Thema Nr. 13: Die „Ringspezies“ an den Beispielen der Silbermöwe (Larus argentatus) und des Grünen Laubsängers



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Thema Nr. 13: Die „Ringspezies“ an den Beispielen der Silbermöwe (Larus argentatus) und des Grünen Laubsängers (Phylloscopus trochiloides)

Fragen:


  • was ist der Unterschied zwischen „isolation by distance“ und „Ringspezies“?

  • warum ist die Silbermöwe keine Ringspezies?

  • warum ist der Grüne Laubsänger eine Ringspezies?



relevante Seiten (u.a.):
Buch-Kap. 7: S. 18 und 19
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Erläuterungen:
Im Referat genügt es, die drei oberen Fragen zu erläutern.

Literatur:
Hierzu folgen ausführliche Auszüge aus 3 Originalarbeiten (die Originalarbeiten selbst brauchen nicht eingesehen zu werden).
Liebers,D., de Knijff,P., and Helbig,A.J. (2004). The herring gull complex is not a ring species. Proc. R. Soc. Lond. B Biol. Sci. 271, 893-894.
Ref ID: 5345

Abstract: Under what circumstances speciation in sexually reproducing animals can occur without geographical disjunction is still controversial. According to the ring-species model, a reproductive barrier may arise through 'isolation by distance' when peripheral populations of a species meet after expanding around some uninhabitable barrier. The classical example of this kind of speciation is the herring gull (Larus argentatus) complex, with a circumpolar distribution in the Northern Hemisphere. Based on mitochondrial DNA variation among 21 gull taxa, we show that members of this complex differentiated largely in allopatry following multiple vicariance and long-distance-colonization events, not primarily through isolation by distance. Reproductive isolation evolved more rapidly between some lineages than between others, irrespective of their genetic distance. Extant taxa are the result of divergent as well as reticulate evolution between two ancestral lineages originally separated in a North Atlantic refugium and a continental Eurasian refugium, respectively. Continental birds expanded along the entire north Eurasian coast and via Beringia into North America. Contrary to the ring-species model, we find no genetic evidence for a closure of the circumpolar ring through colonization of Europe by North American herring gulls. However, closure of the ring in the opposite direction may be imminent, with lesser black-backed gulls about to colonize North America.


Notes: INTRODUCTION


Speciation, i.e. the divergence of an ancestral population into two reproductively isolated daughter populations (Dobzhansky 1937), requires genetic differentiation at least at those loci involved in reproductive (sexual) functions. Ongoing gene flow will oppose differentiation between populations, and it is unclear to what extent gene flow must be reduced for speciation to be completed (Slatkin 1987; Turelli et al. 2001). Ernst Mayr (1942), based on the earlier ideas of Geyr von Schweppenburg (1938), proposed that reproductive isolation may evolve in a single species through 'isolation by distance', i.e. without interruption of gene flow, when peripheral populations meet after expanding around a large uninhabitable area. This mode of speciation through 'circular overlap' (Mayr 1942) was later termed the 'ring species' model (Cain 1954). Geographical overlap between taxa that are elsewhere connected through interbreeding populations is an essential element of this model, because it is ongoing gene flow that distinguishes ring species from cases of allopatric speciation that happen to be arranged in a roughly circular fashion (Irwin et al. 2001a).

The classical example that the ring-species model was based upon is the herring gull (Larus argentatus) complex. This group comprises more than 20 taxa of large gulls (Haffer 1982), which together occupy a circumpolar breeding range in the Northern Hemisphere. The taxa differ most obviously in body size and in the darkness of their dorsal plumage ('mantle'), which varies from pale grey to black. According to the model of Mayr (1942), the group originated in the Aralo-Caspian region, from where gulls spread in three directions (figure 1a): (i) west via the Mediterranean into the Atlantic giving rise to Mediterranean (michahellis) and Atlantic (atlantis) yellow-legged gulls; (ii) east towards Inner Asia giving rise to the Mongolian gull (mongolicus); and (iii) north to the Arctic Ocean. Along the North Eurasian coasts, the ancestral population expanded both ways: (i) west across Scandinavia towards Britain and Iceland differentiating into dark-mantled lesser black-backed gulls ( fuscus, intermedius and graellsii); and (ii) east all the way to the North Pacific, giving rise to the progressively paler-mantled forms taimyrensis (Taimyr), birulai and vegae (eastern Siberia), and into North America (smithsonianus).

Following the Last Glacial Maximum, North American herring gulls are supposed to have crossed the North Atlantic and invaded Europe, where they gave rise to the taxon argentatus and argenteus, which now overlap with lesser black-backed gulls (Geyr von Schweppenburg 1938; Mayr 1942). Mayr envisioned all taxa of the circumpolar chain to be connected by gene flow, while herring gulls and lesser black-backed gulls in Europe, the hypothetical endpoints of the ring, have reached full reproductive isolation and now coexist as distinct species.

If this scenario is true, we would expect to find (i) extensive sharing of mitochondrial haplotypes resulting from gene flow between geographically neighbouring taxa within the circumpolar ring; and (ii) evidence of introgression or closely related haplotypes between North American (smithsonianus) and European (argentatus) herring gulls caused by recent transatlantic colonization. We tested this hypothesis by investigating the phylogeography of 20 Northern Hemisphere gull taxa and one Southern Hemisphere gull taxon using 1.5 kb of mitochondrial DNA (mtDNA) sequence.

Previous attempts to test the ring-species model in these gulls were inconclusive owing to the low amount of variation recovered from allozymes (Snell 1991) and short conservative segments of mitochondrial DNA (Crochet et al. 2002). The latter study did show that 'herring gull' haplotypes differ between North America (smithsonianus) and Europe (argentatus), but sample sizes and sequence variation were insufficient to reject the ring-species model. The hypervariable part of the mitochondrial control region (HVR-I), which has been used extensively to document recent evolutionary differentiation of various organisms including humans (Vigilant et al. 1991; Baker & Marshall 1997), proved highly informative in gulls (Liebers et al. 2001; Liebers & Helbig 2002). Here, we use the HVR-I segment plus the entire cytochrome b (cyt b) gene in the most comprehensive genetic study of the herring gull complex to date.

RESULTS
The concatenated mitochondrial sequences (1.57 kb) of 410 individuals contained 116 polymorphic sites (70 in cyt b, 46 in HVR-I), which defined a total of 160 unique haplotypes. The haplotype network shows extensive genetic divergence within the herring gull complex. The AMOVA indicated strong segregation of haplotypes along taxonomic, i.e. phenotypically defined, boundaries. The taxonomic affiliation of individuals accounted for 54% of the molecular variance. This variance component increased to 71% when geographical structuring within L. argentatus and L. hyperboreus was taken into account. The significant taxonomic and geographical structure enables us to interpret the evolutionary history of these gulls based on mitochondrial genetic variation.

The western gull (Larus occidentalis) of the North American west coast (range not shown in figure 2) was found to have highly divergent haplotypes relative to all other taxa in this study and is regarded as the outgroup. NCA indicated that the ancestral ingroup population was divided by an allopatric fragmentation event, leading to the evolution of two major clades: clade I (haplogroups A-C) centred in the North Atlantic and clade II (haplogroups D-H) with a circumpolar distribution. ….. Larus argentatus and L. cachinnans are the two taxa currently containing the most highly divergent and the earliest-branching haplotype lineages. This indicates that they are direct descendants of the two ancestral populations. If current breeding ranges are any indication, ancestors of clade I probably lived in the northeastern Atlantic (current range of argentatus sensu stricto), and those ancestral to clade II lived in the Aralo-Caspian region (current range of cachinnans).

As indicated by NCA, the Aralo-Caspian population (ancestors of clade II) spread by contiguous range expansion towards the north Eurasian coast, then west up to Britain and Iceland (fuscus range) and east throughout northern Siberia (vegae, schistisagus) and North America (smithsonianus, glaucescens, glaucoides; figure 1b). In accordance with Mayr's theory, the sharing of haplotypes between adjacent taxa in this circumpolar range indicates ongoing gene flow. However, we find no support for the key element of the ring-species hypothesis, i.e. a transatlantic invasion of North American herring gulls (smithsonianus) into Europe. No haplotypes typical of, or derived from, nearctic smithsonianus were found anywhere in the European argentatus population, not even in Iceland. The endpoints of the circumpolar ring of interbreeding taxa, therefore, do not overlap. Furthermore, yellow-legged gulls of the Atlantic islands (atlantis), the Mediterranean Sea (michahellis) and Asia Minor (armenicus) are derived from North Atlantic (clade I), not Aralo-Caspian ancestors, while central Asia (mongolicus range) was colonized from the Pacific coast, not from the Aralo-Caspian basin (contra Mayr 1942). The demographic histories of the two basal lineages were quite different according to the respective mismatch distributions: among descendants of the North Atlantic refugium (clade I), strong bimodality indicates population growth after long periods of allopatric divergence, whereas descendants of the Aralo-Caspian refugium (clade II) experienced more recent contiguous population expansion.

Two more aspects of the mitochondrial phylogeny are surprising. First, three distinct species previously thought to derive from phylogenetically older ancestors are nested within the herring gull complex: the greater black-backed gull (L. marinus) and the glaucous gull (L. hyperboreus), which overlap extensively in breeding range with each other and with other members of the assemblage, and the Southern Hemisphere kelp gull (L. dominicanus). The latter evolved via long-distance colonization from the same ancestral population as the lesser black-backed gull (figure 4), suggesting that its ancestors were highly migratory, as nominate lesser black-backed gulls still are today.

……. Argentatus haplotypes of clade I are rather basal and more widely dispersed (groups B and C) than those in clade II, probably reflecting ancient polymorphism and thus suggesting that clade I haplotypes are ancestral in argentatus. Nuclear amplified fragment length polymorphism markers (de Knijff et al. 2001) also show argentatus to be most closely related to clade I taxa (atlantis and michahellis). By contrast, the geographically widespread occurrence of clade II haplotypes (group F) in the extant argentatus population appears to be the footprint of a past gene-flow episode. Birds derived from the Aralo-Caspian refugium, possibly members of a pre-heuglini population, must have hybridized, perhaps briefly, with the ancestral argentatus population of the North Atlantic. The introgressed clade II mitochondrial lineage then persisted in the argentatus population, where it continued to diversify to this day.

……Hybridization between glaucous and herring gulls has been observed in Iceland (Ingolfsson 1970) and northwestern Canada (Spear 1987) and may have been more frequent in the past than it is today. This may have led to strong mitochondrial introgression, perhaps approaching replacement of mtDNA of one species by that of the other (cytonuclear replacement), as seen in other cases of avian hybridization (Gill 1997; Weckstein et al. 2001). Nuclear markers will be needed to decide in which direction such cytonuclear replacement occurred, i.e. whether clade I or clade II haplotypes are ancestral in hyperboreus.

DISCUSSION



(a) Evolutionary history of the herring gull complex

What earlier authors (Geyr von Schweppenburg 1938; Mayr 1942; Haffer 1982) regarded as 'the herring gull' turned out to be an assemblage of several distinct taxa (argentatus, vegae, smithsonianus), which are not each other's closest relatives (figure 3). Our results show that the ring-species model does not adequately describe the evolution of the herring gull group because, contrary to the proposal of Mayr (1942), there is no overlap between the endpoints of a ring of interbreeding taxa. A circumpolar ring of interbreeding populations does exist, but it is made up entirely of taxa belonging to mitochondrial haplotype clade II, and its endpoints do not overlap. Current coexistence in sympatry, e.g. between argentatus and fuscus in Europe or between marinus and smithsonianus in North America, occurs between clade I and clade II taxa and is caused not by 'circular overlap', but by second expanding westwards and currently breed as far west as Greenland (Boertmann 1994). The species may soon colonize North America, where many birds are already observed annually (Post & Lewis 1995). It will be interesting to see whether graellsii will turn out to be reproductively isolated from North American smithsonianus. If so, the circumpolar ring of interbreeding taxa might yet close to form a classic ring species, not by an invasion of herring gulls from North America to Europe as postulated earlier (Geyr von Schweppenburg 1938; Mayr 1942), but by expansion of lesser black-backed gulls in the opposite direction.

(b) Genetic divergence versus reproductive isolation

Although our results do not support a speciation model involving only isolation by distance, the origin of reproductive barriers in the herring gull complex does provide important insights into the speciation process. An ancestral population may split up into two or more daughter populations, which gradually acquire reproductive isolation over a prolonged period, but at different rates (Grant & Grant 1997). In fact, we found no close relationship between mitochondrial genetic distance, which is roughly proportional to time since divergence, and reproductive isolation. The most divergent taxon in terms of mtDNA, L. occidentalis, is known to hybridize extensively with one of the ingroup taxa, L. glaucescens, along the west coast of North America (Bell 1997). On the other hand, our data do not support the traditional view of the greater black-backed gull (L. marinus) being an outgroup relative of the herring gull complex. Although L. marinus is fully reproductively isolated from all species it co-occurs with, in the mtDNA network it is nested among several taxa that hybridize: argentatus × hyperboreus (Ingolfsson 1970; Spear 1987), michahellis × graellsii (Van Swelm 1998), cachinnans × argentatus (Panov & Monzikov 1999) and earlier in the twentieth century argentatus × fuscus (Tinbergen 1953). Assuming equal mutation rates among lineages, reproductive isolation of L. marinus from its currently sympatric congeners evolved more rapidly [besser: hat schneller reproduktive Barrieren errichtet] than, for instance, between L. occidentalis and L. glaucescens. This must have involved a period of geographical isolation [warum nicht sympatrische Artbildung?: gerade das würde schnelle reproductive Isolation selektiv begünstigen], probably in northeastern North America (cf. figure 1b).

We hypothesize that marinus diverged in allopatry [?] from the rest of clade I, although NCA indicates not allopatric fragmentation but contiguous range expansion for this split. ….

A survey involving experimental cross-mating between Drosophila species showed that, for a given genetic distance, the degree of reproductive isolation is much greater between sympatric than between allopatric species, indicating that reinforcement plays a role in the evolution of reproductive isolation (Coyne & Orr 1997). If this is also true in gulls, it could mean that L. marinus, after a period of allopatric divergence [??], has had a relatively long history of geographical contact with closely related taxa (hyperboreus, argentatus, atlantis, and later with smithsonianus and fuscus), which facilitated the perfection of reproductive barriers through reinforcement. This may explain why L. marinus achieved complete reproductive isolation more rapidly than other taxa in the herring gull group and, thus, why the phylogenetic age of the marinus lineage was previously overestimated. [das ist ein wunderschönes Beispiel dafür, was an Fehldenken dabei rauskommt, wenn man die verschiedenen Artkonzepte zur Kongruenz bringen will. Allerdings dürfte sich nach langzeitlicher Trennung die Diskrepanz zwischen dem Ausmaß der Trennung (Verwandtschaft) und dem Ausmaß der Reproduktionsschranken einebnen, weil (in Sympatrie) das Getrenntsein ja auf den Sexualschranken basiert]. Novel theoretical considerations (Servedio & Saetre 2003) also ascribe a more important role to reinforcement than was hitherto accepted (Turelli et al. 2001).

We conclude that the establishment of reproductive barriers, several of which are still incomplete, occurred at different rates and allowed for some reticulation between lineages, which is reflected in the mitochondrial phylogeny. The degree of reproductive incompatibility, therefore, can serve only as a rough indicator of the phylogenetic distance between gull taxa. This agrees with findings in Drosophila (Coyne & Orr 1997), Plethodon salamanders (Highton 1995) and pigeons (Lijtmaer et al. 2003), which show that, although there is a general correlation between genetic divergence and the degree of postzygotic isolation, this correlation is far from perfect, i.e. in some cases strong isolation occurs even between species that are poorly diverged genetically.



(c) Significance of the ring-species model

Good examples fulfilling the stringent criteria of the ring-species model appear to be rare (Irwin et al. 2001a). The conclusions we derived here for the herring gull complex are similar to those reached in studies of other potential ring species, for example the Ensatina complex of salamanders in western North America (Wake 1997) and the great tit Parus major complex in Eurasia (Kvist et al. 2003). In both cases, ancestral populations expanded in a roughly circular fashion around an uninhabitable area, but there have been intermittent (zeitweilig) periods of allopatric fragmentation and subsequent range expansion. This led to areas of secondary contact, where hybridization currently occurs, often in very narrow zones. Population divergence, therefore, proceeded at least partly in allopatry, not exclusively through isolation by distance throughout a contiguous range. A case that closely approximates a true ring species is that of the Asian greenish warbler Phylloscopus trochiloides group (Irwin et al. 2001b). Ancestors of this complex spread from south of the Himalayas east and west around the Tibetan Plateau, north of which the ring closed between two taxa that are reproductively isolated, primarily through divergent songs that function in mate recognition. Song evolution is driven by sexual selection, which may produce rapid divergence. So reproductive divergence in these warblers seems to have come about through a combination of isolation by distance and sexual selection. The main difference between this and the herring gull case is that the ring-shaped range of the warblers has closed, leading to geographical overlap between the two terminal taxa, whereas in the gulls this is not (yet) the case. In conclusion, although ring speciation is theoretically possible, the few well-studied examples suggest that it occurs infrequently, because the dynamics of species' ranges are more likely to result in fragmentation, i.e. periods of allopatry, before the slow process of isolation by distance leads to sufficient divergence to allow for circular overlap.

__________________________________________________________


Irwin,D.E., Bensch,S., and Price,T. (2001). Speciation in a ring. Nature 409, 333-337.
Ref ID: 5740

Abstract: The evolutionary divergence of a single species into two has never been directly observed in nature, primarily because speciation can take a long time to occur. A ring species, in which a chain of intergrading populations encircles a barrier and the terminal forms coexist without interbreeding, provides a situation in which variation in space can be used to infer variation in time. Here we reconstruct the pathway to speciation between two reproductively isolated forms of greenish warbler (Phylloscopus trochiloides). These two taxa do not interbreed in central Siberia but are connected by a long chain of intergrading populations encircling the Tibetan Plateau to the south. Molecular data and climatic history imply that the reproductively isolated taxa came into contact following expansions northward around the western and eastern sides of the plateau. Parallel selection pressures for increased song complexity during the northward expansions have been accompanied by divergence in song structure. Playback experiments show that the two Siberian forms do not recognize each other's songs. Our results show how gradual divergence in a trait involved in mate choice leads to the formation of new species.


_____________________________________________________
Irwin,D.E., Bensch,S., Irwin,J.H., and Price,T. (2005). Speciation by distance in a ring species. Science 307, 414-416.
Ref ID: 5739

Abstract: Ring species, which consist of two reproductively isolated forms connected by a chain of intergrading populations, have often been described as examples of speciation despite gene flow between populations, but this has never been demonstrated. We used amplified fragment length polymorphism (AFLP) markers to study gene flow in greenish warblers (Phylloscopus trochiloides). These genetic markers show distinct differences between two reproductively isolated forms but gradual change through the ring connecting these forms. These findings provide the strongest evidence yet for "speciation by force of distance" in the face of ongoing gene flow.


Traditional models emphasize geographic separation as a necessary prerequisite to speciation. Although experiments and theory indicate that species can form despite ongoing gene flow, there are very few known examples in nature. Some studies have demonstrated divergence despite gene flow, but they do not enable an assessment of reproductive isolation because the divergent forms remain geographically separated. Species are usually defined as groups of interbreeding populations reproductively isolated from other such groups, and this can only be critically examined if different populations regularly come into contact in nature. [Das ist genau die klassische Mayr’sche Auffassung von der Reproduktionsgemeinschaft.]

There are a few examples where reproductively isolated populations coexist while being connected by apparently gradual variation around geographic barriers ("ring species"). In theory, ring species enable us to trace the process by which one species diverges into two. [Das ist nicht zwangsläufig]. They also potentially show that reproductive isolation can arise in the face of gene flow. However, a clear pattern of a gradual genetic variation has not previously been observed in a ring species. Here, we use molecular markers to show that two reproductively isolated forms of greenish warbler (Phylloscopus trochiloides) are connected by gene flow through a ring of populations, providing the strongest empirical evidence yet for "speciation by force of distance".

Two forms of greenish warbler, one in west Siberia (P. t. viridanus) and one in east Siberia (P. t. plumbeitarsus), coexist without interbreeding in central Siberia and can therefore be considered separate species. [Hier ist klar, was die Mayr’sche Definiton ist und dass Irwin sie übernimmt.] These forms are connected by a chain of populations to the south that encircles the high-altitude desert of the Tibetan Plateau, which is not inhabited by the warblers. Through this chain of populations, traits such as color patterns, morphology, and behaviors (song and song recognition), change gradually, demonstrating a smooth gradient in forms between two species. There is evidence that all of these traits are under selection in the Phylloscopus warblers; it is therefore unclear that such traits can be used to infer gene flow. To directly measure genome-wide genetic relationships, we used amplified fragment length polymorphism (AFLP) markers.

From 105 greenish warblers at 26 sites throughout the breeding range we obtained 62 AFLP markers that were variable and could be scored unambiguously as present or absent in each individual. West Siberian viridanus and east Siberian plumbeitarsus are clearly separated in AFLP genotypes, which confirms that the two taxa are genetically distinct. In contrast, AFLP genotypes change gradually through the ring of populations to the south. The genetic gradient in the AFLP genotypes around the southern ring of populations is best seen in a plot of pairwise AFLP distances versus pairwise geographic distance.

Geographic distances were measured under the assumption that no genes flow across the uninhabited area in the center of the ring or between viridanus and plumbeitarsus in central Siberia. Thus, "corrected" distances between west Siberian (viridanus) and east Siberian (plumbeitarsus) populations were measured through the long chain of populations running to the south of Tibet, through the Himalayas. Genetic distance and corrected geographic distance are strongly correlated (Mantel's r = 0.782, P = 0.0003), consistent with a pattern of isolation by distance around the ring. An alternative analysis based on pairwise FST distances between populations produces similar results (Mantel's r = 0.677, P = 0.0012; table S1 and fig. S3).

On the basis of these results, we conclude that there is no break in gene flow through the ring of populations, except between the divergent forms viridanus and plumbeitarsus in central Siberia. Thus all populations have been recently connected by at least some gene flow. The simplest historical scenario for this result is that short-distance dispersal in a continuously distributed species has resulted in a pattern of isolation by distance.

This interpretation may at first seem inconsistent with previously published patterns of variation in mitochondrial DNA (mtDNA), in which there are several deep phylogeographic breaks around the ring, the deepest of which is in the western Himalayas. In fact, the mtDNA and AFLP patterns are compatible. Short-distance dispersal in a continuously distributed species is expected to cause phylogeographic structuring in mtDNA clades and a pattern of isolation by distance in AFLP markers. The shape of the greenish warbler range is particularly suited to creating this pattern; the birds breed in a narrow string of treeline habitat through the Himalayas, where dispersal distances are likely shorter than in more broadly distributed forest habitat further north. Ticehurst hypothesized that the greenish warblers were at one time confined to the Himalayas and then expanded northward along two pathways into Siberia.



Theory predicts that the pattern of isolation by distance should be weaker in regions of recent range expansion compared with regions that have been inhabited over a long period of time. The steeper genetic change seen in AFLPs, mtDNA, and two microsatellite loci through the Himalayas than through regions to the north is consistent with this prediction. It is also possible that these patterns were influenced by temporary breaks in gene flow due to geographic barriers in the Himalayas; however, such barriers, if they existed, did not cause reproductive isolation to evolve in that region.

Greenish warblers provide the only known example of a smooth genetic gradient between two genetically differentiated and reproductively isolated forms, providing rare insight into how speciation can occur. Patterns of variation in ecologically and sexually selected traits of greenish warblers suggest that latitudinal gradients in environmental characteristics, such as forest density and seasonal migration distance, during the two northward expansions into Siberia have resulted in rapid evolutionary adaptation, divergence, and reproductive isolation.

Several authors have suggested on theoretical grounds that ring species or "sexual continua" are unstable and will fairly quickly break into two or more species that do not exchange genes. The two models that show this effect do not apply well to the greenish warbler, because one does not include local adaptation throughout a continuous geographic range and the other does not include different geographic locations for different populations. We suggest that ring species such as the greenish warbler, in which local adaptation occurs along a long and nearly continuous ring of populations, could be stable indefinitely. This stability could be interrupted by processes such as habitat change, which could increase the likelihood of parapatric speciation, or habitat destruction, which could divide the continuous range and thereby increase the likelihood of additional species boundaries forming.



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