Use of Host-Plant Trait Space by Phytophagous Insects during Host-Associated Differentiation: The Gape-and-Pinch Model

Table 1

Relationships between patterns in trait-space use (Figures 2(a)–2(d)), the GAP model, and statistical tests implemented for analysis of attacked trait spaces.

Pattern in trait-space use	Hypothesis	Attacked trait spaces marginal?^1	Attacked trait spaces restricted?^1	Attacked trait spaces distant?
Monophagous, attacked trait space marginal and adjacent to alternative host (Figure 2(a))	Adjacency favours host shifting (adjacent errors hypothesis)	Ancestral host: marginal Novel host: marginal but rare^2	—	Attacked spaces close^2
Oligophagous, attacked trait spaces marginal and adjacent (Figure 2(b))	Adjacency persists after host shifting (adjacent oligophagy hypothesis)	Ancestral host: marginal Novel host: marginal	—	Attacked spaces close
Nascent host races, attacked trait spaces marginal and distant (Figure 2(c))	Distance permits, and is also symptomatic of, genetic isolation (trait distance-divergence hypothesis)	Ancestral host: marginal Novel host: marginal	—	Attacked spaces distant
Pair of monophagous species, attacked trait spaces central on each host (Figure 2(d))	Other isolating mechanisms reduce importance of trait-space distance (distance relaxation hypothesis)	Ancestral host: not marginal; Novel host: not marginal	Ancestral host: restricted (central) or nonselective Novel host: restricted (central) or nonselective	Attacked spaces neither close nor distant

¹More strictly, only restricted or marginal trait-space use along the PC axis (or axes) defining the difference between available hosts is directly relevant to the GAP model.
²But statistical detection is difficult because attack on the novel host is rare.