Abstract
New Zealand biodiversity has often been viewed as Gondwanan in origin and age, but it is increasingly apparent from molecular studies that diversification, and in many cases origination of lineages, postdate the break-up of Gondwanaland. Relatively few studies of New Zealand animal species radiations have as yet been reported, and here we consider the species-rich genus of carabid beetles, Mecodema. Constrained stratigraphic information (emergence of the Chatham Islands) and a substitution rate for Coleoptera were separately used to calibrate Bayesian relaxed molecular clock date estimates for diversification of Mecodema. The inferred timings indicate radiation of these beetles no earlier than the mid-Miocene with most divergences being younger, dating to the Plio-Pleistocene. A shallow age for the radiation along with a complex spatial distribution of these taxa involving many instances of sympatry implicates recent ecological speciation rather than a simplistic allopatric model. This emphasises the youthful and dynamic nature of New Zealand evolution that will be further elucidated with detailed ecological and population genetic analyses.
Highlights
Biologists have long been perplexed with New Zealand’s biotic composition which cannot be classed as typically oceanic or continental [1,2]
M. alternans may be better treated as a species complex [47], no morphological characters have yet been described that distinguish Chatham Island populations from those in mainland New Zealand
Incomplete resolution of some specieslevel relationships in Mecodema reflects the low levels of DNA sequence divergence in some cases, for example, at the cytochrome oxidase I (COI) locus, a maximum of 3.6% among more than 8 species in one sub-clade is observed
Summary
Biologists have long been perplexed with New Zealand’s biotic composition which cannot be classed as typically oceanic or continental [1,2]. Its enigmatic biota includes so-called relicts (tuatara Sphenodon), a supposed Gondwanan element (e.g. leiopelmatid frogs, Onychophora, kauri Agathis) and recent colonists [6,7,8,9,10] This eclectic biotic composition has singled New Zealand out as being a intriguing example of island biology [11]. There appears to be little congruence in phylogeographic patterns expressed by New Zealand taxa with no overwhelming pattern among lineage formation, landscape history and distribution of taxa (see reviews in [8,9,20]) In retrospect this may not be surprising, for the early expectation that species level phylogeography would reveal scaled-down patterns of the type seen in the northern hemisphere have rarely been realised (but see [21,22,23]). Even within species, genetic diversity is often very high [24] and spatial patterns often complex, indicating that even on the relatively small scale of New Zealand population diversity relates to older events and/or involves persistence of larger populations than those commonly identified for European taxa [18,20]
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