Natural selection and artificial selection (selective breeding) both have important roles in evolutionary change. (a) Seahorses are fish with an unusual appearance and many specialised features. Fig. 8.1 shows a seahorse. [Figure 8.1] Two species of seahorse, Hippocampus erectus and H. zosterae, are found in the coastal waters of the Gulf of Mexico and the Caribbean Sea. • It is thought that these two species of seahorse have a common ancestor. • The ranges of the two species overlap in many areas. • Male and female seahorses stay together for the whole of the breeding season. • H. erectus is much larger than H. zosterae. • Few seahorses occur that are intermediate in size between the two species.