Conrad Hal Waddington (1905–1975)

Abstract Conrad Hal Waddington (1905–1975) took a degree in Geology at Sidney Sussex College, Cambridge. He began but never finished a graduate degree; genetics and experimental embryology replaced palaeontology. He began experimental studies on the chemical nature of the primary organiser discovered by Spemann and Mangold in 1924. From collaboration with Joseph and Dorothy Needham and through the Theoretical Biology Club, came the concepts of evocation and individuation. Establishment of a Unit on Animal Breeding and Genetics in Edinburgh after the second World War provided Waddington with his professional home where he sought to integrate genetics and development into an evolutionarily relevant discipline. Adhering to the organic philosophy initiated by Alfred Whitehead (organicism) Waddington identified three major modes of explanation for the development of organisms as integrated wholes: canalisation, genetic assimilation and epigenetics. The metaphorical epigenetic landscape became the way most developmental biologists ‘saw’ the organisation of embryonic development. Integrated, heritable, epigenetic organisation of embryonic development is Waddington's lasting legacy to development and evolution. Key Concepts Canalisation is a homeostatic mechanism by which development is buffered against environmental or genetic variation, allowing the same feature to form under changing conditions. Genetic assimilation is the process by which embryos respond to changes in the environment by producing a different phenotype, which, through selection, can arise in the absence of the environmental change. Epigenetics (Waddingtonian epigenetics) is the term for the interactions among genes and between genes and environmental signals that result in the formation of elements of the phenotype. Molecular epigenetics is the study of how mechanisms such as methylation or imprinting regulate gene action. Interactions between parts of developing animal embryos (embryonic induction) are essential for the development of major body parts.