Abstract
The current ease of obtaining thousands of molecular markers challenges the notion that full phylogenetic concordance, as proposed by phylogenetic species concepts, is a requirement for defining species delimitations. Indeed, the presence of genomic islands of divergence, which may be the cause, or in some cases the consequence, of speciation, precludes concordance. Here, we explore this issue using thousands of RAD markers on two sister species of surgeonfishes (Teleostei: Acanthuridae), Zebrasoma flavescens and Z. scopas, and several populations within each species. Species are readily distinguished based on their colors (solid yellow and solid brown, respectively), yet populations and species are neither distinguishable using mitochondrial markers (cytochrome c oxidase 1), nor using 5193 SNPs (pairwise Φst = 0.034). In contrast, when using outlier loci, some of them presumably under selection, species delimitations, and strong population structure follow recognized taxonomic positions (pairwise Φst = 0.326). Species and population delimitation differences based on neutral and selected markers are likely due to local adaptation, thus being consistent with the idea that these genomic islands of divergence arose as a consequence of isolation. These findings, which are not unique, raise the question of a potentially important pathway of divergence based on local adaptation that is only evident when looking at thousands of loci.
Highlights
Defining species has important theoretical and practical implications, yet no unified concept and approach have emerged (De Queiroz, 1998, 2007; Hausdorf, 2011)
Taking into account the lack of resolution at the CO1 sequence level, the presence of hybrid forms, and geographic range overlaps, we considered the Zebrasoma scopas/flavescens system ideal to study questions pertaining to understanding the different dynamics of genomic regions
The original goal of Darwinian principles was to classify organisms according to their descent, which translates into genetic lineages traceable by their molecular markers (Cracraft, 1983; Wiley, 1981)
Summary
Defining species has important theoretical and practical implications, yet no unified concept and approach have emerged (De Queiroz, 1998, 2007; Hausdorf, 2011). Phylogenetic studies based on the universal barcoding region (mitochondrial cytochrome c oxidase 1, CO1) have highlighted two main findings: First, the species Zebrasoma scopas most likely corresponds to two paraphyletic species restricted to either the Indian Ocean or the Pacific Ocean (Figure 2) (Hubert et al, 2012); and second, CO1 markers could not separate Z. scopas (the Pacific form) and its closest relative Z. flavescens (Figure 2) (Steinke, Zemlak, & Hebert, 2009). This lack of species delimitation was interpreted as either a case of incomplete lineage sorting or a case of species oversplitting (Steinke et al, 2009). Our goals were first to determine whether genomic regions of divergence were present, and in the affirmative, if there was a signature associated with past or present speciation events in this group of fishes
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