Since the time of Charles Darwin, studies of interspecific hybridization have been a major focus for evolutionary biologists. Although this phenomenon has often been viewed as problematic in the fields of ecology, taxonomy and systematics, it has become a primary source of data for studies on speciation and adaptation. Effects from genetic/evolutionary processes, such as recombination and natural selection, usually develop over extended periods of time; however, they are accelerated in cases of hybridization. Interspecific hybrids exhibit novel genomes that are exposed to natural selection, thus providing a key to unravel the ultimate causes of adaptation and speciation. Here we provide firstly a historic perspective of hybridization research, secondly a novel attempt to assess the extent of hybridization among animals and thirdly an overview of the reviews and case studies presented in this theme issue.

The overenrichment (eutrophication) of aquatic ecosystems with nutrients leading to algal blooms and anoxic conditions has been a persistent and widespread environmental problem. Although there are many studies on the ecological impact of elevated phosphorus (P) levels (e.g., decrease in biodiversity and water quality), little is known about the evolutionary consequences for animal species. We reconstructed the genetic architecture of a Daphnia species complex in 2 European lakes using diapausing eggs that were isolated from sediment layers covering the past 100 years. Changes in total P were clearly associated with a shift in species composition and the population structure of evolutionary lineages. Although environmental conditions were largely reestablished after peak eutrophication during the 1970s and 1980s, original species composition and the genetic architecture of species were not restored but evolved along new evolutionary trajectories. Our data demonstrate that anthropogenically induced temporal alterations of habitats are associated with long-lasting changes in communities and species via interspecific hybridization and introgression.biological archive ͉ eutrophication ͉ hybridization ͉ introgression ͉ invasiveness

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BackgroundThe Euthyneura are considered to be the most successful and diverse group of Gastropoda. Phylogenetically, they are riven with controversy. Previous morphology-based phylogenetic studies have been greatly hampered by rampant parallelism in morphological characters or by incomplete taxon sampling. Based on sequences of nuclear 18S rRNA and 28S rRNA as well as mitochondrial 16S rRNA and COI DNA from 56 taxa, we reconstructed the phylogeny of Euthyneura utilising Maximum Likelihood and Bayesian inference methods. The evolution of colonization of freshwater and terrestrial habitats by pulmonate Euthyneura, considered crucial in the evolution of this group of Gastropoda, is reconstructed with Bayesian approaches.ResultsWe found several well supported clades within Euthyneura, however, we could not confirm the traditional classification, since Pulmonata are paraphyletic and Opistobranchia are either polyphyletic or paraphyletic with several clades clearly distinguishable. Sacoglossa appear separately from the rest of the Opisthobranchia as sister taxon to basal Pulmonata. Within Pulmonata, Basommatophora are paraphyletic and Hygrophila and Eupulmonata form monophyletic clades. Pyramidelloidea are placed within Euthyneura rendering the Euthyneura paraphyletic.ConclusionBased on the current phylogeny, it can be proposed for the first time that invasion of freshwater by Pulmonata is a unique evolutionary event and has taken place directly from the marine environment via an aquatic pathway. The origin of colonisation of terrestrial habitats is seeded in marginal zones and has probably occurred via estuaries or semi-terrestrial habitats such as mangroves.

We studied two Corbicula morphotypes in a syntopic population in the Rhine River in order to reveal their taxonomic, reproductive and phylogenetic relationship, using morphometrics, DAF-fingerprinting, mitochondrial COI and nuclear ITS1 sequence variation. Morphometric analysis showed that two statistically distinguishable morphotypes with few intermediates were present. Mitochondrial sequence analysis detected two divergent clades. DAF-fingerprinting revealed three highly distinctive multilocus genotypes. Two of the multilocus genotypes were significantly associated with different morphotypes and mitochondrial lineages. The third genotype B, however, was found in both morphotypes, intermediates and mitochondrial lineages. Conclusive evidence for hybridization came from RFLP analysis of the nuclear ITS1 locus. We interpret the hybrids as F1 hybrids between different evolutionary lineages. Integration of Corbicula sequences from all over the world into Maximum Parsimony analysis suggested a simultaneous radiation resulting in several evolutionary lineages whose species status remained doubtful. An unequivocal taxonomic assignment of the two evolutionary lineages in the Rhine population was therefore not possible.

Biodiversity studies require species level analyses for the accurate assessment of community structures. However, while specialized taxonomic knowledge is only rarely available for routine identifications, DNA taxonomy and DNA barcoding could provide the taxonomic basis for ecological inferences. In this study, we assessed the community structure of sediment dwelling, morphologically cryptic Chironomus larvae in the Rhine-valley plain/Germany, comparing larval type classification, cytotaxonomy, DNA taxonomy and barcoding. While larval type classification performed poorly, cytotaxonomy and DNA-based methods yielded comparable results: detrended correspondence analysis and permutation analyses indicated that the assemblages are not randomly but competitively structured. However, DNA taxonomy identified an additional species that could not be resolved by the traditional method. We argue that DNA-based identification methods such as DNA barcoding can be a valuable tool to increase accuracy, objectivity and comparability of the taxonomic assessment in biodiversity and community ecology studies.

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The European wildcat, Felis silvestris silvestris, serves as a prominent target species for the reconnection of central European forest habitats. Monitoring of this species, however, appears difficult due to its elusive behaviour and the ease of confusion with domestic cats. Recently, evidence for multiple wildcat occurrences outside its known distribution has accumulated in several areas across Central Europe, questioning the validity of available distribution data for this species. Our aim was to assess the fine-scale distribution and genetic status of the wildcat in its central European distribution range. We compiled and analysed genetic samples from roadkills and hundreds of recent hair-trapping surveys and applied phylogenetic and genetic clustering methods to discriminate wild and domestic cats and identify population subdivision. 2220 individuals were confirmed as either wildcat (n = 1792) or domestic cat (n = 342), and the remaining 86 (3.9 %) were identified as hybrids between the two. Remarkably, genetic distinction of domestic cats, wildcats and their hybrids was only possible when taking into account the presence of two highly distinct genetic lineages of wildcats, with a suture zone in central Germany. 44 % of the individual wildcats where sampled outside the previously published distribution. Our analyses confirm a relatively continuous spatial presence of wildcats across large parts of the study area in contrast to previous analyses indicating a highly fragmented distribution. Our results suggest that wildcat conservation and management should take advantage of the higher than previously assumed dispersal potential of wildcats, which may use wildlife corridors very efficiently.