Radiating on Oceanic Islands: Patterns and Processes of Speciation in the Land Snail Genus Theba (Risso 1826)

Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany.
PLoS ONE (Impact Factor: 3.53). 09/2012; 7(4):e34339. DOI: 10.1371/journal.pone.0034339
Source: PubMed

ABSTRACT Island radiations have played a major role in shaping our current understanding of allopatric, sympatric and parapatric speciation. However, the fact that species divergence correlates with island size emphasizes the importance of geographic isolation (allopatry) in speciation. Based on molecular and morphological data, we investigated the diversification of the land snail genus Theba on the two Canary Islands of Lanzarote and Fuerteventura. Due to the geological history of both islands, this study system provides ideal conditions to investigate the interplay of biogeography, dispersal ability and differentiation in generating species diversity. Our analyses demonstrated extensive cryptic diversification of Theba on these islands, probably driven mainly by non-adaptive allopatric differentiation and secondary gene flow. In a few cases, we observed a complete absence of gene flow among sympatrically distributed forms suggesting an advanced stage of speciation. On the Jandía peninsula genome scans suggested genotype-environment associations and potentially adaptive diversification of two closely related Theba species to different ecological environments. We found support for the idea that genetic differentiation was enhanced by divergent selection in different environments. The diversification of Theba on both islands is therefore best explained by a mixture of non-adaptive and adaptive speciation, promoted by ecological and geomorphological factors.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Time-trees are commonly calibrated based on fossils attributed to internal nodes, thus defining the minimum age of these nodes. However, in the absence of synapomorphies, the phylogenetic position of a fossil can only be inferred based on similarity. In this study, we objectively allocated fossil shells to internal nodes based on the reconstruction of ancestral shapes and sizes in a geometric morphometric framework. Our phylogenetic analysis of 24 (putative sub)species was based on 2524 amplified fragment length polymorphism loci. In this well-supported tree the taxa occurring in north-west Africa and on the Iberian peninsula were paraphyletic with respect to the (sub)species from the Canary and Selvagen Islands, indicating a continental origin of the genus in contrast to our earlier sequence-based account. Ancestral shell shapes and sizes were inferred based on landmark data using squared-change parsimony. In a subsequent principal component analysis, only three of 20 fossil shells could be unequivocally allocated to internal nodes. However, these fossils were all Quaternary, and thus too young to infer meaningfully narrow confidence intervals for divergence estimates that probably reach back into the Miocene or even Oligocene. The apparent failure of allocating older fossils to internal nodes and achieving a reliable tree calibration was caused by the absence of phylogenetic signal in the shells of Theba due to extensive, sometimes rapid, convergent evolution, including reversals. © 2014 The Linnean Society of London
    Zoological Journal of the Linnean Society 04/2014; 171(1). DOI:10.1111/zoj.12123 · 2.66 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: AimOur aim was to determine the relative contribution of species replacement and richness differences to overall beta diversity in Macaronesian spiders, the influence of several biogeographical drivers in shaping such dissimilarity patterns, and how these change according to the dispersal ability of spiders.LocationFour Macaronesian archipelagos: the Azores, Madeira, Selvagens and the Canary Islands.Methods Each spider species was assigned to a group relative to its ballooning propensity (frequent, occasional or rare), used as a surrogate of dispersal ability, based on its family membership. Beta diversity was decomposed for each group, by disentangling all compositional differences (overall beta diversity, βtotal) into two components, species replacement (βrepl) and species richness differences (βrich). The effects of island area, environmental heterogeneity, geological age, distance to mainland and inter-island distances on βrepl and βrich were tested by partial Mantel tests and hierarchical partitioning of variation for each mobility group.ResultsThe archipelagos studied had similar intra-archipelagic richness differences, but species replacement was lower within the Azores for the three groups of spiders. The variation in community composition among the archipelagos was determined by species replacement for frequently ballooning spiders, while richness differences dominated for less mobile spiders. Island area was more important for species with higher mobility, while less mobile species were mostly affected by the distance to mainland and inter-island distances. Environmental heterogeneity had an effect, mostly on richness differences, across the three spider groups. Time had only a weak effect on species replacement for less mobile species.Main conclusionsThe partition of βtotal into βrepl and βrich identified different causes of beta-diversity patterns as driven by the dispersal ability of spiders. Dispersal-limited species responded more strongly to isolation than more mobile species. In contrast, the latter were influenced more by island area. Thus our findings emphasize the importance of interspecific traits and dispersal limitation for structuring species assemblages on islands.
    Journal of Biogeography 05/2014; 41(10). DOI:10.1111/jbi.12348 · 4.97 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The population structure of three endemic land snail species of the genus Discula (Lowe,1831) on the archipelago of Madeira were studied. Ten allozyme loci were screened in 13 populations (11 from Madeira Island and two from the adjacent Desertas Islands) of D. polymorpha (Lowe, 1831), two populations of D. attrita (Lowe, 1831) and one population of D. calcigena (Lowe, 1831) from Porto Santo Island. All populations, except those from the Desertas, showed high levels of genetic variability. Deviation from Hardy-Weinberg equilibrium due to heterozygote defi ciency was detected in all D. polymorpha populations from Madeira and populations of D. attrita. This defi ciency was mainly attributed to inbreeding within the populations. No heterozygote defi ciency was found in D. calcigena, probably due to the larger population size of this species in the recent past. The FST values were signifi cant in many pairwise comparisons between the populations of D. polymorpha, with the highest values occurring between populations from Deserta Grande and the peninsula of Madeira. All interspecifi c FST values were highly signifi cant. FST values between populations of D. polymorpha from Deserta Grande and the Madeiran peninsula were comparable to observed interspecifi c values. Based on the factorial analysis all Madeiran populations grouped together, the populations from Deserta Grande formed a separate group, while the two populations of D. attrita show clear structuring.
    American Malacological Bulletin 02/2014; 1(32):52 - 61. DOI:10.4003/006.032.0105 · 0.84 Impact Factor

Full-text (2 Sources)

Available from
May 30, 2014