A hypothetical new phenotype, or monstrosity, that arises due to mutations that radically alter an individual’s developmental pattern. Such an individual, it is claimed, could possess major innovations that equip it for an environment quite different from that of its immediate antecedents. The concept, which was introduced in 1933 by the geneticist R. Goldschmidt, presents a way in which, theoretically, a new group could arise in a single macroevolutionary leap, rather than by the gradual process of natural selection producing many small adaptive changes—the route favoured by orthodox neo-Darwinians. There are some well-documented examples of single-gene mutations causing large morphological changes that appear to be of adaptive value, such as the loss of pelvic spines in freshwater three-spined sticklebacks bred from their marine counterparts. In this light, Goldschmidt’s ideas, which were initially ridiculed, have been reappraised.