Synopsis: Several commenters have asked me to discuss the Modern Synthesis. I allude to it fairly frequently but I have not offered a working definition or idea of what it is. This made me realize that I have a fairly vague conception of it as well and thus decided to work out a post that gives some details.
In the early 20th century, biology was still fractured into several disciplines. Darwin’s theory of natural selection itself was in “eclipse.” The work of Fisher, Wright, and Haldane resolved the major dispute between Darwinian evolution and Mendelian genetics, forming population genetics. However, the disciplines of genetics, systematics, paleontology, and others still disagreed on how evolution worked. This was apparently not a real problem as the architects of what came to be known as the Modern Synthesis realized that the disparate fields actually did agree. These architects include Theodosius Dobzhansky (genetics), Ernst Mayr (systematics), GG Simpson (paleontology), and G. Ledyard Stebbins (botany). Some idea of a “unified biology” was reached in the 1940s and its subsequent popularization was carried out by Huxley and Mayr.
(I deeply apologize for this “textbook history.” As I strongly dislike these kinds of histories, I would normally not have written this up but since I was requested to discuss it, I decided to at least throw my vague understanding out there for reference and for corrections. The history of biology during this time period is extraordinarily interesting but I have only begun to understand its development. This is a problem I am actively working on.)
Anyway, what the Synthesis did was show that the fields of biology were unified by evolution. As Dobzhansky said, “Nothing in biology makes sense except in the light of evolution.” Some fields were left out, most notoriously embryology/developmental biology, and perhaps ecology, and that is a major problem for some scientists like Sean B Carroll. There are now calls for an extended synthesis but I can’t comment on their merits.
The historian William Provine has also described the Synthesis as a constriction – while combining various biological subfields, the architects also rejected several alternative theories of evolution. These included:
- Neo-Lamarckism (inheritance of acquired characteristics)
- teleological autogenetic theories or those that advocated “some sort of intrinsic, direction-giving force” such as orthogenesis, nomogenesis, aristogenesis, the Omega Point
- saltationism (mutations by large leaps)
(Before reading these sections by Mayr, I had only heard of orthogenesis and seen allusions to nomogenesis. Anyone know of good sources that discuss these alternative theories, especially aristogenesis? The way Mayr describes it makes it sound related to internalism, developmental constraints, mutationism, etc.)
What is perhaps the most important part of this discussion is knowing exactly what the Synthesis entails – what are its major tenets? Here is a partial list given by Ernst Mayr in his The Growth of Biological Thought (1988), p. 532:
- Evolution is change in allele frequencies in a population plus “changes in adaptation and diversity”
- Macroevolution is a result of microevolutionary processes plus the role of geography.
- Evolution is gradual.
- Variation is a result mostly of recombination, not mutation.
- Selection is not the only evolutionary process, but it is predominant.
- “Selection is probabilistic, not deterministic.”
- Selection acts on an individual (with inclusive fitness).
- Neutral and nearly neutral mutations are common.
This list largely conforms to other descriptions of the Synthesis, although I find his inclusion of neutral mutations intriguing. That seems to be the most critical post-Synthesis tenet Mayr has added and I feel like not everyone would agree with its inclusion (in that it argues against the synthesis and is not an addition).
Taking the list from the very vocal and opinionated Mayr introduces substantive bias in any evaluation of what the Modern Synthesis is although he was the major popularizer and defender of the Synthesis. Were the fellow architects largely in agreement with his views or did they come to differ over time? I don’t know.
In regards to Stoltzfus’ work, Mayr may have voiced a critical opinion:
“The Synthesis was a reaffirmation of the Darwinian formulation that all adaptive evolutionary change is due to the directing force of natural selection on abundantly available variation” (Mayr 1988, 527).
Natural selection is a two-step process. One, “production of an unlimited amount of new variation” via recombination (but also mutation). Two, natural selection. (Mayr 1982, 591).
The work done by Hubby, Lewontin, and Harris – finding large degree of standing variation in natural populations – was a “vindication of Darwin’s faith in the existence of a virtually inexhaustible supply of genetic variation” (Mayr 1982, 592).
As far as I understand, these three quotes – favoring the gene pool and an “unlimited amount of variation” while disregarding the role of mutation in the evolutionary process – seems to largely confirm Stoltzfus’ conception of the Modern Synthesis. I do this to show that I derived this idea of the Modern Synthesis from Mayr, not Stoltzfus, but also to double check (to a limited degree) Stoltzfus’ interpretations.
Mayr, Ernst. The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge, MA: Belknap, 1982. Print.
Mayr, Ernst. Toward a New Philosophy of Biology: Observations of an Evolutionist. Cambridge, MA: Harvard UP, 1988. Print.