3. Drivers of island arthropod speciation
Traditional approaches to speciation research typically analyse a limited number of species with great detail, necessitating accurate estimation of population and phylogenetic histories, and the limitations of single-locus markers for such purposes are well recognized (e.g. Bazin, Glémin, & Galtier, 2006; Toussaint et al., 2015). However it has been shown that, across multi-species comparisons, potentially idiosyncratic single marker signals within a subset of arthropod species may be outweighed by common community level signatures (e.g. Salces-Castellano et al., 2020; Scalercio et al., 2020). As the histories of individual species complexes may themselves be idiosyncratic, having noisy data across hundreds or even thousands of species may, in some cases, be more revealing of general patterns and processes in a region than deeper sequencing of a more limited number. With the ability to now obtain reliable haplotype-level data from metabarcode sequence output (Andújar et al., 2021), it is possible to implement both multiplex barcode and wocDNA metabarcoding to explore both the patterns and drivers of diversification and speciation across arthropod assemblages. Andújar et al. (2022, this issue) demonstrate the implementation of such a metaphylogeographic approach with wocDNA barcoding to understand the relative roles of ecological and geographical drivers for diversification among soil arthropods within a single oceanic island. Extending such an approach across multiple islands within archipelagos can provide baseline data on the relative importance of ecological and geographical speciation within many as yet largely unstudied fractions of arthropod diversity. When contrasted with existing data for plants, vertebrates, and more easily studied invertebrate groups, a fuller understanding of: (i) dispersal dynamics within and among islands; (ii) the role of environment in structuring genetic variation within species, and; (iii) their implications for speciation, will emerge. Recent work also demonstrates how a single locus community-level approach can inform about the relative importance of specific traits for diversification within islands (Salces-Castellano et al., 2021). In conjunction with barcode reference libraries with trait data (e.g. body size, dispersal ability, niche) multiplex barcoding and wocDNA metabarcoding can be used to scale up both geographic and taxonomic sampling to identify functional traits associated with arthropod diversification within and across island systems.