Introduction
Past population demographics shape present genetic structure (Avise, 2004). Population genetic methods unravelling current allelic patterns may thus provide insight into past and extant species distribution dynamics (Avise et al., 1987; Crisci, Katinas, & Posadas, 2003). Hybrid zones, where genetically distinct populations meet and admix, are characterised by gene flow through time and space (Barton & Hewitt, 1985; Hewitt, 2001). These ‘natural laboratories’ are often established when recently diverged species meet, for example following secondary contact during range expansion of species from glacial refugia (Excoffier, Foll & Petit, 2009; Hewitt, 1988; Taberlet, Fumagalli, Wust-Saucy, & Cosson, 1998).
Spatio-temporal dynamics among hybridising taxa can result in the formation of enclaves (Buggs, 2007; Wielstra, Burke, Butlin, & Arntzen, 2017a). Enclaves form when the population of one species is surrounded by populations of a competing related species, becoming genetically isolated from the remainder of the receding species’ range (Arntzen, 1978). Enclaves can therefore illustrate historical species replacement, particularly in ground-dwelling organisms with low dispersal capabilities. Moving hybrid zones may leave a specific spatial signature, in the form of a molecular genomic footprint (Scribner & Avise, 1993; Wielstra, 2019). As an advancing species spreads into a contact zone, neutral alleles may flow from the retreating to the invading taxon and introgression will be more pronounced in the advancing than in the receding species (Moran, 1981; van Riemsdijk, Butlin, Wielstra, & Arntzen, 2019). Asymmetric introgression may reflect neutral alleles left in the wake of the moving hybrid zone and eventually become geographically stable over time, as the genomic footprint is solely dependent on drift (Barton & Hewitt, 1985; Currat, Ruedi, Petit, & Excoffier, 2008). Hence, introgression patterns of selectively neutral traits can be used to reconstruct the history of hybrid zones (Wielstra et al. , 2017b).
We previously documented an enclave and limited interspecific gene flow in the marbled newt Triturus marmoratus (Latreille, 1880) around Caldas da Rainha in the northwest of the Lisbon Peninsula, and suggested that the observation was best explained by the competitive advance of its sister-species, the pygmy marbled newt T. pygmaeus(Wolterstorff, 1905) (Espregueira Themudo & Arntzen, 2007; Wielstra, Sillero, Vörös, & Arntzen, 2014). Long-distance colonisation or human introduction seem unlikely to have originated the enclave, since the minimum distance to the contact zone exceeds the dispersal capacities ofTriturus newts and there is no tradition of newt husbandry in Portugal (Espregueira Themudo & Arntzen, 2007). The potential for interspecific gene flow in these species allows exploring genomic signals and reconstructing hybrid zone movement (Arntzen, Wielstra, & Wallis, 2014). We here test for the presence of a genomic footprint ofT. marmoratus in T. pygmaeus south of the enclave employing newly developed single-nucleotide polymorphism (SNP) markers with species diagnostic allele variants (Garvin, Saitoh, & Gharrett, 2010; Meilink, Arntzen, van Delft, & Wielstra, 2015).