Endemic evolutionary radiation of Rhagada land snails (Pulmonata: Camaenidae) in a continental archipelago in northern Western Australia

Abstract

In the Dampier Archipelago of Western Australia's Pilbara Region, several locally endemic, morphologically distinctive species of Rhagada land snails occur, contrasting with the morphologically conservative species with wider distributions on the adjacent mainland. To test alternative origins of this unusual local diversity in a continental archipelago, we examined sequences of the cytochrome oxidase subunit 1 and 16S mitochondrial genes in 22 described species and eight undescribed forms, including all known morphospecies from the Pilbara Region's Dampier Archipelago and adjacent mainland. Phylogenetic analyses consistently resolved four, deep clades within the Pilbara Region, with a mean sequence divergence of 15–18%. All but one of the species from the Dampier Archipelago formed one of the major clades, indicating that the morphological radiation in the archipelago evolved locally, rather than through multiple, relictual mainland lineages. Morphological divergence spanning almost that of the entire genus was within a subclade with sequence divergence < 4%, highlighting the disconnection between morphological diversification and levels of molecular genetic divergence. This in situ morphological radiation in the Dampier Archipelago, which transcends variation seen over much larger distances on the mainland, is unusual for a continental archipelago, and may relate to local heterogeneity of land forms. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106, 316–327.

Rhagadarevisited: on the taxonomy of species from the Kimberley and Dampierland, Western Australia (Pulmonata, Camaenidae)

“…sp. Mundungum' (in error for Marndungum Island, southwest Kimberley) by Johnson et al (2012). Both candidate taxa were found to be closely related to Rhagada cygna Solem, 1997 and Rhagada bulgana Solem, 1997 from the Dampier Peninsula but their morphology had not been studied previously to clarify their identity.…”

“…In accordance with their extensive ranges, Rhagada species may be associated with a large variety of habitats, from monsoonal vine thickets to xeric habitats, such as sandy plains dominated by spinifex grasses (Triodia spp.) or more complex, rocky outcrops with shrubby vegetation (Stankowski 2011;Johnson et al 2012).…”

“…Molecular phylogenetic studies suggested that the morphological radiation in this archipelago evolved locally, rather than through multiple introductions of relictual mainland lineages. There seems to be a general disconnection between morphological diversification and levels of molecular genetic divergence within this radiation (Johnson et al 2004;Stankowski 2011;Johnson et al 2012;Stankowski and Johnson 2014 represent another undescribed species ('R. cf.…”

Camaenid land snails on Barrow Island: distributions, molecular phylogenetics and taxonomic revision

“…Broad-scale phylogenetic analyses within both Quistrachia and Rhagada, including representatives of the Barrow Island species, have already been carried out using sequences of the mitochondrial genes 16S rRNA (16S) and cytochrome c oxidase subunit 1 (COI) (O'Neill 2008 for Quistrachia; Johnson et al 2012 for Rhagada). Here, we added specimens specifically required to understand the relationships of the taxa on Barrow Island, and analysed these with the relevant subset of species in each genus.…”

“…Trees are presented in linearized form, to indicate clearly the levels of divergence. Methods of DNA extraction, PCR, sequencing and analysis were as detailed in Johnson et al (2012). COI is the usual gene used in DNA barcoding, and its use here to quantify genetic divergence allows consistent comparisons with other studies.…”