Depicted are the two ecotype pairs and the species pair studied for the degree of phenotypic and evolutionary divergence in relation to the number of niche dimensions subject to divergent selection. A1 and C1 refer to ecotypes of T. cristinae (A = Adenostoma and C = Ceanothus hereafter). A2 and C2 refer to ecotypes of T. podura. A3 and C3 refer to the species pair T. podura and T. chumash, respectively. The ecotype pairs exhibit weaker divergence in morphology, host preference, and mtDNA than the species pair, and are also subject to divergent selection on fewer niche dimensions. A) Photographs of the three taxon pairs, and divergence in colour-pattern between them (brightness contrast, mean±95% C.I.). Host plants are also shown. B) Summary of divergence in host plant preferences and mtDNA (colours represent host plant use). Δ%C refers to the difference between each taxon pair in the percent of individuals choosing Ceanothus over Adenostoma in host preference trials [data from 78, 72, and the current study for T. cristinae ecotypes, T. podura ecotypes, and the species pair, respectively]. The phylogenetic trees are schematic for simplicity. The patterns depicted were robust to alternative methods for tree construction [75], [81 for details]. C) The nature of selection on crypsis and physiology for each taxon pair. For crypsis, the term ‘survival’ is used as a general y-axis label, representing the fitness of each insect host form on each host species. For ecotypes, the y-axes specifically represent 1- the proportion of insects eaten in predation trials with scrub jays [data from 72]. For the species pair, the y-axis specifically represents the proportion of each insect species on each host plant at the end of the field experiment (shown in more detail in Fig. 2). Further evidence that selection is exerted by visual predation stems from the observation that: (a) survival was measured using predation trials, or (b) divergent selection in manipulative field experiments was detected in the presence, but not in the absence, of visual predation [see also 70], [71]. For physiology, the y-axis represents lifetime fecundity in all cases (data on survival for the species pair are also reported in Fig. 2). The data depicted can be used to infer the presence versus absence of divergent selection, but should not be used to quantitatively compare the strength of selection (because somewhat different experimental procedures were used among taxa). For simplicity, error bars were removed for the current figure, but are depicted in Fig. 2. See text for statistical details.

A) Host-plant preferences of Timema collected from Adenostoma (T. podura) or Ceanothus (T. chumash). Shown for each insect species is the percent of individuals choosing each host species in host choice trials. Numbers of individuals are denoted above the bars. Each Timema species preferred its native host. B) Results of the predation experiment. T. podura and T. chumash were released at equal proportion onto Ceanothus and Adenostoma bushes. Four weeks later the relative proportion of each insect species had diverged, but to a much larger extent when predation was present versus absent. Shown is the proportion of T. chumash (±1 S.E.) on each host species. C) Results of the physiology experiment. Norm-of-reaction plots showing means and standard errors (±1 S.E.) of survival of walking-sticks from Ceanothus or Adenostoma raised on their native or the alternative host-plant species. Survival was estimated as the mean number of insects observed alive within an enclosure, averaged across the multiple census periods. Physiological trade-offs in host plant use were evident.

The graphs depict fitness functions (y-axis is fitness, x-axis is trait value/habitat of origin), with crossing lines indicative of divergent selection. Population pairs using the same host (left, e.g., two populations in different geographic location that both use Ceanothus) are not exposed to divergent selection and show no progress towards speciation. Ecotype pairs (center) are exposed to divergent selection along a single axis (crypsis), and show only partial progress towards speciation. Species pairs (right) are exposed to divergent selection along both axes.