Search for: All records

Abstract Secondary contact between previously allopatric lineages offers a test of reproductive isolating mechanisms that may have accrued in isolation. Such instances of contact can produce stable hybrid zones—where reproductive isolation can further develop via reinforcement or phenotypic displacement—or result in the lineages merging. Ongoing secondary contact is most visible in continental systems, where steady input from parental taxa can occur readily. In oceanic island systems, however, secondary contact between closely related species of birds is relatively rare. When observed on sufficiently small islands, relative to population size, secondary contact likely represents a recent phenomenon. Here, we examine the dynamics of a group of birds whose apparent widespread hybridization influenced Ernst Mayr’s foundational work on allopatric speciation: the whistlers of Fiji (Aves:Pachycephala). We demonstrate two clear instances of secondary contact within the Fijian archipelago, one resulting in a hybrid zone on a larger island, and the other resulting in a wholly admixed population on a smaller, adjacent island. We leveraged low genome-wide divergence in the hybrid zone to pinpoint a single genomic region associated with observed phenotypic differences. We use genomic data to present a new hypothesis that emphasizes rapid plumage evolution and post-divergence gene flow. 

Abstract Many organisms possess multiple discrete genomes (i.e. nuclear and organellar), which are inherited separately and may have unique and even conflicting evolutionary histories. Phylogenetic reconstructions from these discrete genomes can yield different patterns of relatedness, a phenomenon known as cytonuclear discordance. In many animals, mitonuclear discordance (i.e. discordant evolutionary histories between the nuclear and mitochondrial genomes) has been widely documented, but its causes are often considered idiosyncratic and inscrutable. We show that a case of mitonuclear discordance inTodiramphuskingfishers can be explained by extensive genome‐wide incomplete lineage sorting (ILS), likely a result of the explosive diversification history of this genus. For these kingfishers, quartet frequencies reveal that the nuclear genome is dominated by discordant topologies, with none of the internal branches in our consensus nuclear tree recovered in >50% of genome‐wide gene trees. Meanwhile, a lack of inter‐species shared ancestry, non‐significant pairwise tests for gene flow, and little evidence for meaningful migration edges between species, leads to the conclusion that gene flow cannot explain the mitonuclear discordance we observe. This lack of evidence for gene flow combined with evidence for extensive genome‐wide gene tree discordance, a hallmark of ILS, leads us to conclude that the mitonuclear discordance we observe likely results from ILS, specifically deep coalescence of the mitochondrial genome. Based on this case study, we hypothesize that similar demographic histories in other ‘great speciator’ taxa across the Indo‐Pacific likely predispose these groups to high levels of ILS and high likelihoods of mitonuclear discordance.