Fig. 1

Two models explaining the formation of spatially non-overlapping (or near non-overlapping) ecotypes. Under the sympatric host-shift model (a) insects from a population (Eco 1) shift to a novel host-plant (Host 2) and become a new ecotype (Eco 2). Note that the lack of competition [13] on the novel host-plant is often invoked to explain why such shifts occur (see introduction). After the host-shift occurs the new ecotype expands into the distribution range of the novel host (Host 2). Under the allopatric host-shift model (b) insects from a population (Eco 1) disperse into a region where its ancestral host-plant is absent (Host 1). It is then forced to use an alternative host-plant (Host 2). It adapts to the novel host-plant and becomes a new ecotype (Eco 2). While the allopatric host-shift model can occur when host-plant distribution ranges are completely non-overlapping or partially non-overlapping, the sympatric host-shift model can only occur when host-plants have partially overlapping distribution ranges. Note that we do not attempt to cover all possible models. See, for example, the more complex specialisation oscillation hypothesis [69]