Diversification in a fluctuating island setting: rapid radiation ofOhomopterusground beetles in the Japanese Islands

Abstract

The Japanese Islands have been largely isolated from the East Asian mainland since the Early Pleistocene, allowing the diversification of endemic lineages. Here, we explore speciation rates and historical biogeography of the ground beetles of the subgenus Ohomopterus (genus Carabus) based on nuclear and mitochondrial gene sequences. Ohomopterus diverged into 15 species during the Pleistocene. The speciation rate was 1.92 Ma -1 and was particularly fast (2.37 Ma) in a group with highly divergent genitalia. Speciation occurred almost solely within Honshu, the largest island with complex geography. Species diversity is highest in central Honshu, where closely related species occur parapatrically and different-sized species co-occur. Range expansion of some species in the past has resulted in such species assemblages. Introgressive hybridization, at least for mitochondrial DNA, has occurred repeatedly between species in contact, but has not greatly disturbed species distinctness. Small-island populations of some species were separated from main-island populations only after the last glacial (or the last interglacial) period, indicating that island isolation had little role in speciation. Thus, the speciation and formation of the Ohomopterus assemblage occurred despite frequent opportunities for secondary contact and hybridization and the lack of persistent isolation. This radiation was achieved without substantial ecological differentiation, but with marked differentiation in mechanical agents of reproductive isolation (body size and genital morphology).

Application of RAD-based phylogenetics to complex relationships among variously related taxa in a species flock

“…Among these species groups, the iwawakianus-insulicola group (Sota and Vogler, 2003) exhibits the most marked diversification in genital morphology. The six species are estimated to have diverged 0.76 million years ago, representing the most rapid speciation in this subgenus (Sota and Nagata, 2008). Previous molecular phylogenetic studies revealed that phylogenetic reconstruction of this subgenus is extremely difficult owing to introgressive hybridization between species and/or incomplete lineage sorting of ancestral polymorphism Vogler, 2001, 2003;Sota and Sasabe, 2006;Sota and Nagata, 2008).…”

“…Ohomopterus diverged from its sister subgenus Isiocarabus in East Asia 2.14 million years ago during the Pleistocene (Sota and Nagata, 2008). Molecular phylogenetic studies using nuclear DNA revealed that this subgenus diverged into several species groups during the early Pleistocene, followed by differentiation within the species groups leading to the extant species (Sota and Vogler, 2003;Sota and Nagata, 2008).…”

“…The subgenus Ohomopterus of the genus Carabus is one of the best models of rapid diversification resulting in a species flock, with 15 or more species on the Japanese islands in which diverged body size and genital morphology play a major role in reproductive isolation between species (Sota and Nagata, 2008). Ohomopterus diverged from its sister subgenus Isiocarabus in East Asia 2.14 million years ago during the Pleistocene (Sota and Nagata, 2008).…”

Ecological differentiation and habitat unsuitability maintaining a ground beetle hybrid zone

“…In a hybrid zone between Carabus maiyasanus and C. iwawakianus , a physical incompatibility between the male and female genitalia is an agent of endogenous selection (Kubota 1988; Kubota and Sota 1998; Sota and Kubota 1998). These two species are closely related (Sota and Nagata 2008; Takahashi et al. 2014) and exhibit little mate discrimination between conspecifics and heterospecifics; accordingly, interspecific mating occurs frequently (Sota and Kubota 1998).…”

Introduction. Evolution on Pacific islands: Darwin's legacy

“…These deal with beetles in Japan (Sota & Nagata 2008), land snails in Belau (Rundell 2008) and the Ogasawara Islands (Davison & Chiba 2008), iguanas in Fiji (Keogh et al 2008), crickets in New Zealand, Australia and New Caledonia (Pratt et al 2008), weevils in the Galápagos (Sequeira et al 2008) and moths in the Galápagos (Schmitz et al 2008) and Hawaii (Rubinoff 2008).…”