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Competition, Coexistence and Character Displacement: in A List of Papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Vallin, N., Rice, A. M., Arntsen, H., Kulma, K. and Qvarn- ström, A. (2011) Combined effects of interspecific competition and hybridization impede local coexistence of Ficedula fly- catchers. Submitted manuscript. II Qvarnström, A., Wiley, C., Svedin, N. and Vallin, N. (2009) Life-history divergence facilitates regional coexistence of com- peting Ficedula flycatchers. Ecology 90:1948-1957. III Vallin, N., Nonaka, Y., Feng, J. and Qvarnström, A. (2011) Life-history divergence and environmentally dependent relative fitness of hybrid nestlings in Ficedula flycatchers. Manuscript. IV Vallin, N., Rice, A. M., Bailey, R. I., Husby, A. and Qvarn- ström, A. (2011) Positive feedback between ecological and re- productive character displacement in a young avian hybrid zone. Submitted manuscript. V Vallin, N. and Qvarnström, A. (2011) Learning the hard way: imprinting can enhance enforced shifts in habitat choice. Sub- mitted manuscript. Reprints were made with permission from the publisher. Cover picture: variation in male pied flycatcher plumage coloration. Artwork by Måns Hjernquist. Contents 1. Introduction ................................................................................................. 7 2. Study system ............................................................................................. 10 The Ficedula flycatcher hybrid zone on Öland ................................... 10 3. Aims of the thesis...................................................................................... 13 3.1. The interaction between competition and hybridization in causing local extinction .................................................................................... 13 3.2. The role of life history divergence in facilitating regional coexistence ........................................................................................... 15 3.3. The role of life history divergence for the relative fitness of hybrids across different environmental conditions .............................. 17 3.4. The interplay between ecological and reproductive character displacement ........................................................................................ 19 3.5. The role of early learning in causing shifts in habitat choice ....... 21 4. Conclusions and future perspectives ......................................................... 23 4.1. Competition and coexistence ........................................................ 23 4.3. Evolutionary implications ............................................................. 24 5. Sammanfattning på svenska ...................................................................... 26 6. Acknowledgements ................................................................................... 31 7. References ................................................................................................. 33 1. Introduction Since Darwin (1859), the definition of species has shifted from being based on morphological differences between them towards a definition based on reproductive isolation, i.e. the degree of reduced interbreeding between them through different isolating barriers (Dobzhansky 1937, Mayr 1942). Howev- er, Darwin’s (1859) general ideas on the importance of natural selection for the origin of species have gained increasing support and attention in recent years (Schluter 2000, Coyne and Orr 2004, Price 2007). The evolution of reproductive isolation through ecologically based divergent selection is commonly referred to as ecological speciation (Schluter 2001, Rundle and Nosil 2005), and predicts that reproductive isolation should evolve between populations when adapting to contrasting environments but not between populations when adapting to similar environments (Schluter 2009). Reproductive isolation comes in many forms, which fall into two main categories; prezygotic and postzygotic. Prezygotic isolation acts before ferti- lization and can be ecological (e.g. habitat or temporal isolation), behavioral (e.g. divergence in mate preferences), mechanical (e.g. incompatibilities in reproductive structures), or gametic (e.g. incompatibilities between ga- metes). Postzygotic isolation acts through reduced fitness of hybrids, and can be further divided into extrinsic (e.g. hybrids are unable to find an ecological niche and/or to obtain a mate) and intrinsic (e.g. hybrids are inviable or ste- rile) forms (Coyne and Orr 2004). To what extent intrinsic postzygotic isolation in terms of genetic incom- patibilities between populations (e.g. the “Dobzhansky-Muller” model, Dobzhansky 1937, Muller 1942) arise as a result of adaptation remains an open question (Coyne and Orr 2004). By contrast, extrinsic postzygotic iso- lation is often a direct result of adaptive evolution (Hatfield and Schluter 1999). Hybrids are often phenotypically intermediate between pure species, and may therefore be unfit in either parental habitat (reviewed in Coyne and Orr 2004). Hybrids may also be intermediate in behavior, as in the hybrids between two populations of blackcaps who undertake an intermediate migra- tory route as compared to the parental populations (Helbig 1991). Behavioral sterility is another form of extrinsic hybrid inviability where hybrids are rejected by potential mates due to their intermediate phenotype. Allopatric speciation appears to be a non-controversial mode of speciation. Given enough time apart, pairs of isolated taxa are likely to evolve reproduc- 7 tive barriers. However, sooner or later, these pairs are also likely to expe- rience secondary contact with each other. The topic of coexistence between similar species has fascinated the scientific community for decades (Volterra 1926, Lotka 1932, Gause 1934, Hardin 1960, Kuno 1992, Gröning and Hochkirch 2008). Can closely related species coexist in the same habitat, and if so - how? How important are interactions such as competition and hybri- dization between species in driving speciation events or extinctions? These types of questions are more relevant today than ever as global envi- ronmental changes are increasingly affecting species distributions and diver- sity in a number of direct and indirect ways. Climate change, translocations, and habitat disturbance are expected to increase the prevalence of biological invaders (Sala et al. 2000), which in turn will increase the probability for hybridization and competition between formerly separated closely related species. There are already several examples of hybridization driving species to extinction (Rhymer and Simberloff 1996), but replacement of species and hybridization are often treated as independent subjects in conservation biol- ogy (Konishi and Takata 2004). By contrast, there is also a more positive viewpoint on hybridization and biodiversity. For example, hybridization can be seen as a natural process with the potential to genetically enrich rare species (Arnold 1997), as a source for rapid adaptive diversification (Lewontin and Birch 1966, Seehau- sen 2004), and as a means to facilitate microevolutionary changes in re- sponse to climatic extremes (Grant and Grant 1993). Secondary contact be- tween closely related species is “revitalizing the ghosts of competition and evolution” (Confer 2006), and is generally considered to result in either the extinction of one population, stable coexistence with hybridization, or for- mation of distinct species by the buildup of reproductive isolation and niche separation (Liou and Price 1994). When the enhancement of reproductive isolation in sympatry is driven by natural selection against hybridization the process is referred to as rein- forcement (Dobzhansky 1940). As reinforcement might be an important process in finalizing speciation, it has received considerable empirical and theoretical attention (reviewed by Servedio and Noor 2003). One of the ma- jor criticisms against reinforcement is that it can only work under a rather narrow window of conditions: hybridization must be fairly common to exert a significant selection pressure (Moore 1957), but not so prevalent that re- combination breaks down the association between genes influencing hybrid fitness and genes coding for traits important for assortative mating (Barton and Hewitt 1985). According to the advocates of reinforcement (see Serve- dio and Noor 2003 and references therein), the process of reinforcement, i.e. selection to avoid hybridization, can lead to a pattern of divergent resource- use or reproductive phenotypes between populations. 8 However, divergence in resource-use or in reproductive phenotypes, i.e. character displacement (Brown and Wilson 1956), can also reduce other harmful interactions between populations. Ecological character displacement (evolution resulting from selection to reduce interspecific resource competi- tion) and reproductive character displacement (evolution resulting from se- lection to minimize interspecific reproductive interference) can result in a pattern of geographical variation where populations in sympatry with a closely related heterospecific differ from conspecific populations in allopa- try. One difficulty with inferring process from pattern is that character dis- placement driven by competition could create a similar pattern of greater divergence in sympatry than in allopatry as character displacement through reinforcement would. Hence, reinforcement could also be seen rather as a special case of the process of
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