A Systems View of Waddington’s Genetic Assimilation

Lamarck believed that traits acquired during an organism’s lifetime could be passed onto the next generation. Although the idea of the inheritance of acquired characters was discarded due to lack of experimental evidence, Conrad H. Waddington realized its significance. In 1953, he showed that Drosophila melanogaster (wild-type) flies that were heat-shocked produced a Crossveinless (cve; disrupted posterior crossveins) trait. Through repeated selection of this trait with heat-shock, he not only increased its frequency in the population, but also found that individuals, from the untreated stock, showed the phenotype. This apparent inheritance of an acquired character is important to evolutionary theory, because it provides a mechanism whereby the environment may influence future evolutionary change. Despite the long history of this experiment, genetic assimilation remains elusive. The main aim of this work was to examine genetic assimilation and understand it as an evolutionary theory. Revisiting the experiment indicated that there is much that remains unclear. We have shown that production of cve is strain specific, with the white-eyed lines being vulnerable and the wild-type not. Though the frequency of the cve allele increased in every generation, there was a fitness cost for acquiring crossveinless. Assimilation of cve was found to be heritable but, unlike Waddington’s classic work, it did not tend towards fixation; appearing more like a transient, low penetrance effect