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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on published and unpublished works by Michael Dietrich, John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
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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.
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