CONCEPT
Modern Synthesis (Evolutionary Biology)
The mid-twentieth-century integration of Darwinian selection with Mendelian genetics — the gradualist framework Margulis challenged by demonstrating that symbiogenesis, not mutation, produced the major transitions in complexity.
The Modern Synthesis (also called the neo-Darwinian synthesis) is the early-to-mid-twentieth-century unification of Charles Darwin's theory of natural selection with Gregor Mendel's genetics, population genetics, and paleontology into a comprehensive theory of evolution. Developed by figures including Ronald Fisher, J.B.S. Haldane, Sewall Wright, Theodosius Dobzhansky, Ernst Mayr, and George Gaylord Simpson between the 1920s and 1950s, the synthesis established that evolutionary change occurs through shifts in allele frequencies in populations, driven by mutation, selection, genetic drift, and gene flow. The framework is mathematically rigorous, predictive, and empirically supported. It is also, in Margulis's assessment, incomplete: it explains microevolution (changes within species) and adaptation (refinement of existing designs) but cannot adequately account for macroevolution (the origin of major new body plans and metabolic capabilities). The Modern Synthesis assumes gradualism — that all evolutionary change is decomposable into small steps — and Margulis spent her career demonstrating that the most consequential changes were not gradual but discontinuous, produced by symbiotic mergers.
