Tuesday, September 13, 2005

Evolutionary Synthesis

The Three Pioneers

One of the greatest scientific achievements of the last century was the integration of Darwin's theory of evolution with Mendel's theory of heredity to form what is known as the "Modern Synthesis" of evolutionary theory or "Evolutionary Synthesis" which is now the backbone of all biological thought.

Ernst Mayr, one of the principal architects of the Evolutionary Synthesis, wrote a superb retrospective on how this achievement came about in his "80 Years of Watching Evolutionary Scenery"--on his 100th birthday, few months before he died.

"Having reached the rare age of 100 years, I find myself in a unique position: I'm the last survivor of the golden age of the Evolutionary Synthesis. That status encourages me to present a personal account of what I experienced in the years (1920s to the 1950s) that were so crucial in the history of evolutionary biology.

"Evolutionary biology in its first 90 years (1859 to the 1940s) consisted of two widely divergent fields: evolutionary change in populations and biodiversity, the domains of geneticists and naturalists (systematicists), respectively...

"Fortunately, there was one evolutionist who had the background to be able to resolve the conflict between the geneticists and the naturalists. It was Theodosius Dobzhansky. He had grown up in Russia as a naturalist and beetle taxonomist, but, in 1927, he joined Morgan's laboratory in America where he became thoroughly familiar with population genetics. He was ideally suited to show that the findings of the population geneticists and those of the European naturalists were fully compatible and that a synthesis of the theories of the two groups would provide a modern Darwinian paradigm, subsequently referred to as the 'Evolutionary Synthesis.' "

In Mayr's account of the Evolutionary Synthesis, he acknowledges the critical role Theodosius Dobzhansky played in achieving the synthesis of genetics with the theory of evolution. Here is a detailed account of Dobzhansky's achievement by one of his students.

"In 1972, Theodosius Dobzhansky addressed the convention of the National Association of Biology Teachers on the theme 'Nothing in biology makes sense except in the light of evolution'... The place of biological evolution in human thought was, according to Dobzhansky, best expressed in a passage that he often quoted from Pierre Teilhard de Chardin: 'Evolution is a general postulate to which all theories, all hypotheses, all systems must hence forward bow and which they must satisfy in order to be thinkable and true. Evolution is a light which illuminates all facts, a trajectory which all lines of thought must follow--this is what evolution is'.

"Dobzhansky's 'Genetics and the Origin of Species' advanced a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical arguments. It had an enormous impact on naturalists and experimental biologists, who rapidly embraced the new understanding of the evolutionary process as one of genetic change in populations. Interest in evolutionary studies was greatly stimulated and contributions to the theory soon began to follow, extending the synthesis of genetics and natural selection to a variety of biological fields."

By integrating the field of paleontology with natural selection and genetics, George G. Simpson played a vital role in building the Evolutionary Synthesis. Following is a classic paper by Simpson analyzing the geographical dispersal of various species over time.

"This I believe to be the type of pattern that would be shown by almost any form of life that had run its entire course from origin to extinction. A form appears in some center or 'cradle,' not an exact spot that could be marked with a monument but, say, a single biotic district or province. Thence it tends to spread steadily in all directions until it encounters insuperable barriers. After a time it begins to contract, possibly but not usually toward its center of origin and often splitting into disjunctive spots as it contracts. Finally it disappears."

-- Shrikant Rangnekar

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