Phylogeny and biogeography of the Laurasian genus Agonum Bonelli (Coleoptera, Carabidae, Platynini)

Abstract

The World fauna of Agonum Bonelli, 1810 was analyzed by cladistic parsimony implementing Nixon's ratchet, based on 135 terminals representing 131 species, and 138 unit-coded characters, including 98 external morphological characters, 21 characters of the female reproductive tract and ovipositor, and 19 male genitalic characters. The cladogram was outgroup-rooted to the African Agonidium Jeannel, 1948. Phylogenetic data were summarized via strict consensus; this consensus was used as the basis for cladistic classification. A single preferred cladogram was chosen from among 4576 unambiguously optimized, equally parsimonious cladograms using Goloboffs fit value as an optimality criterion, and converted to a taxon-area cladogram for biogeographic analysis. Coxal and femoral setation has generally increased during phylogenesis, and served to define natural groups. The configuration of the female bursa copulatrix helped define the Agonum ingroup. Spermathecal duct length has increased dramatically during diversification of Agonum, though reversal to a short, broad duct has also occurred. A balanced cladistic classification based on relationships maintained in the strict consensus cladogram recognized four subgenera: Platynomicrus Casey, 1920 (2 spp.), Europhilus Chaudoir, 1859 (28 spp.), Agonum (29 spp.), and Agonothorax Motschulsky, 1850 (72 spp.). Nomenclatural changes necessitated by the cladistic analysis include: 1, Megalonychus Chaudoir, 1843 = Agonum Bonelli, 1810; 2, Agonops Bousquet, 2002 = Agonum Bonelli, 1810; 3, 12 species previously combined with Megalonychus are NEWLY COMBINED with Agonidium, whereas the remaining 32 species previously combined with Megalonychus are provisionally assigned to Agonidium pending future study. Fossil locality data were used to determine Pliocene and Pleistocene distributional limits, and to establish minimum ages for extant taxa. Estimates of species duration for three species preserved in the Beaufort Formation of Arctic Canada range from 3–4,000,000 yr. Pliocene and Pleistocene fossil localities in eastern North America and Greenland augmented extant species distributions analyzed biogeographically. Dispersal-vicariance biogeographic analysis (DIVA), which optimized area relationships among major areas of endemism in the northern hemisphere using the taxon-area cladogram, indicated that Agonum diversification began in Eocene time when Europe and Eastern North America were connected by North Atlantic Land Bridges. Eocene origin of Agonum is consistent with a Paleocene-constrained biotic connection to the African outgroup Agonidium. Subsequently, Beringian exchange was associated with Agonum diversification through Late Miocene and Pliocene. Dispersal across Pleistocene-aged Beringia is associated strictly with extant Holarctic and circumpolar species. Plesiomorphically, Agonum beetles were marsh dwellers. Subsequent ecological specialization has involved use umbrotrophic bogs, gravel and cobble streambank habitats, shores of saline lakes and seashores, and moist forest floors. Speciation has followed both an allopatric vicariant mode, and peripatric isolation of peripheral populations. Both Mayr's centripetal speciation and Brown's centrifugal speciation models were supported by geographic relationships of various sister species, indicating that environmental factors have greatly influenced speciation mode, especially of peripheral isolates.