Micro‐evolution in the weevil genus Larinus : the formation of host biotypes and speciation

Data are presented on allozyme variation between 15 populations of the stenophagous capitulum weevil, Larinus cynarae , and three populations of its congener, L. latus , that had been collected throughout the northern mediterranean range of these species. A phenetic analysis of these data revealed no direct relationship between genetic variation and host‐plant association within L. cynarae , but there was a strong geographical structuring of allozyme patterns. Most of the genetic variation was due to differences between geographical regions and variation within these was small. Wright's F ST values showed that Italian and Greek populations of L. cynarae were most distinct from L. latus , with southern Iberian, northern Spanish and French populations increasingly less so. This pattern was associated with a cline in the frequencies of certain alleles along this geographical arc from France to Greece. A phenogram of Nei's genetic distances indicated the close genetic relationship between the two species of Larinus and separated the populations of L. cynarae into three allopatric groups. These groups have different host‐plant spectra ‐ dominated by Cynara cardunculus in Italy and Greece, Cynara humilis/Onopordum in southern Iberia and Onopordum spp. in France/Northern Spain ‐ and can be considered to be host biotypes of L. cynarae. L. latus , which occurs in Greece and further east is also an Onopordum specialist. An analysis of the phylogeny of this group of Larinus indicates a primary separation into eastern ( L. latus ) and western ( L. cynarae ) taxa, with further branching of the L. cynarae lineage into the putative host‐biotypes. An hypothesis for the evolution of these taxa is given, based on the evolutionary history of host‐plant taxa and geographical constraints.