Abstract
Using the nudibranch genus Amphorina as a model, ongoing speciation is demonstrated, as well as how periodic-like patterns in colouration can be included in an integrated method of fine-scale species delimitation. By combining several methods, including BPP analysis and the study of molecular, morphological, and ecological data from a large number of specimens within a broad geographic range from northern Europe to the Mediterranean, five species are recognised within the genus Amphorina, reviewed here for the first time. Two new species from the southwestern coast of Sweden are described, A. viriolasp. nov. and A. andrasp. nov. Evidence is provided of a recent speciation process between the two closely related, yet separate, species which inhabit the same geographic localities but demonstrate strict water depth differentiation, with one species inhabiting the shallow brackish top layer above the halocline and the other species inhabiting the underlying saltier water. The results presented here are of relevance for currently debated issues such as conservation in relation to speciation, fine species delimitation, and integration of molecular, morphological and ecological information in biodiversity studies. The periodic approach to biological taxonomy has considerable practical potential for various organismal groups.
Highlights
Species delimitation, and the degree of separation between different groups of biological organisms, is a pivotal concept for modern biology, despite the fact that there is no universal agreement about the species concept itself (Stanton et al 2019)
Eleven A. farrani specimens from the UK, France, and Spain clustered in a well-supported clade (PP = 1, BS = 98 %) and sister to another wellsupported clade (PP = 1, BS = 97) containing the new species of the genus Amphorina
The genus Amphorina is a suitable model for studying the link between a “static” taxonomic system and the underlying evolutionary processes fuelled by ontogenetic periodicity due to both the morphological uniformity across the genus on the one hand, and to the large degree of variation in external colouration on the other
Summary
The degree of separation between different groups of biological organisms, is a pivotal concept for modern biology, despite the fact that there is no universal agreement about the species concept itself (Stanton et al 2019). The universal species concept proposed by de Queiroz (2007) – i.e., that species represent separately evolving evolutionary lines without any other defining characters – potentially implies the impossibility of taxonomically defining characters at a general scale. It makes species delimitation a significant modern problem because of the considerable proportion of hidden diversity that is often seemingly impossible to detect by morphological examination alone. For species as a systematic unit we only need to represent a firm taxonomic diagnosis (Winston 1999), whereas underlying natural phenomena may be represented by multilevel organism diversity fuelled by a dynamic evolutionary process (Korshunova et al 2019a) in a species-population complex continuum (Coates et al 2018). Developing approaches that will help reveal the multilevel nature of organism diversity is highly desirable, since that would place the issue within a more complex framework than traditional strictly hierarchical and diagnosis-based taxonomy
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