The carabid beetle genus Agonum comprises species with an aggregate Holarctic distribution, plus one clade found in Asia and Africa. Using morphological characters we have deduced a phylogenetic hypothesis for the natural entities comprising this group, rooting the cladogram at the African sister group Agonidium. Distributional ranges vary tremendously among the species; some are peripheral isolates occupying only a fraction of the range of the adelphotaxon, others are broadly sympatric assemblages of related species, which in some instances span hemispheres. This occurrence of many widespread, sympatric species is typical for many Holarctic groups, who have diversified prior to the great perturbations of Pleistocene glaciation. We compare the results of several biogeographic methods-including reconciled tree analysis and dispersal-vicariance analysis-to illuminate the reticulate pattern of Holarctic area relationships exhibited by Agonum. Pleistocene fossil records are used to assist the definition of species distributions. The inclusion of Pliocene fossils allows the dating of internal nodes on the cladogram, supporting an initial Eocene or pre-Eocene diversification within the genus - including amphi-Atlantic area relationships-and later trans-Beringian diversification associated with Oligocene to Pleistocene sea-level fluctuations.
Species 1: Coleoptera Carabidae Agonum
Species 2: Coleoptera Carabidae Agonidium
Keywords: Holarctic biogeography
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