Snails in the desert: Species diversification of Theba (Gastropoda: Helicidae) along the Atlantic coast of NW Africa

Abstract

The spatial subdivision of species often plays a pivotal role in speciation. Across their entire range, species are rarely panmictic and crucial consequences of spatial subdivision are (1) random genetic drift including historical factors, (2) uniform selection, and (3) divergent selection. Each of these consequences may result in geographic variation and eventually reproductive isolation, but their relative importance in speciation is still unclear. In this study, we used a combination of genetic, morphological, and climatic data to obtain a comprehensive picture of differentiation among three closely related, parapatrically distributed taxa of the land snail genus Theba occurring along the Atlantic coasts of South Morocco and Western Sahara. We conducted Mantel and partial Mantel tests to relate phenotypic and genotypic variation of these species to geography and/or climate. As null hypothesis for an evolutionary scenario, we assumed nonadaptive speciation and expected a pattern of isolation by distance among taxa. Rejection of the null hypothesis would indicate isolation by environment due to adaptation. Generally, genetic drift plays an important role but is rarely considered as sole driver of speciation. It is the combination of drift and selection that predominantly drives speciation. This study, however, provides a potential example, in which nonadaptive speciation, that is, genetic drift, is apparently the main driver of shaping the diversity of Theba in NW Africa. Restriction of gene flow between populations caused by geographic isolation probably has played an important role. Climate oscillations during the Plio‐ and Pleistocene may have led to repeated ecological changes in NW Africa and disruptions of habitats promoting differentiation by geographic isolation. The inferred evolutionary scenario, however, did not fully explain the incongruence between the AFLP‐ and mtDNA‐tree topologies. This incongruence might indicate past hybridization among the studied Theba forms.

How many single-copy orthologous genes from whole genomes reveal deep gastropod relationships?

“…Gastropods have a worldwide distribution, from the near Arctic and Antarctic zones to the tropics ( Crame, 2013 ) and ubiquitous in the oceans. They have colonized even extreme environments, such as the deep sea ( e.g ., faunas associated with hydrothermal vents) ( Govenar, Fisher & Shank, 2015 ) and deserts ( Greve et al, 2017 ). Gastropods are highly diverse with regard to their body forms, shells, and functions, and in many ways, they affect human life.…”

Multiple Introductions of the Pestiferous Land Snail Theba pisana (Müller, 1774) (Gastropoda: Helicidae) in Southern California

“…Such studies also highlight the phenomenon of biotic homogenization [80], identify anthropogenic pathways of introduction [81], may improve protocols for species interception and quarantine, and inform strategies for invasive species eradication or control [81]. Likewise, the distinct parentage of T. pisana in Southern California could be an important component of studies focused on diversification in the genus Theba [82] and varied investigations of genetic drift [83] and the founder effect [84], resource use [85], niche partitioning [86] and adaptation [87][88][89] in introduced gastropods. We anticipate that the CO1 barcodes and 16S and ITS2 sequences from T. pisana specimens generated for this study will be useful in assessing the provenance of new populations and interceptions of T. pisana in California, elsewhere in North America, and worldwide.…”

Peer Review #2 of "How many single-copy orthologous genes from whole genomes reveal deep gastropod relationships? (v0.2)"

“…Gastropods have a worldwide distribution, from the near Arctic and Antarctic zones to the tropics (Crame 2013) and ubiquitous in the oceans. They have colonized even extreme environments, such as the deep sea (e.g., faunas associated with hydrothermal vents) (Govenar et al 2015) and deserts (Greve et al 2017). Gastropods are highly diverse with regard to their body forms, shells, and functions, and in many ways, they affect human life.…”