La perception du paysage dans les images viatiques des Alpes

Etudiant les images viatiques des Alpes présentes dans la base de données VIATIMAGES, au tournant des XVIIIe-XIXe siècles, l'article montre que la prépondérance du sens de la vue dans la perception des paysages est une construction historique et anthropologique. En analysant plus précisément ces images, on peut y repérer des personnages identifiés aux habitants des lieux visités par les voyageurs. Au-delà des stéréotypes du "berger des Alpes", ces images nous disent quelque chose sur la perception pluri-sensorielle et sur l'immédiateté de la présence au monde des paysans alpins. Cependant, une équivoque demeure, que les sources viatiques ne permettent pas de lever, car les qualités attribuées aux paysans des Alpes sont celles mêmes dont les élites urbaines voyageantes sont privées. Dans les zones de contact que sont les sites touristiques au début du XIXe siècle, le regardant et le regardé mettent en miroir leurs qualités sans les partager.

Timeframe of speciation inferred from secondary contact zones in the European tree frog radiation (Hyla arborea group).

BACKGROUND: Hybridization between incipient species is expected to become progressively limited as their genetic divergence increases and reproductive isolation proceeds. Amphibian radiations and their secondary contact zones are useful models to infer the timeframes of speciation, but empirical data from natural systems remains extremely scarce. Here we follow this approach in the European radiation of tree frogs (Hyla arborea group). We investigated a natural hybrid zone between two lineages (Hyla arborea and Hyla orientalis) of Mio-Pliocene divergence (~5 My) for comparison with other hybrid systems from this group. RESULTS: We found concordant geographic distributions of nuclear and mitochondrial gene pools, and replicated narrow transitions (~30 km) across two independent transects, indicating an advanced state of reproductive isolation and potential local barriers to dispersal. This result parallels the situation between H. arborea and H. intermedia, which share the same amount of divergence with H. orientalis. In contrast, younger lineages show much stronger admixture at secondary contacts. CONCLUSIONS: Our findings corroborate the negative relationship between hybridizability and divergence time in European tree frogs, where 5 My are necessary to achieve almost complete reproductive isolation. Speciation seems to progress homogeneously in this radiation, and might thus be driven by gradual genome-wide changes rather than single speciation genes. However, the timescale differs greatly from that of other well-studied amphibians. General assumptions on the time necessary for speciation based on evidence from unrelated taxa may thus be unreliable. In contrast, comparative hybrid zone analyses within single radiations such as our case study are useful to appreciate the advance of speciation in space and time.