Summary1. The endophagous weevil Lixus elongatus was studied on two Carduus species in order to look for the impact of a structural gradient in host plant stem diameter on the life cycle and the genetic structure of this species.2. Body size of L. elongatus females was correlated positively with their fecundity and the stem diameter at the location of oviposition.3. Emerging individuals showed a significantly positive relationship of body size with the stem diameter at their pupation and emergence site. Mating behaviour was positively size‐assortative.4. Allozyme analyses indicated a gradient of genetic differentiation along body size classes and a decreasing heterozygosity with decreasing body size.5. The results led to the conclusion that a structural gradient in plants can promote body size variability in an endophagous herbivore through phenotypic constraints of the host plant and the herbivore. As size‐assortative mating translates this variability into differential gene flow between individuals of different body size, this could provide the raw material for speciation processes. Within a single host plant patch, however, further processes are necessary to split the existing genetic gradient into separate gene pools.

Synchronous speciation of hosts and herbivorous insects predicts a congruent topology of host and insect phylogenies and similar evolutionary ages of host and insect taxa. To test these predictions for the specialized herbivorous fly genus Urophora (Diptera: Tephritidae), we used three different approaches. (i) We generated a phylogenetic tree of 11 European Urophora species from allozyme data and constructed a phylogeny of their hosts from published sources. Superimposing the Urophora tree on the host‐plant tree we found no evidence for general congruence. (ii) We correlated genetic distances (Nei distances) of the host plants vs. the genetic distances of associated Urophora species. Overall, the relationship was not positive. Nevertheless, for some pairs of Urophora species and host plants genetic distances were in the same order of magnitude. (iii) We collected allozyme data for pairs of thistle taxa and pairs of herbivores on thistles together with independent time estimates. With these data we calibrated a molecular clock. There was a non‐linear relationship between phylogenetic age and genetic distance, rendering the dating of deep events in thistle–insect evolution difficult. Nevertheless the derived molecular clock showed that the split of insect taxa lagged behind the split of hosts. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84, 775–783.

The distribution of genetic variance was investigated in two closely related Oreina leaf beetle species, Oreina cacaliae and Oreina speciosissima. Populations of these alpine beetles were sampled in mountainous areas of Western Europe, the total sampling area ranging from the Pyrenees to the Czech Republic. Allozyme electrophoresis of 21 (O. cacaliae) and 16 (O. speciosissima) loci revealed high genetic variability as expressed in a high percentage of polymorphic loci (only one monomorphic locus was found in each species) and high heterozygosities. No overall linkage disequilibrium was found in either species. Extensive heterozygote deficits were observed in several samples as reflected by high F IS-values and high overall inbreeding coefficients (F IT) of 0.349 (O. cacaliae) and 0.503 (O. speciosissima). The overall inbreeding coefficient was mainly attributable to within-population differentiation. The high heterozygote deficits are explained by a combination of inbreeding resulting in kinship groups and a sampling effect over several such kinship groups. No explanation of the observed patterns could be found in the host plant use or altitudinal location of the samples. For O. cacaliae, isolation by distance was found, but not for O. speciosissima. Gene flow estimates were in the range of Nm = 0.8 to 1.5.

The distribution of genetic variance was investigated in two closely related Oreina leaf beetle species, Oreina cacaliae and Oreina speciosissima. Populations of these alpine beetles were sampled in mountainous areas of Western Europe, the total sampling area ranging from the Pyrenees to the Czech Republic. Allozyme electrophoresis of 21 (O. cacaliae) and 16 (O. speciosissima) loci revealed high genetic variability as expressed in a high percentage of polymorphic loci (only one monomorphic locus was found in each species) and high heterozygosities. No overall linkage disequilibrium was found in either species. Extensive heterozygote deficits were observed in several samples as reflected by high F IS -values and high overall inbreeding coefficients (F IT ) of 0.349 (O. cacaliae) and 0.503 (O. speciosissima). The overall inbreeding coefficient was mainly attributable to within-population differentiation. The high heterozygote deficits are explained by a combination of inbreeding resulting in kinship groups and a sampling effect over several such kinship groups. No explanation of the observed patterns could be found in the host plant use or altitudinal location of the samples. For O. cacaliae, isolation by distance was found, but not for O. speciosissima. Gene flow estimates were in the range of Nm = 0.8 to 1.5.