How does the environment affect evolution? Waddington's example of genetic assimilation

Conrad Waddington, throughout his long and varied career as a developmental biologist, was foundational to several aspects of modern evolutionary theory, such as epigenetics, developmental canalization, and genetic assimilation. One of the great puzzles of evolution is how organisms can become so specifically and heritably adapted to their environment. Random genetic mutations can sometimes serve as the sole explanation, but not always. Through several rather cleverly simple experiments, Waddington demonstrated that phenotypes elicited by a specific environmental cue (such as heat shock or ether treatment) could be "assimilated" into the genotype. This means that the phenotype could eventually be expressed even if the corresponding cue was absent. He argued that this phenomena, which he witnessed in Drosophila in less than 30 generations, could play a powerful and vital role in evolution.
Background paper: Genetic assimilation of cross-veinless phenotype (1953)

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