Figure 2: Some possible relationships between attacked trait spaces on two alternative host plants. Illustrated are hypothetical relationships for four stages during a host shift and subsequent host-associated differentiation. (a) Specialist on host 1, predisposed to host-choice errors due to marginal attack on trait space adjacent to the alternative host. (b) Following diet expansion, herbivore is oligophagous accepting both hosts; selection favours central attack on the combined trait space. (c) Early evolution of host-associated differentiation: use of marginal and distant trait spaces on the two hosts reduces host-choice errors and hence gene flow. (d) Reproductively isolated monophagous specialists on each host: selection favours central attack by each specialist on the trait space of its host. See text for further discussion of these scenarios.