Genetic Diversity of Melica Transsilvanica Schur (poaceae) at Its Northern Range Limit

Marginal populations are commonly expected to be less genetically variable than populations in the central range, which can make them less viable than the latter (Lesica and Allendorf, 1995). The degree of environmental and geographical marginality influences the genetic structure of populations (Johannesson and André, 2006). According to Mayr's (1970) model, the isolation of marginal populations is accompanied by a genotype change resulting from natural selection or genetic drift. Studies provide empirical evidence that genetic erosion, genetic drift and accumulation of deleterious alleles in small and isolated populations can reduce species viability (Lynch et al., 1995; Rasmussen and Kollmann, 2004; Johannesson and André, 2006). Both marginal and central-range populations effectively isolated over a long period have been found to be characterized by high interpopulational variability and diverge from the typical variability pattern formed by random breeding (Mitka, 1997). Many studies, however, have not confirmed the predicted consequences of population isolation by habitat fragmentation, or have even found evidence to the contrary (Llorens et al., 2004). Genetic diversity within the endangered shrub Grevillea caleyi (Proteaceae) did not vary with population size or degree of isolation, and the current genetic structure of populations may be almost entirely the result of historical events (Llorens et al., 2004). The consequences of habitat fragmentation for the genetic diversity of Anthyllis vulneraria (Fabaceae) were shown to be relatively minor in comparison with historical high levels of seed exchange between habitat fragments. Hence, seed flow is responsible for conservation of the relatively high genetic diversity of this species even within populations in small fragments (Honnay et al., 2006). Melica transsilvanica Schur is a submediterranean-continental, mainly steppe and steppe-forest species (Hempel, 1970). Its main geographical range covers Central Asia, the Middle East, eastern Siberia, the Caucasus, Western Asia, China, Eastern and Central Europe, reaching southern France and northern Italy in the west (Hultén and Fries, 1986). In the Polish flora it represents the Sub-IranoTuranian geographical element (Zając and Zając,