Modern evolutionary synthesis
The evolution of flatfish (pleuronectiformes), such as plaice, sole, turbot and halibut. Their young are perfectly symmetrical, but the head is remodelled during a metamorphosis. One eye moves to the other side, close to the other eye. Some species have both eyes on the left (turbot), some on the right (halibut, sole); all living and fossil flatfish to date show an 'eyed' side and a 'blind' side. Darwin predicted a gradual migration of the eye in evolution, mirroring the metamorphosis of the living forms. A recent examination of two fossil species from the Eocene shows "the assembly of the flatfish bodyplan occurred in a gradual, stepwise fashion". The intermediate stages were fully viable: these forms ranged over two geological stages, and are found in sites which also yield flatfish with the full cranial asymmetry. The evolution of flatfish falls squarely within the evolutionary synthesis.