| dc.description.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. | en |
