Beatty, Dobzhansky, draft 20 April
Theodosius Dobzhansky
Theodosius Dobzhansky (b. 1900 Nemirov, Ukraine; d. 1975 Davis, California) is
perhaps most famous for the slogan, “Nothing in biology makes sense except in the light of
evolution,” which is endlessly invoked by evolutionists in their battles with creationists. But that
is only half the battle that Dobzhansky was waging on behalf of the importance of evolutionary
thinking. He had two targets in mind: creationists on the one hand, and those would-be
evolutionists who naively suppose that molecular biology is the key to life when in fact nothing,
not even the processes of molecular biology can be understood except in the light of evolution.
Ironically, Dobzhansky himself was obsessed in a religious or at least moralistic way with the
“meaning” of everything, evolution included. Ernst Mayr may have believed that evolutionary
biology provided the “ultimate” explanations of life (while molecular biological explanations are
at best “proximate”), but for Dobzhansky, the ultimate understanding of life was deeper still than
evolution (more on this at the end).
To those familiar with his work, Dobzhansky is best known not just as a flag-bearer for
evolutionary thinking, but in particular for his contributions to population genetics. He did as
much or more than anyone else to establish that field and set its early agenda, especially through
his influential textbook, Genetics and the Origin of Species (hereafter GOS). One of the recurrent
themes of GOS was that the study of gene frequency changes is an autonomous field, not
reducible to genetics per se (the genetics of individuals). And its domain is the entirety of
evolution. What Dobzhansky could not prove in the latter regard he stipulated by definition:
“Evolution is [no more than] a change in gene frequencies.”
The main problem of population genetics, as Dobzhansky elaborated it in GOS, is the
“paradox of viability.” On the one hand, each species needs to have variation present in order to
adapt to environmental changes. On the other hand, the particular variations that might prove
adaptive in the future are very likely maladaptive at present. As he concluded in typically
dramatic fashion, “Evolutionary plasticity can be purchased only at the ruthlessly dear price of
continuously sacrificing some individuals to death from unfavorable mutations.”
Dobzhansky’s own insights into the paradox were based on his studies of variation in
natural (non-laboratory) populations, which began in the 1920s in Russia. At that time he was
studying intraspecific variation in ladybird beetles. But it was his studies of variation in natural
populations of the fruitfly, Drosophila, that would prove most influential. These studies were
undertaken in the 1930s, after he had moved to the U.S. and was working with the great
Drosophila geneticist, T.H. Morgan.
Drosophila at first seemed very different from ladybugs. Phenotypically, there seemed to
be no intraspecific variation at all. One need only recall Morgan’s excitement at finally
discovering an observable mutant whose pattern of inheritance could be studied. Subsequently
variations were found in ever greater numbers in his lab. But it was believed by many that these
variations were artifacts. Drosophila in the wild seemed to have little if any intraspecific
variation, and hence perplexingly little material for future adaptive evolution. In trying to make
sense of the difference between laboratory and wild Drosophila, Dobzhansky followed the
suggestion of Sergei Chetverikov, who proposed that most mutations were recessive and would
Beatty, Dobzhansky, draft 20 April
result in observable differences only when doubled-up in the homozygous state. This would
occur more often in laboratory stocks than in nature, since in the former case there is
significantly more inbreeding than in wild populations. In this way, natural populations soak up
variations “like a sponge.”
Dobzhansky (originally in collaboration with Alfred Sturtevant) found a way to observe
otherwise hidden variation in Drosophila by focusing his microscope on variations in
chromosome structure--“inversion” variations--which involve differences in gene arrangement,
and are often associated with gene differences as well. In both respects, these variations
constitute material for evolutionary change. But how was the observed variation maintained?
Dobzhanksy originally adopted (part of) Sewall Wright’s “shifting balance” theory to explain
this variation. He reasoned that the variations were adaptively insignificant and that their
frequencies were drifting randomly. This might lead to a reduction in variation within each
population of a species, but would lead to an increase in variation between populations and hence
within the species.
Dobzhansky was quite surprised, then, to find evidence that the inversion differences
were adaptively significant after all, which led him to explore a range of variation-maintaining
forms of natural selection. He finally settled on the idea that heterozygotes are adaptively
superior to homozygotes, since having two genes for a trait, instead of two copies of the same
gene, allows more adaptability at the individual level. Selection for adaptability at the individual
level in turn preserves evolutionary adaptability at the species level.
Dobzhansky’s account came to be known as the “balance” theory of evolution. His main
detractor was H. J. Muller, whose view of evolution was dubbed by Dobzhansky the “classical”
(old-fashioned) theory. According to Muller, there was no paradox of viability. The vast majority
of mutations are detrimental for the individuals in which they occur, and also for the species in
which they accumulate. There was, in Muller’s opinion, only one species--humans--in which
mutations accumulate to any great extent. Through the amenities of civilization we have
managed to escape natural selection, to our ultimate detriment. Only through a conscious
eugenics program can we now successfully reverse the rising tide of deleterious mutations that
will otherwise cripple our species. Dobzhansky, for his part, criticized “the eugenical Jeremiahs”
who overlook the importance of evolutionary plasticity, and who also seem unaware that natural
populations of many species possess considerable reserves of variation.
Dobzhansky and Muller were also at the center of controversies in the 1950s - early
1970s concerning the impact of radiation-induced mutation from atomic weapons tests (both men
received considerable funding from the Atomic Energy Commission during this period). Adding
yet more mutations to the human species was straightforwardly negative from Muller’s point of
view; less so from Dobzhansky’s. Interestingly though, Dobzhansky was strongly opposed to the
testing and use of atomic weapons, while the once pro-Soviet Muller believed that, up to a point,
such weapons had an important role to play in the containment of Communism.
Dobzhansky also used his studies and perspectives on variation to support liberal
democratic ideals. As he and Leslie Dunn argued in their popular Heredity, Race, and Society,
“the absolute uniqueness of every human individual . . . translated into metaphysical and political
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terms is fundamental for ethics and democracy.” In his widely read Mankind Evolving,
Dobzhansky argued that, far from genetic diversity undermining equality, it actually serves as an
excellent rationale: “Equality of opportunity tends to make the occupational differentiation
comport with the general polymorphism of the population, and would be meaningless if all
people were genetically identical.”
As much as he claimed to understand evolution, Dobzhansky remained deeply puzzled
and uncomfortable about the means by which evolution proceeded. He had found a way to
reconcile the selection of the fittest with the preservation of variation for future adaptive
evolution, but not in a way that reconciled the good of the species with the welfare of its
members. The greater fitness of heterozygotes means that lesser fit homozygotes will continue to
be born in each generation. It has been suggested that Dobzhansky understood this situation
religiously, as a necessary evil--along the lines of free will, which gives us moral freedom, but at
the cost of many sad choices. Perhaps. But he also explicitly regarded it as an “unpleasing”
“imperfection of nature” that so many individuals should suffer such “misery” for the good of
the group.
Dobzhansky drew productively on his Russian heritage and Russian sources in his
investigations and explanations of intraspecific variation. This was a major reason for the
originality of GOS in an Anglo-American context. Dobzhansky’s understanding of the
religious/moralistic significance of intraspecific variation also reflected his Russian heritage,
although this influence has been more difficult to reconstruct, and many of the same concerns-like the evils of natural selection--had also been raised in the Anglo-American context (e.g., by
the atheist T.H. Morgan, in conversations with Dobzhansky).
Suggested readings:
Adams, MB, editor. 1994. The Evolution of Theodosius Dobzhansky. Princeton: Princeton
University Press.
Beatty, J. 1987. “Dobzhansky and drift: Facts, values and chance in evolutionary biology. In The
Probabilistic Revolution, vol 2. Edited by L Kruger et al. Cambridge, Massachusets: Harvard
University Press.
Dobzhansky, T. 1937. Genetics and the Origin of Species. New York: Columbia University
Press.
Dobzhansky, T. 1962. Mankind Evolving: The Evolution of the Human Species. New Haven,
Connecticut.
Lewontin, RC et al., editors. 1981. Dobzhansky's Genetics of Natural Populations I-XLIII. New
York: Columbia University Press.
van der Meer, J. 2007. “Theodosius Dobzhansky: Nothing in evolution makes sense except in the
light of evolution:” Religious interpretation of nature in the population genetics of Theodosius
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Dobzhansky. In Eminent Lives in Twentieth-Century Science & Religion. Edited by N. Rupke.
Frankfurt: Peter Lang.