Morphological and molecular characters to describe a marbled newt hybrid zone in the Iberian peninsula

I document the contiguous distribution of the marbled newt species Triturus marmoratus and T. pygmaeus over the western part of the Iberian peninsula with a suite of morphological and molecular genetic data from altogether 141 populations. Morphological characters that identify the species are body size and the colour pattern character ‘Links’. Links is the number of transversal connections following the green surface at the lateral sides of the newts’ body. Large adults with few Links are T. marmoratus and small adults with many Links are T. pygmaeus. However, no morphological identification criterion is entirely adequate. Eight molecular genetic markers show markedly bimodal character state distributions that give rise to sharp species range descriptions, with T. marmoratus in the north of the Iberian peninsula and T. pygmaeus in the south and along most of the Atlantic coast of Portugal. I encountered ten genetically admixed populations that are all located at the T. marmoratus - T. pygmaeus species range interface, suggesting widespread but limited interspecific hybridization. A latitudinal transect across Portugal confirmed the narrow and steep transition from one to the other species for morphological and molecular characters alike. In central Portugal the position of the hybrid zone coincides with the river Tejo. However, the cline for mitochondrial DNA is relatively wide and shallow and its centre is positioned south of the river. In view of published data that reconstruct the northward advance of T. pygmaeus along the Portuguese coast at the expense of T. marmoratus, I propose that T. marmoratus had a wider range in central Portugal too, where it was eventually superseded by T. pygmaeus. I hypothesize that ‘marmoratus’ mtDNA haplotypes found south of the Tejo constitute a ‘genetic footprint’ left behind in T. pygmaeus by the receding species T. marmoratus.