What is Evo-devo?

The genes that build animals are an ancient and shared toolkit: a few hundred transcription factors and signalling pathways present in the common ancestor of all animals, and animal diversity comes mostly from changes in when, where and how much they are expressed rather than in what they encode. Deep homology: the gene Pax6 directs eye formation in flies, squid and mice, whose eyes are not homologous as organs — a mouse Pax6 expressed on a fly’s leg makes an ectopic fly eye there (Halder, Gehring, 1995). Cis-regulatory evolution: the loss of pelvic spines in freshwater sticklebacks is the deletion of one enhancer of the Pitx1 gene, the protein untouched and still serving elsewhere; the loss of wing spots in some fruit flies, a change in an enhancer of yellow; the loss of legs in snakes, a broken enhancer of sonic hedgehog in the limb. Hox shifts: the boundary of Hoxc6 expression marks the first thoracic vertebra in mouse, chick, goose and snake alike, at vertebra 7, 14, 23 and 3; insects’ loss of abdominal legs was the Hox protein Ubx acquiring a repressive domain that switches off the leg gene Distal-less, where in crustaceans it does not. Heterochrony, a change in the timing of a developmental programme, made the axolotl a permanent larva and the human face a juvenile ape’s. The old question of how new forms arise becomes a question about regulatory DNA: most of the genome that matters for form is not gene but switch.