Nuclear and mitochondrial sequences confirm complex colonization patterns and clear species boundaries for flightless weevils in the Galápagos archipelago

Abstract

Nuclear sequence data were collected from endemic Galápagos species and an introduced close relative, and contrasted with mitochondrial DNA sequences, continuing investigation into the colonization history and modes of diversification in the weevil genus Galapaganus. The current combined phylogeny together with previously published penalized likelihood age estimates builds a complex picture of the archipelago's colonization history. The present reconstruction relies on submerged platforms to explain the early divergence of the young southern Isabela endemics or the Española or San Cristobal populations. Diversity is later built through inter-island divergence starting on older islands and continuing on two simultaneous tracks towards younger islands. The amount of diversity generated through intra-island processes is skewed towards older islands, suggesting that island age significantly influences diversity. Phylogenetic concordance between nuclear and mitochondrial datasets and well-supported monophyletic species in mitochondrial derived topologies appear to reject the possibility of inter-species hybridization. These clear species boundaries might be related to the tight host associations of adult weevils in discrete ecological zones. If shared hosts facilitate hybridization, then host- or habitat-promoted divergences could prevent it, even in the case of species that share islands, since the altitudinal partitioning of habitats minimizes range overlap.

Figure 1

The Galápagos archipelago with island age estimates and distribution of Galapaganus species. Numbers in parentheses after species names indicate numbers of localities and total numbers of specimens for each species sampled. The asterisk indicates an introduced species.

Figure 2

MP strict consensus tree derived from (a) mitochondrial and (b) nuclear datasets. Topological differences from the Bayesian majority rule consensus tree in each analysis are indicated by grey bars. Numbers above branches indicate bootstrap support values; numbers below branches indicate BP expressed as percentages. Grey dotted lines indicate differences between topologies.

Figure 3

MP strict consensus tree derived from a concatenated mitochondrial and nuclear dataset. Topological differences from the Bayesian majority rule consensus tree are indicated by dashed grey lines. Numbers above and below branches as in figure 2. The insets illustrate the hypothesized order of island colonization suggested by the (a) MP and (b) Bayesian topologies. Dotted arrows indicate equally parsimonious scenarios in each case. Numbers below the branches in the topology correspond to the arrows in the inset for two alternative colonization scenarios: MP: normal font, BI: italics. IC indicates initial colonization and asterisks indicate intra-island speciation.