Harvey's doctor prescribed a heart medicine that led to digestive problems in harvey. he saw a different doctor for that issue and was prescribed another drug, which led to headaches. yet another doctor prescribed another drug for his headaches. harvey has just experienced

The answer is prescribing cascade.

Prescribing Cascade can be defined as the the process in which the side effects of the drugs are not diagnosed precisely. In this case the symptoms of the problem results in further prescription and further side effects.This cascade results in generation of prescription for every symptom and this results in unanticipated drug interaction. This is an example of feedback loop.

on island x the species needed to adapt because they were being subject to predators which didn't inhabit island y.