Sir Gavin Rylands de Beer FRS[1] (1 November 1899 – 21 June 1972) was a British evolutionary embryologist, known for his work on heterochrony as recorded in his 1930 book Embryos and Ancestors.

Born on 1 November 1899 in Malden, Surrey (now part of London), de Beer spent most of his childhood in France, where he was educated at the Parisian École Pascal.

His early work was in experimental embryology; some of it was done in collaboration with Huxley, who would go on to be one of the leading figures of the modern synthesis.

De Beer worked on paleornithology and general evolutionary theory, and was largely responsible for elucidating the concept of mosaic evolution, as illustrated by his review of Archaeopteryx in 1954.

De Beer's also reviewed Haeckel's concept of heterochrony, with particular emphasis on its role in avian evolution, especially that of the ratites, in 1956.

[10] His thesis received support in 2016 when Mahaney et al. reported that sediments had been identified at the pass that had been churned up by "the constant movement of thousands of animals and humans" and dated them to the time of Hannibal's invasion.

[10] The conventional view had been that developmental biology had little influence on the modern synthesis, but the following assessment suggests otherwise, at least as far as de Beer is concerned: In a series of remarkable books that established the synthetic theory of evolution, Gavin de Beer's Embryology and evolution was the first and the shortest (1930; expanded and retitled Embryos and ancestors, 1940; 3rd ed 1958).