 Perturbations can be genetic or epigenetic rather than environmental.For example, "Drosophila" fruit flies have a heat shock protein, Hsp90, which protects the development of many structures in the adult fly from heat shock.If the protein is damaged by a mutation, then just as if it were damaged by the environmental effects of drugs, many different phenotypic variants appear; if these are selected for, they quickly establish without further need for the mutant Hsp90.