Starting “The Modern Synthesis”: Theodosius Dobzhansky (1 of 2)

Ronald Fisher and his colleagues set Darwin’s concept of natural selection on a new foundation of genetics. They left an equally major project open for later biologists: to explain in the language of genes, what species are and how they originate. The answer only began to emerge in the 1930s, thanks in large part to the work of a Soviet-born geneticist named Theodosius Dobzhansky (right).

Dobzhansky, who emigrated to the United States in 1928, worked in Thomas Hunt Morgan’s “Fly Room,” where mutations were being studied closely for the first time. He also paid careful attention to the work of population geneticists such as Sewall Wright, who were showing how the size of a population affects the rate at which a mutation can spread. Dobzhansky was interested in discovering the genetics that determined the differences between populations of a species.

Genetically Variable Populations
At the time, most biologists assumed that all of the members of any given species had practically identical genes. But these were assumptions bred in the lab. Dobzhansky began analyzing the genes of wild fruit flies, traveling from Canada to Mexico to catch members of the species Drosophila pseudoobscura. He found that different populations of D. pseudoobscura did not have identical sets of genes. Each population of fruit flies he studied bore distinctive markers in its chromosomes that distinguished it from other populations.

Dobzhansky helped discover that different fruit fly populations have different frequencies of two different versions of the same chromosome; chromosome A might be more frequent in one population while chromosome A' is more frequent in a neighboring population.

If there was no standard set of genes that distinguished a species, what kept species distinct from each other? The answer, Dobzhansky correctly realized, was sex. A species is simply a group of animals or plants that reproduces primarily among themselves. Two animals belonging to different species are unlikely to mate, and even if they do, they will rarely produce viable hybrids. Dobzhansky ran experiments on fruit flies that demonstrated that this incompatibility is caused by specific genes carried by one species that clash with the genes from another species.