Dispersal and speciation in the avian archipelago

Abstract

Islands play a key role in both the evolution of new species and our understanding of that evolution. The colonisation of islands, followed by different levels of gene flow between populations, has produced a diverse array of bird species scattered around the world, from which generations of biologists have drawn their insights. Like the islands on which they have evolved, these birds are divided into an “archipelago” of populations of different shapes and sizes, with varying levels of isolation between them. Their fragile ecosystems are under increasing threat, however, and may be lost before their diversity is fully catalogued or the evolutionary processes that birthed it are understood. Though island birds have informed many important theories in evolutionary biology and biogeography, our knowledge of them is subject to many shortfalls. We have not named all of the species that exist (“Linnean shortfalls”) and we do not know how species are distributed (“Wallacean shortfalls”). These shortfalls are particularly concerning when it comes to islands with highly endemic faunas, such as Madagascar and Sulawesi. In this thesis I have worked to help address some Linnean and Wallacean shortfalls in our knowledge of island birds, as all biogeographic theory relies on accurate knowledge of the biota itself. I have also explored the population structure of birds in the important biodiversity hotspot of Sulawesi and its surrounding regions, supplying insights into evolutionary processes. From there, I have used the patterns uncovered to refine certain concepts from biogeography, in particular our understanding of how differences in the dispersal ability of birds affect gene flow. This, in turn, drives patterns of speciation and thus the generation of biodiversity. The first two data chapters of this thesis (Chapters 2 and 3) contribute new natural history and distribution data for a number of island bird species. Chapter 2 contains the first detailed inventory of the bird species of Ankobohobo Wetland in Madagascar, improving our knowledge of the distribution and conservation status of some threatened and endemic species. In Chapter 3 I provide evidence of cryptic sexual dimorphism in the Sulawesi Babbler (Pellorneum celebense), an understorey bird endemic to Sulawesi and surrounding islands in Indonesia. Distribution and sexual dimorphism are two aspects of natural history that can help to inform biogeographic theory. The core of the thesis is made up of three chapters (Chapters 4 to 6) that delve deeper into biogeographic patterns around Sulawesi and surrounding regions, using DNA sequencing and integrative taxonomy. Chapter 4 returns to the story of the Sulawesi Babbler, finding that some populations show notable differences in DNA and song, even though they have been connected by land in recent geological history. In chapter 5 we consider another Sulawesi endemic, the Pale-blue Monarch (Hypothymis puella), along with its widespread relative the Island Monarch (Monarcha cinerascens). The Island Monarch was one of the original examples of a “supertramp species”, hypothesised to be a dispersal specialist excluded from larger islands by sedentary competitors like the Pale-blue Monarch. This chapter, however, argues for a slightly different interpretation, as I found that Island Monarch populations have diverged on even relatively small geographic scales, while the Pale-blue Monarch exhibits signs of gene flow across its range. The last of these core chapters (Chapter 6) presents a case that populations of two sunbird species, the Olive-backed Sunbird (Cinnyris jugularis) and the Black Sunbird (Leptocoma aspasia) exhibit such strong divergence that the species should be split. One of these populations (the “Wakatobi Sunbird Cinnyris infrenatus”, currently named as a subspecies) warrants recognition as an endemic species on the grounds of both genetic divergence and integrative taxonomy. The divergence of sunbird populations appears to be driven by both small islands, with their unique evolutionary conditions, and large biogeographic barriers inhibiting gene flow between regions. Chapter 7, the final data chapter, offers a broad examination of gene flow between islands all over the world using previously published sequence data. Bird populations on pairs of islands are seen to be more genetically distinct from each other when their islands are separated by wider expanses of water. This chapter reinforces a conclusion suggested by all the preceding chapters: diversification of island birds is driven by both the geology of islands (producing barriers to gene flow) and the ecology and behaviour of birds (making some more likely to overcome the barriers than others). Knowledge of biodiversity, classic biogeographic theory, and modern methods of data collection and analysis are all required if we are to address the fundamental question: how species have been, and are being, evolved.