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The theory, first proposed by Hugo de Vries in 1903, that suggests that new forms of organisms (mutations) arise suddenly in a population, and that evolution proceeds by natural selection operating on these. The theory was based on de Vries' observations of the relatively frequent occurrence of markedly different forms of Oenothera erythrosepala (evening primrose). (It was later demonstrated that the different forms were triploids or tetraploids and hence gave an exaggerated idea of the rate and effects of mutations.) The theory was first validated experimentally when it was found that x-ray induced heritable mutations occurred in Drosophila. It is now recognized that although new allelic forms can only arise by mutation much of the variation on which natural selection acts is generated by crossing over and recombination during meiosis.
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