In the first few decades of the 20th century, most field naturalists continued to believe that Lamarckian and orthogenic mechanisms of evolution provided the best explanation for the complexity they observed in the living world. However, as the field of genetics continued to develop, those views became less tenable. Theodosius Dobzhansky helped to bridge the divide between the foundations of microevolution developed by the population geneticists and the patterns of macroevolution observed by field biologists. Dobzhansky examined the genetic diversity of wild populations and showed that, contrary to the assumptions of the population geneticists, these populations had large amounts of genetic diversity, with marked differences between sub-populations. In Great Britain E.B. Ford, the pioneer of ecological genetics, continued throughout the 1930s and 1940s to demonstrate the power of selection due to ecological factors. Ford’s work would contribute to a shift in emphasis during the course of the modern synthesis towards natural selection over genetic drift.
Mayr followed up on Dobzhansky’s work with the 1942 book “Systematics and the Origin of Species”, which emphasized the importance of allopatric speciation in the formation of new species. This form of speciation occurs when the geographical isolation of a sub-population is followed by the development of mechanisms for reproductive isolation. Mayr also formulated the biological species concept that defined a species as a group of interbreeding or potentially interbreeding populations that were reproductively isolated from all other populations.
In the 1944 book “Tempo and Mode in Evolution”, George Gaylord Simpson showed that the fossil record was consistent with the irregular non-directional pattern predicted by the developing evolutionary synthesis. In 1950, G. Ledyard Stebbins published “Variation and Evolution in Plants”, which helped to integrate botany into the synthesis.
The evolutionary synthesis provided a conceptual core that tied together many, but not all, biological disciplines. It helped establish the legitimacy of evolutionary biology in a scientific climate that favoured experimental methods over historical ones. The synthesis also resulted in a considerable narrowing of the range of mainstream evolutionary thought. By the 1950s, natural selection acting on genetic variation was virtually the only acceptable mechanism of evolutionary change (pan-selectionism), and macroevolution was simply considered the result of extensive microevolution.