Larvae of the leaf beetle Chrysomela lapponica feed on birches or willows. Both beetle populations utilize precursor molecules of the plants to produce chemical defenses. As a toxin, the willow population produces salicylaldehyde. The birch population does not produce salicylaldehyde, since birches do not contain the precursor salicin. During evolution this resulted in a defect aldehyde producing enzyme, a salicylalcohol oxidase. The strong association of the leaf beetles with their host plants can be considered as the beginning of a newly emerging species.