(Introduction to ichthyological osteology)
As fish evolved so did skeletal changes and nowhere as quickly as in the jaws, head and bones that support the fins; today these are as if not more important as DNA evidence.
Evolution of Cichlids
By 250mya there were around 1200 species of fish. Between then and now around 27,000 species of fish evolved.
Our multi finned brackish friend
As marine fish currently entering fresh waters sticklebacks are being studied to observe the micro-evolution demonstrable in contemporary time frames.