Archipelago-Wide Patterns of Colonization and Speciation Among an Endemic Radiation of Galápagos Land Snails

Abstract

Newly arrived species on young or remote islands are likely to encounter less predation and competition than source populations on continental landmasses. The associated ecological release might facilitate divergence and speciation as colonizing lineages fill previously unoccupied niche space. Characterizing the sequence and timing of colonization on islands represents the first step in determining the relative contributions of geographical isolation and ecological factors in lineage diversification. Herein, we use genome-scale data to estimate timing of colonization in Naesiotus snails to the Galápagos islands from mainland South America. We test inter-island patterns of colonization and within-island radiations to understand their contribution to community assembly. Partly contradicting previously published topologies, phylogenetic reconstructions suggest that most Naesiotus species form island-specific clades, with within-island speciation dominating cladogenesis. Galápagos Naesiotus also adhere to the island progression rule, with colonization proceeding from old to young islands and within-island diversification occurring earlier on older islands. Our work provides a framework for evaluating the contribution of colonization and in situ speciation to the diversity of other Galápagos lineages.

Does island ontogeny dictate the accumulation of both species richness and functional diversity?

“…Species richness is expected to build in mid-ontogeny as species accumulate by both colonization and intra-island speciation. Previous phylogenetic work supports both modes of species accumulation in Galápagos Naesiotus (Parent & Crespi, 2006;Phillips et al, 2020). The decline in species richness on older islands is thought to result from the loss of physical space, ecological habitat, or both, as islands erode and subside.…”

“…All OTUs we consider here are recognizable by their island of origin and a fixed (albeit arbitrary) degree of morphological difference, most frequently by unambiguous speciesspecific apomorphy in at least one shell trait. For the majority of OTUs, the taxonomic utility of these characters for the purposes of delimitation is supported directly by mitochondrial and genomic phylogenetic data (Parent & Crespi, 2006;Phillips et al, 2020), although the robust delimitation of species is neither the object nor the result of the present paper.…”

Comparing Adaptive Radiations Across Space, Time, and Taxa

“…In Hawaiian Tetragnatha spiders, for example, the sister lineage on the American mainland is widespread but restricted to riparian habitats, building flimsy webs over water, whereas the species radiation in Hawaii is found in almost every forest habitat and microhabitat (Gillespie 2016). Likewise, the most probable sister group to Galapagos Naesiotus snails is restricted to dry forest habitats, whereas Galapagos snails have adapted to a much wider range of habitats (C. Parent, unpubl) (Phillips et al 2020). Similarly, phylogenetic reconstruction of extant Hawaiian honeycreepers suggests that the Cardueline colonizer was a finch-billed, seed-eating specialist; this morphology seems to have been lost at the onset of the honeycreeper radiation (Campana et al 2020), with the finch morphology subsequently regained from a thin-billed ancestor (Lerner et al 2011).…”

“…Divergence in isolation without (initial) ecological selection-A second mechanism through which initial divergence can occur is through intrinsic reproductive incompatibility that is ecologically independent (32% responses, or 46% in conjunction with divergent selection). Thus, anoles (Losos 2009; Stroud and Losos 2020), Hawaiian spiders (Cotoras et al 2018;Gillespie 2005), and Galapagos snails (Phillips et al 2020), all appear to demonstrate initial divergence in the same environment, though in allopatry presumably through intrinsic incompatibility. Ecological shifts are associated with subsequent secondary contact (Cotoras et al 2018;Stroud and Losos 2020).…”

Effect of topographic complexity on species richness in the Galápagos Islands

“…For mammals, reptiles and birds, SIE was not analysed due to the small sample sizes which resulted in several of the islands not having a single SIE. Additional analyses used a data set of SIE terrestrial snails (limited to the genus Naesiotus ), comprising a single endemic adaptive radiation (Parent & Crespi, 2006; Phillips et al, 2020) for which higher spatial resolution range maps are available (Table S1.2). The differences between the data sets are that the snail data set: (1) includes the island of Rábida and (2) Isabela Island is split into the six individual volcanoes forming this island, which arose independent of one another (Geist, 1996).…”