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.
Highlights
A major problem for the study of speciation is that the formation of new and genetically isolated species is in most cases a slow and continuous process, lasting many generations
MOTU assignment We considered the criterion of reciprocal monophyly to infer possible boundaries of molecular operational taxonomic units (MOTU)
In the run that had the highest likelihood at K = 13, nine clusters corresponded to MOTUs identified by the prior molecular analyses: Theba sp. 5 ‘‘Sand’’, T. cf. clausoinflata ‘‘Rock’’, Theba sp. 4, Theba sp. 1a, Theba sp. 1b, T. grasseti, T. geminata, T. impugnata and Theba sp. 2
Summary
A major problem for the study of speciation is that the formation of new and genetically isolated species is in most cases a slow and continuous process, lasting many generations. Speciation has yet to be studied by comparing many snap-shots of this continuous process from the divergence of populations to fully reproductively isolated species [2]. The study of island radiations have helped to establish our current understanding of allopatric, sympatric and parapatric speciation, but the relative importance of these three modes is still unclear. In this context, it is remarkable that species divergence often correlates with island size. This phenomenon has been interpreted as evidence for the important role of geographic isolation (allopatry) in speciation [7]
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