Literally ‘different form’: changes in developmental outcome arising from mutations in key regulatory molecules that control embryonic pattern formation and tissue specification. Such changes can lead to dramatic changes in form during evolution. A classic example is the suppression of limb growth in the abdominal segments of insects. This is thought to have arisen in an ancestral arthropod through mutation of a gene called Ultrabithorax (Ubx). The mutant gene expressed a Ubx protein capable of repressing the gene Distalless required for leg development in the abdomen. Hence, the descendants had legs confined to the thoracic segments, and became the insects. Compare heterochrony; heterometry; heterotony.