Molecular Evolution Activities

Neutral Theory of Molecular Evolution

Origins of Molecular Evolution

The following essay is a brief outline of developments in evolutionary biology and molecular biology just before the emergence of the field of molecular evolution. It is based on published and unpublished works by Michael Dietrich, John Beatty, and Jay Aronson. This work is not to be taken as a definitive history. Rather, we hope it stimulates scientists who were active during this tumultuous period in biology to contribute their recollections and related documents to the website.

Progress in Molecular Biology

The 1950s and early-1960s was a time of tremendous advance in molecular biology. Most importantly, the structure of DNA unraveled and its role in protein synthesis was elucidated. Encouraged by their progress, many molecular biologists developed a faith in their emerging discipline that was described by E.O. Wilson as an "imperialistic zeal." Indeed, many molecularly inclined biologists began to argue that the best way to answer questions about evolution was to study them at the molecular level. Using newly developed techniques, including protein sequencing, immunoelectrophoresis and micro compliment formation, molecular biologists were for the first time able to compare proteins of different species at the level of single amino acid substitutions. For the first time in history, biologists were no longer reliant solely on morphological characteristics or microscopic phenomena like blood coagulation and chromosomal rearrangements in systematics research. Indeed, as the physical anthropologists John Buettner-Janusch and Robert Hill declared in the introduction of their 1965 article entitled "Molecules and Monkeys:"

"The era of the molecule, the protein molecule, is upon us. Anthropology, as it attempts to reconstruct the phylogeny of man and his fellow members of the order Primates, must take cognizance of molecules. It is unlikely that significant quantities of proteins will ever be extracted from fossil primates. But we can study the differences in many proteins of the living primates, a group of mammals that exhibit a remarkable degree of evolutionary stratification."

The Evolutionary Synthesis (Back to top)

The place of molecular evolution within evolutionary biology can only be understood within the context of the evolutionary synthesis, the unification of a disparate group of biological disciplines-systematics, paleontology, botany and zoology-through the reinterpretation and integration of Mendelian genetics with Darwinian evolution in the 1930s and 1940s. The synthesis can be seen as the construction of compatibility arguments among the various fields of biology; as well as the professionalization of evolutionary biology in various institutional contexts including The Society for the Study of Evolution, its official journal, Evolution, and the National Research Council's Committee on Common Problems of Genetics, Paleontology, and Systematics.

Although historians and scientists have been debating the meaning of the synthesis since almost the day it began, a few definitive statements can be made about this time period in the history of biology. First, the architects of the synthesis sought to create an interpretive framework and institutional network through which workers in seemingly unconnected biological disciplines could communicate with one another and strive to solve problems of common interest. They found this framework in Darwin's theory of evolution by the natural selection. Second, a major stimulus for the evolutionary synthesis was the increasing difficulty that many non-experimentally-inclined biologists faced in securing funding and other resources for their work. And third, many biologists were concerned that as more and more funding was devoted to experimental sciences, biology was in danger of being reduced to physics and chemistry. These efforts were spearheaded by Theodosius Dobzhansky, G.G. Simpson, and Ernst Mayr, who came to be known as the "architects of the synthesis," as well as other biologists including G. L. Stebbins and H.J. Muller. Although each architect had a mix of personal, professional, and epistemological reasons for taking part in the synthesis, they were united by the idea that eventually became the central dogma of the newly "unified" biology:

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 transpecific evolution is nothing but the extrapolation and magnification of the events that take place within population and species.


Evolution Goes Molecular (Back to top)

Just a few short years after the Dobzhansky, Mayr, Simpson and others claimed that they had succeeded in unifying biology under the rubric of natural selection, molecular biologists began making claims that evolution at the molecular level was radically different than evolution at the morphological and organismal level. Most obviously, they claimed that molecular evolution occurred at a steady rate and was largely unaffected by the forces of natural selection (this idea led to the formulation of the molecular evolutionary clock). This issue would come to have a profound impact within the world of systematics.

At least one molecular evolutionist, Emile Zuckerkandl claimed that molecular data were "'cleaner' material for phyletic investigations than morphological characters" because changes at the molecular level were not likely to be affected by the perturbing influence of the organism's interaction with its environment. This notion allowed molecular evolutionists to make sweeping claims about evolution and speciation based solely on the comparison of one or a few protein sequences. To give but one example, Emanuel Margoliash reconstructed the entire evolutionary history of humanity-starting with its one-celled ancestors-by comparing amino acid differences in the cytochrome-c molecule of a wide variety of living organisms.

As studies of evolution at the molecular level proliferated, the importance of traditional evolutionary biology began to be questioned by molecular biologists. In his opening address of the 1964 conference on "Evolving Genes and Proteins," E. L. Tatum of the Rockefeller Institute proudly proclaimed:
This symposium in a certain sense is a milestone on our travels along uncharted paths of molecular biology. Not too many years ago it would have been presumptuous to have undertaken a serious discussion of the molecular bases of evolution…We are now in a position of having maps…indicating some of the guiding landmarks along the road leading to an understanding of the molecular events involved in the evolution of pathways, proteins and genes….It is tempting to try framing our possible travels toward this goal, in terms of an analogy, as a "Biologist's Pilgrims Progress."

Organismal Biologists React (Back to top)

Tatum was not the only molecular biologist to articulate the apparent ascendance of molecular biology over older approaches to the study of life. At least in the eyes of Mayr, Simpson, and Dobzhansky, there was increasing sentiment in both the scientific and lay communities that traditional biology was merely "stamp-collecting" and naturalists were "old-fashioned." In response to these beliefs, the architects of the synthesis launched an "unprecedented counterattack" aimed at protecting organismal biology from the "imperialistic zeal" of molecular biologists. These maneuvers were carried out in popular scientific journals of the day (including Science, American Naturalist, and American Scholar) and portrayed molecular biologists in a less-than-flattering light. G.G. Simpson, for instance, described molecular biology as a "gaudy bandwagon" that was "manned by reductionists, travels on biochemical and biophysical roads, and carries a banner with a strange device: DNA." In his writings throughout the 1960s, Simpson insisted that molecular biology was subservient to evolutionary biology, and not vice versa.

Mayr and Dobzhansky took a slightly different approach, claiming that the molecular approach was better suited than the organismal approach to tackle certain types of problems in biology. They did this primarily by setting up a dualism in the biological sciences. For Dobzhansky, this involved articulating a difference between Cartesian biology (which sought to reduce all biological phenomena to their physical and chemical roots) and Darwinian biology (which sought to make sense of the great mass of biological facts by explaining them in light of evolution). While both biologies were important for studying life, Dobzhansky made it clear that Cartesian biology could only be understood in terms of Darwinian biology and not vice versa. Hence his famous statement that "nothing makes sense except in light of evolution."

Mayr made his case for the importance of molecular and organismic approaches in biology by distinguishing between the proximate and ultimate causes of biological phenomena. Proximate causes, which Mayr believed could often be profitably answered using molecular approaches, dealt with biology at the functional level. For instance, a bird begins its migration on a certain day because fluctuations in daylight have a specific impact on the bird's endocrine system. This type of explanation is profitably reducible to the molecular level (e.g. hormone X binds to receptor Y which leads to a release of hormone Z which causes the bird to leave its summer nesting site and fly south, etc.). However, Mayr argued, it tells us absolutely nothing about the crucial question of why the bird migrates. Such ultimate causes simply cannot be reduced to the molecular level.

As we have documented elsewhere, the architects of the evolutionary synthesis did not argue that molecular biology had no value. Rather, they hoped to integrate the study of biologically important macromolecules into the study of organisms without reducing biology to physics and chemistry. They also hoped to persuade molecular biologists to focus on how all of their molecular data fit within the framework of evolution by natural selection. In more specialized journals and at conferences, they sought to spread the gospel of the evolutionary and organismal approach among the increasingly molecularly-inclined researchers of the day.


Molecular Biology and Evolution