The Modern Synthesis of Evolutionary Theory

Overview

During the 1930s and 1940s a group of biologists and scientists in a variety of related fields assembled a new picture of biological change, mutation, and variation, merging genetics with Charles Darwin's (1809-1882) vision of natural selection, and refining and altering understanding of both. Contributions to the new understanding of evolutionary processes came from population geneticists, paleontologists, ornithologists, mathematical geneticists, and naturalists. Because it drew upon so many fields, the view of evolution that emerged was called the Modern Synthesis or New Synthesis.

Background

In some cases, the synthesis was also known as neo-Darwinism, or new Darwinism, because it helped resolve many of the problems some scientists had found in the mechanics of Darwin's view of evolution. (In its most proper usage, neo-Darwinism was the movement that flourished between the death of Darwin and the arrival of the Synthesis; this movement attributed all changes in species to the effects of natural selection.)

More than half a century after Darwin presented his theory of the evolution of life by natural selection, the actual processes by which that evolution took place were still not understood. While few scientists questioned evolution as a process, natural selection itself had fallen into some disrepute as the engine of biological change over time.

Put simply, natural selection argues that plants and animals develop and pass on to succeeding generations those characteristics that best adapt the organisms for survival in their environment. The dilemma for scientists seeking to explain natural selection lay in determining how those characteristics that best suited an organism to a particular environment were passed from one generation to the next, as well as how new characteristics could be acquired and, in turn, passed on.

Some scientists, notably Jean Lamarck (1744-1829), had wrestled with the question of inherited characteristics even before Darwin prepared his theory of evolution. Lamarck argued that survival characteristics could be acquired or developed by a living organism, and once acquired, passed on to succeeding generations. That is, a characteristic such as stretching to reach high leaves, could be developed by an animal in response to its environment and, once the characteristic proved effective, it would be passed on to that animal's offspring, which would possess longer necks.

Lamarckism, as the theory of acquired characteristics came to be known, enjoyed much popularity early in the nineteenth century, but by the time Darwin published On the Origin Of Species by Means of Natural Selection in 1859, the theory had begun to be doubted. By the twentieth century, Lamarckism had become almost completely discounted.

But no clear view of inheritance and change emerged to take its place. Scientists and philosophers found themselves confronted with fundamental questions, including uncertainty as to how species arrive in the first place. Ornithologist Ernst Mayr (1904- ), who would be one of the major figures in the Modern Synthesis, pointed out that despite the title of his great book, Darwin did not actually address how species were originated. Scientists also questioned how existing species change, how those changes are transmitted to offspring, and, above all, what the rate of change was over time.

Several evolutionary theories perceived purpose and direction in the march of changes throughout the history of life. This interpretation of evolution, known as orthogenesis, implied that changes in species were directed toward the development of ever-higher species, culminating in human beings. The process of evolution, however it worked, guided life from the simpler to the more advanced, the humblest to the most sophisticated. Orthogenesis found, perhaps not surprisingly, its most devoted followers among scientists with a religious or philosophical bent.

Others were not so sure. Some suspected that species emerged gradually, as a result of small changes that compounded with subsequent generations. Again and again, the focus of scientific inquiry returned to the search for the exact mechanism of change within a species. Darwin himself had postulated a specialized type of cellular structure, which he called a "gemmule," that was responsible for inherited characteristics. Unfortunately, Darwin's "gemmule" also allowed for the inheritance of acquired characteristics. Darwin died still pondering the process of inheritance.

Ironically, though, the very mechanisms that would go far to explain the workings of evolution and inheritance, had been observed by a Silesian (now in the Czech Republic) scientist named Gregor Mendel (1822-1884), whose paper on inherited characteristics was published only six years after Darwin's Origin of Species.

Whereas Darwin had achieved worldwide fame immediately upon the publication of his Origin of Species (the book sold more than 1,000 copies the day it was printed,) Mendel's work went largely unnoticed. Having experimented with the breeding and cross-pollination of peas, Mendel showed that the species characteristics, encoded in genes, of a set of parents do not simply blend in offspring, as Darwin theorized, but are passed on independently of each other. That independence allows for many different ways for the genes to be recombined in offspring.

Around the turn of the twentieth century, Mendel's insights into the operation of genetics began to receive increasing attention as a mechanism that explained not only how traits and characteristics could be inherited, but also how changes in the genetic makeup of an organism could be passed on to successive generations, ultimately becoming a part of the entire species' genetic pattern.

The introduction of those changes into the genes became the focus of much scientific research and experimentation. Around the turn of the twentieth century, Dutch botanist Hugo de Vries (1848-1935), in addition to helping restore Mendel's insights to prominence, began to formulate a theory of the role played by mutation in the process of evolution.

According to de Vries, sudden evolutionary jumps, the appearance of new species, were the results of alterations, or mutations, in the parents' genetic material. The transmission of these alterations, and the ways in which they in turn combined with existing genetic material, could cause the immediate appearance of a whole new species.

In short, de Vries postulated that evolution moved rapidly, if fitfully, in great leaps, called mutationism. De Vries viewed evolutionary change as a process of large and dramatic changes in genetic material, macromutations, which resulted in new species. A similar theory, saltationism, also proposed large jumps as powering the engine of evolution, and attracted many adherents in the years after the publication of Darwin's Origin of Species.

Already, though, some scientists were beginning to question these interpretations of both the fossil record and the ways genes worked. Particularly important was the work of Sewall Wright (1889-1988), who in the 1920s developed a mathematical model for the effects of genes and genetic change in populations. His studies, along with those of J.B.S. Haldane (1892-1964) and Ronald A. Fisher (1890-1962), among others, laid the groundwork for an entire and important field of study—population genetics—as well as demonstrating the vital contributions mathematics could make to biological investigation.

Despite the increasing importance of genetics to evolutionary studies—some scientists felt that the rediscovery of Mendel had rendered Darwin's insights all but useless—a great debate raged on. Part of the problem derived from a split in the biological sciences between naturalists and experimentalists. Naturalists, such as Darwin, based their theories on careful observation of the natural world. Experimentalists derived their findings from laboratory work, often isolating the object of their investigations from its natural environment. The two branches rarely shared information.

Impact

By the 1930s, it was becoming clear that the various theories not only did not fit well with observations of nature, they were also becoming increasingly problematic in the lab. The stage was set for a new approach to gathering information and insight from diffuse sources, and assembling the whole into a more unified vision of the fundamental operations of inheritance, change, and evolution.

In the opinion of many, the great catalyst for this unification flowed from the Columbia University laboratory of Russian immigrant Theodosius Dobzhansky (1900-1975). Working with Drosophila, a common species of fruit fly whose short 10-day reproductive cycle permitted experiments with evolution in action, Dobzhansky studied the role of genetics in whole populations of a species, as well as the ways in which populations could become isolated from one another. As a result of his experiments and his observations of species in the natural world, Dobzhansky was able to show that small genetic changes in a population could result in that population's inability to interbreed with other populations of the same organism. In short, small changes, along with the effects of natural selection, could result in the emergence of a new species.

Dobzhansky's 1937 book Genetics and the Origin of Species attracted excited attention throughout the scientific community, and helped shift efforts toward unifying various fields of evolutionary inquiry into higher gear.

At roughly the same time, paleontologist George Gaylord Simpson (1902-1984) was using his studies of fossils as a basis for recreating the breeding patterns of prehistoric species. Specifically focusing upon the horse, Simpson showed, from the fossil record, how the species spread, how its population changed as its environments changed or as it entered new environments, how geographic separations resulted in the emergence of new species, the role natural selection played in adaptation to environments, and so on.

Ernst Mayr, working at Harvard during the same period, produced sharp insights into the effects of geographic and other factors on species populations. He was able to show that members of species, isolated from others of the same species, could as a result of mutation and natural selection eventually become an entirely new species, unable to interbreed with the original population from which it had become separated.

In England, Julian Huxley (1887-1975), the grandson of Darwin's great defender Thomas Henry Huxley (1825-1895), built upon his own substantial biological work, combining it with the insights of Dobzhansky and others to produce the 1942 book Evolution: The Modern Synthesis, which gathered together and presented the unified (although not completely in agreement) views of the architects of the revolution in evolutionary biology.

With the publication of Huxley's book, the great revolutionary period of the Synthesis gave way to the refinements, disagreements, reinterpretations, and ongoing scientific inquiries that have marked the field ever since.

What had been shown by all of the scientists involved in the development of the Synthesis was that evolution was not an either/or proposition. Both genetics and natural selection, as demonstrated by both Darwin and Mendel (and hundreds of thousands of subsequent scientists and researchers), were required.

As a result of the Modern Synthesis, natural selection returned once more to the heart of evolutionary studies, although this time informed by a more accurate understanding of how those processes worked, and the role of genetics and population in the emergence of new species.

But the Synthetic theory did more than just revolutionize the ways in which scientists thought about evolution. The development of Synthetic theory is one of the great examples of cooperative study by scientists in various disciplines. Communication was revealed to be as important as research.

Nor did the development of the synthesis end the debates over the workings of evolution. Rather, it provided a more level field on which scientists from all disciplines could present their theories and interpretations, which is still going on, with occasional vehemence, today.

The Modern Synthesis proved that evolution itself continues to evolve.

KEITH FERRELL