Mixed Pair of South Polar Skua Male (Left) and Brown Skua Female
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Mixed pair of South Polar Skua male (left) and Brown Skua female Speciation and hybridisation in skuas (Catharacta spp.) Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt dem Rat der Biologisch-Pharmazeutischen Fakultät der Friedrich-Schiller- Universität Jena am 4.12.2008 und am 16.3.2009 verteidigt von Diplom-Biologe Markus Ritz geboren am 17.08.1976 in Dresden Gutachter: Prof. Stefan Halle, Jena Prof. Gerald Heckel, Bern, Schweiz Prof. Peter de Knijff, Leiden, Niederlande Markus Ritz: Speciation and hybridisation in skuas CONTENT INTRODUCTION ............................................................................................................................ 1 Speciation and hybridisation .................................................................................................. 1 Phylogeography and glaciation .............................................................................................. 3 Skua diversity ......................................................................................................................... 4 Hybridisation between southern skua taxa ............................................................................. 7 AIMS OF THE STUDY .................................................................................................................... 8 MANUSCRIPT 1 .......................................................................................................................... 10 Ritz, M.S., Millar, C., Miller, G.D., Phillips, R.A., Ryan, P., Sternkopf, V., Liebers- Helbig, D. & Peter, H-U. (2008) Phylogeography of the southern skua complex – rapid colonization of the southern hemisphere during a glacial period and reticulate evolution. Molecular Phylogenetics and Evolution 49(1): 292-303 MANUSCRIPT 2 .......................................................................................................................... 23 Votier S.C., Kennedy M., Bearhop S., Newell R.G., Griffiths K., Whitaker H., Ritz M.S. & Furness R.W. (2007) Supplementary DNA evidence fails to confirm presence of Brown Skuas Stercorarius antarctica in Europe: a retraction of Votier et al. (2004). Ibis 149(3): 619-621 MANUSCRIPT 3 .......................................................................................................................... 27 Ritz M.S., Hahn S., Janicke T. & Peter H.-U. (2006) Hybridisation between South polar skua (Catharacta maccormicki) and Brown skua (C. antarctica lonnbergi) in the Antarctic Peninsula region. Polar Biology 29(3): 153-159 MANUSCRIPT 4 .......................................................................................................................... 35 Ritz M.S. & Peter H.-U. (re-submitted to Molecular Ecology on 30.11.2008) Hybrid identification using AFLP markers in a hybrid zone between two recently diverged seabird species – strong admixture and biased gene flow DISCUSSION ............................................................................................................................... 61 Methods ................................................................................................................................ 61 Phylogeographic study (manuscript 1) ............................................................................ 61 SNP between South Polar Skua and all other southern skuas (manuscript 2) ................ 62 Hybrid zone description (manuscript 3) ........................................................................... 63 AFLP hybrid assignment (manuscript 4) ......................................................................... 63 Phylogeography of southern hemisphere skuas ................................................................... 65 Mitochondrial mutation rate ............................................................................................ 65 Colonisation of the southern hemisphere ......................................................................... 66 Diversification of the southern skua complex .................................................................. 66 Glaciation and skua diversification ................................................................................. 68 Speciation mode ............................................................................................................... 68 Educated guesses concerning the skua enigma ................................................................ 68 Hybridisation between South Polar Skua and Brown Skua ................................................. 69 Hybrid zone characterisation ........................................................................................... 69 Preliminary fitness aspects ............................................................................................... 70 Degree of admixture and direction of introgression ........................................................ 71 Mate choice in skuas ........................................................................................................ 72 Taxonomic status of southern skuas ................................................................................. 73 OPEN QUESTIONS, ONGOING PROJECTS ...................................................................................... 75 SUMMARY ................................................................................................................................. 77 ZUSAMMENFASSUNG ................................................................................................................. 79 REFERENCES ............................................................................................................................. 81 Markus Ritz: Speciation and hybridisation in skuas Introduction INTRODUCTION Speciation and hybridisation Speciation, the process of species formation, is a central topic in evolutionary biology. Whilst the foundations of evolution, variability and selection, were quickly accepted amongst scientists after the publication of “On the origin of species” (Darwin 1859), it proved much harder to find how species actually diverge and evolve into distinct groups. One reason is that the emergence of a new species can usually not be observed during time scales comparable to a researcher’s career. Studies on speciation will thus have to try to reconstruct the past or to confine to a specific stage in speciation. This raises the question at which stage two taxa can be called separate species. So, Darwin went too far in his conclusions in the “Origin” that by studying evolution “… we shall at least be freed from the vain search for the undiscovered and undiscoverable essence of the term species.” Whilst it is now widely accepted that taxa are variable and it is worthwhile to study biodiversity irrespective of the level of variation (e.g. genera, species, subspecies, ecotypes,…), there are still reasons to adopt a definition for “species” (Coyne and Orr 2004). Most scientists agree that species are real entities in nature and not human constructs to classify organisms. However, in compliance with the saying that “ in a room with ten evolutionary biologists you will hear at least eleven different species concepts” I will not discuss any of the numerous species concepts but only describe the “comprehensive biologic species concept” (CBSC, Johnson et al. 1999) which was introduced with special emphasis on birds. It is a broader version of the biological species concept (Dobzhansky 1937, Mayr 1942) and accounts for the observed stability of species in spite of hybridisation. According to the CBSC an avian species “…is a system of populations representing an essentially monophyletic, genetically cohesive, and genealogically concordant lineage of individuals that share a common fertilisation system through time and space, represent an independent evolutionary trajectory, and demonstrate essential but not necessarily complete reproductive isolation from other such systems.” Three main routes to speciation have been described based on the geographic circumstances during divergence. Sympatric speciation assumes that divergence takes place within a population due to for example disruptive selection. Numerous models have been developed to explore the conditions under which sympatric speciation may occur (e.g. Higashi et al. 1999, 1 Markus Ritz: Speciation and hybridisation in skuas Introduction Via 2001, Kirkpatrick and Ravigne 2002, Gavrilets 2003). The very restrictive conditions allowing for evolution of reproductive isolation in the presence of gene flow argue against sympatric speciation being a common mode of speciation in animals. Some scientists even deny the possibility of sympatric speciation but a few model groups are discussed as candidates for divergence in the presence of gene flow (Coyne and Orr 2004, Bolnick and Fitzpatrick 2007). Parapatric speciation describes the process, where upon a taxon evolves into two taxa with adjacent ranges. This is a likely scenario if the species has a wide range with ecological breaks. Taxa may then diverge along an ecotone or perhaps a migratory divide. The resulting area of contact would be a primary hybrid zone. However, it is hard, or even impossible, to prove that the two groups were not geographically isolated from each other for at least some time during the past. The probably most common process in birds, allopatric speciation, is based on a geographic separation of groups of organisms. The independent evolution of these