Author: Toby Heath Galligan
Galligan, Toby Heath, 2011 Adaptive divergence, genetic connectivity, and post-parasitism morbidity in Darwin's small ground finch, Geospiza fuliginosa, on the island of Santa Cruz, Galapagos Archipelago., Flinders University, School of Biological Sciences
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Speciation is arguably the most important problem in evolutionary biology. Following the biological species concept, speciation is the process by which populations of one species reduce inter-population mating - that is, gene flow - to the point where they become two reproductively isolated species. Gene flow can be reduced more or less incidentally by geographical isolation (i.e., allopatry), or by strong divergent selection on intrinsic barriers (e.g. immigrant inviability, divergent mate preference, or divergent mate recognition) in the same (symparty) or adjacent (parapatry) locations. In birds, the beak is used for foraging and mate recognition (e.g. song production); thereby, divergent niches or habitats can directly select for adaptive divergence in beak dimensions, while indirectly selecting for divergence in mate recognition. The significance of allopatric divergence has been long appreciated; however, the significance of sympatric and parapatric divergence remains debated (particularly in birds). Darwin's finches of the Galapagos Archipelago are a model system in which to study evolution in nature. On the island of Santa Cruz, Darwin's small ground finch G. fuliginosa has recently expanded its range from the arid lowlands into the humid highlands; the ecological contrast between these zones providing strong disruptive selection. Previous studies have shown evidence for adaptive divergence in this system (i.e., morphological clines along the ecological cline, environment-phenotype matching at the extreme zones, and more resightings across years of individuals with predicted trait values for each zone). My thesis has expanded on this work in five ways. First, I have used neutral molecular data to show high gene flow among all ecological zones on Santa Cruz; rejecting non-adaptive divergence in this system (Chapter 2). Second, I have shown the predicted breakdown of morphological clines under relaxed selection in a 'benign' high rainfall year; which infers a central role for alternating strong and weak selection against immigrants as a mechanism of divergence in this system (Chapter 3). Third, I have revealed a loss of assortative pairing within highland-colonist G. fuliginosa in response to ecological opportunities and reduced interspecific competition that have followed range expansion (Chapter 4). Fourth, I have demonstrated the importance of ecological contrasts in the formation of barriers to gene flow, by showing greater divergence in song and song discrimination between lowland and highland zones, than between localities within each zone, while controlling for geographical distance (Chapter 5). Fifth, I have shown that the introduced parasitic botfly P. downsi, which is causes high nestling mortality in Darwin's finches, also causes beak malformations that may significantly influence adaptation, mate recognition, and divergence in this system and this group of birds as a whole (Chapter 6). In synthesising my findings, I conclude while strong divergent selection exists between lowland and highland zones, intrinsic aspects of G. fuliginosa (e.g. high mobility) and Santa Cruz (e.g. no physical barriers between zones) can permit high levels of active dispersal, and probably gene flow, between zones (Chapter 3). In low rainfall periods, divergent selection and adaptive divergence is predicted to be strongest; whereas, in high rainfall years divergent selection is weakest and immigration of otherwise ill-adapted individuals is high, effectively reshuffling phenotypes among zones (Chapter 3). The long-term product of these counter processes requires further research. Yet, song discrimination in lowland G. fuliginosa in a high rainfall year suggests that partial barriers to gene flow may have arisen (Chapter 5).
Keywords: Darwin's small ground finch,Geospiza fuliginosa,Santa Cruz,Galapagos,adaptive divergence,genetic connectivity,post-parasitism morbidity,speciation,gene flow,parapatirc,sympatric,beak,ecological contrast,disruptive selection,morphological clines,microsatellite,panmixia,clinal breakdown,relaxed selection,selection against immigrants,assortative pairing,range expansion,song divergence,song discrimination,playback,botfly,Philornis downsi,beak malformation,habitat matching choice,phenotypic plasticity,alternatiing selection,barriers to gene flow
Subject: Biological sciences thesis, Biology thesis
Thesis type: Doctor of Philosophy
Completed: 2011
School: School of Biological Sciences
Supervisor: Professor Sonia Kleindorfer