If the island on which the iguanas lived had flooded and the food had become much more scarce, than the iguanas in order to survive will have to adapt to the new conditions.
The chances are very big that the iguanas will develop webbed feet so that they can spend more time in the water and be more efficient swimmers. Since the food is rare, the size of the iguanas will decrease. Also, they will have to become more opportunistic in their diet, which will most likely result in the change of the jaw structure and the muscles that support it, thus giving the head much different appearance.
