene-centric view of organisms. It also created a specific, quantifiable assumption: if mutation is simply damage or copying error in a physical molecule, and if the rate of that damage is roughly constant, then the number of mutations separating two species should be roughly proportional to the time since they diverged from a common ancestor. That idea, the molecular clock, was proposed by Emile Zuckerkandl and Linus Pauling in 1962 and given its rigorous population-genetic footing by Motoo Kimura's neutral theory later that decade.⁶ The molecular clock is not a minor corollary: it is the tool used to date when species split from one another. It depends directly on the assumption that the mutation rate is a constant. In the Variable Evolution section we will examine that assumption.

The gene's-eye view

By the 1960s and 70s the synthesis was sharpened by some into the "gene's-eye view," developed by W. D. Hamilton and George C. Williams and given its popular voice by Richard Dawkins in The Selfish Gene (1976), where organisms are vehicles that genes build to carry themselves forward.⁷ That view was well received in many quarters — though not all the synthesis architects accepted it. Mayr, for one, insisted selection acts on whole organisms, not isolated genes. That's my view as well. I ask "what is the unit of selection?" I'll discuss this more in later sections of the website.

What has the Modern Synthesis accomplished, and what is next?

The synthesis was accepted because it made predictions that data confirmed. It better accounted for the fossil record, the distribution of species, the genetic code, and the classification of life from a single set of principles. When molecular biology and, later, genomics arrived, their tools were largely absorbed into the synthesis rather than overturning it. Surveys of the field today still describe it as the dominant paradigm of evolutionary biology.⁸

What does the Modern Synthesis leave out? Proponents of the Extended Evolutionary Synthesis ask whether factors like development, plasticity, and inheritance beyond the gene deserve more consideration — the subject of its own section on the website. Plus, are mutation rates a constant? Building on the variable tempos that Simpson noticed in the fossil record, the paleontologists Stephen Jay Gould and Niles Eldredge argued that the history of life does not, after all, trace that steady, straight line.⁹ If true, what is the mechanism?

• In preparation
  The Architects of the Modern SynthesisThe figures in full — Fisher, Dobzhansky, Mayr, Simpson, Huxley — and the forgotten Russian school of Chetverikov and Filipchenko.
• In preparation
  Genetics: from Mendel to the double helix, in depth A fuller treatment of the story sketched above — Mendel's laws, the rediscovery, and what molecular biology added once DNA's structure was known.
• In preparation
  Population genetics and the mathematics of selection What Fisher, Haldane, and Wright's equations actually say about how populations change.