In the late 1930s and early 1940s, a professionally disparate group of biologists forged an evolutionary synthesis that placed natural selection at the center of the changes in genes and in morphology that have produced the diversity of organisms in the biosphere. The chief architects of this famous synthesis included population geneticists, zoologists, botanists, and paleontologists, but not developmental biologists. Among the tenets of the synthesis was the finding that evolution generally proceeded in populations by selection from among a variety of alleles of structural genes that were provided by various forms of mutation, and which had small but cumulative effects. Some workers concluded that even the evolution of major novelties, such as the distinctive body-plans of different animal phyla, could be produced by the accumulation of such small changes in gene products. Whether the high rates of morphological change inferred from the fossil record could have actually been achieved by these processes alone was not clear.