DOI: 10.1111/j.1472-4642.2007.00373.x © 2007 The Authors 580 Journal compilation © 2007 Blackwell Publishing Ltd www.blackwellpublishing.com/ddi Diversity and Distributions, (Diversity Distrib.) (2007) 13, 580–586 BIODIVERSITY RESEARCH ABSTRACT The newt species Triturus marmoratus and Triturus pygmaeus are both present in central Portugal where they have parapatric distributions. We used four genetic markers to determine which species was present in 31 populations. In the centre of the study area we found a T. marmoratus enclave. Despite small interpopulation distances, hybridization is locally rare. We built several models to try to explain this distribution using environmental data. The best model, chosen by Akaike’s Informa- tion Criterion, relates the presence of T. marmoratus with the temperature in July, the relief of the landscape, and a higher use of the land for orchards. The current distribu- tion can best be explained by T. pygmaeus expanding north and replacing T. marmoratus, the latter only persisting where ecological conditions are relatively favourable. Keywords Akaike’s Information Criterion, allozymes, Amphibia, enclaves, mosaic hybrid zones, parapatry. INTRODUCTION Secondary contact between closely related species often generates spectacular events and provides insight into the evolutionary process. Hybrid zones formed this way are considered ‘natural laboratories’ or ‘windows on evolutionary processes’ (Hewitt, 1988; Harrison, 1990). Evolution is not a static process and to understand it, we must look into dynamic systems like these. The dynamics of species ranges are not independent of ecological conditions, competition with sister species, or dispersal capabili- ties. Present-day distributions are echoes from past events. One particular event that provides clues to the relative movement of species is the formation of enclaves. Enclaves are here defined as populations of one species completely surrounded by popula- tions of closely related species and genetically isolated from other populations of the same species (Arntzen, 1978; in geographical terms, they are simultaneously exclaves and enclaves). This is reminiscent of the ‘internal parapatry’ concept of Key (1981) that, however, does not deal with disjunct distributions. Mosaic hybrid zones are bimodal hybrid zones with few hybrids and predominantly parental genotypes present. The contact between the (sub)species is more strongly shaped by ecological constraints than by genetic interactions. In a recent review, Jiggins & Mallet (2000) go one step further and suggest that ecology contributes more to speciation than genetic incompatibility. Well-known mosaic hybrid zones are for example those in Gryllus crickets (Rand & Harrison, 1989), Chorthippus grasshoppers (Bridle et al., 2001), and Mytilus mussels (Bierne et al., 2003). Examples of mosaic hybrid zones in salamanders are Triturus cristatus and Triturus marmoratus in western France (Arntzen & Wallis, 1991) and Plethodon cinereus and Plethodon shenandoah in the Appala- chian Mountains of North America (Jaeger, 1970, 1971; see also Sites et al ., 2001). Although they present patches of populations of one species distributed among patches of the other, these are not all necessarily enclaves because dispersal among patches may be frequent. To the best of our knowledge the only enclaves recorded in the literature are those for Bombina toads in central Europe (Arntzen, 1978) and Triturus newts in western France and the northern Balkans (Arntzen & Wallis, 1991, 1999). Perhaps enclaves are more likely to arise in organisms with structured populations and low dispersal capability than in organisms that disperse well. Amphibian populations in particular are well delimited because of their dependence on water for reproduction and they have low individual mobility. Once formed, enclaves will take some time to dissolve, or be stable or disappear by reconnection to the main distribution. The two species of marbled newts living in the Iberian Penin- sula, T. marmoratus (Latreille, 1800) and Triturus pygmaeus (Wolterstorff, 1905), have a parapatric distribution. Some reports, however, indicated the presence of T. marmoratus where only T. pygmaeus was expected, near Caldas da Rainha and a spatial-environmental model for the two species, suggests that the local conditions may indeed be favourable to T. marmoratus (see the southernmost record in Fig. 1c in Arntzen, 2006). This would indicate an area of sympatry or a mosaic distribution. A mosaic distribution would point to differential ecological requirements with patches where the environmental conditions are more suitable for one species than for the other. An intuitive 1 CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrário de Vairão, 4485-661 Vairão, Portugal, 2 National Museum of Natural History, PO Box 9517, 2300 RA Leiden, the Netherlands *Correspondence: G. Espregueira Themudo, CIBIO, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrário de Vairão, 4485-661 Vairão, Portugal. E-mail: themudo@mail.icav.up.pt Blackwell Publishing Ltd Newts under siege: range expansion of Triturus pygmaeus isolates populations of its sister species G. Espregueira Themudo 1 * and J. W. Arntzen 2