Insect—Host-plant Relationship — Co-evolution or Sequential Evolution?

Abstract

Co-evolution is the generally accepted theory for the evolution of insect—host-plant relationships, however, it can be shown that its main premisses are inadequate: (1) most phytophagous insects have very low population densities compared to the biomass of their host-plants, therefore, they can hardly be important selection factors for the plant; (2) insect—host-plant interactions are not necessarily antagonistic: mono- and oligophagous insects, if their number is fairly high, may ideally regulate the abundance of their host-plants (mutual advantage); consequently, (3) resistance to insects is not a general necessity in plants and it cannot explain the presence of secondary plant substances; (4) parallel evolutionary lines of plants and insects which should result from co-evolutionary interactions are rare, while many closely related insects feed on botanically very distant plant taxa — a relationship which cannot be related to co-evolution.