Data from: 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.