The primary focus of both evolutionary and conservation biology is to have the knowledge of population genetic structure and consequently define its causes and consequences. The major determinants causing genetic variability in plant species are historical aspects, human activity, narrow habitat preferences, natural selection and gene flow among populations, drift, founder effects, geographical distribution and biological properties of a given species (especially mating system) (Karron et al. 1988; Case 1993; Case 1994; CHUNG 1994). The importance of these above mentioned factors differs between species and populations. In rare plants, such as orchids, population size and the problem of isolation are often very important in shaping the genetic structure of populations. It has been reported that species which are more continuously distributed experience more gene flow than species with discrete, isolated populations leading to relatively lower variation among the populations (Hamrick and Godt 1989; Ellstrand and Elam 1993; Chung 1996). Moreover, geographically restricted species generally have lower levels of genetic variation within populations (Loveless and Hamrick 1984; Karron 1987; Hamrick and Godt 1989) although some of them possess relatively high level of genetic variation (Gitzendanner and Soltis 2000).
Dendrobium nobile Lindl., an epiphytic orchid has its distribution primarily in tropical and subtropical Asia (e.g. Northeast India, China, Japan, Malaysia, Philippines and Borneo) and North Australia usually growing at cooler climatic conditions at higher altitudes. Besides from horticultural significance, it has got a profound use in various herbal preparations. In Taiwan, its stem is used for treating night sweat, kidney disorder and to cure impotence. Chinese have been using it as a tonic and strengthening medicine for generations. The dried stems are used in the treatment of chronic fatigue, disorders related to restlessness, convalescence (Faria and Illg 1995) and are also used in promotion of body fluid production. This immense medicinal importance of D. nobile is primarily because of the bibenzyl compounds (Gigantol and Moscatilin) and one alkaloid (Dendrobine) (Suzuki et al. 1973; Faria and Illg 1995; Miyazawa et al. 1997; Zhao et al. 2001). Various active principles like 180-kDa 2-O-acetylgalactomannoglucan, SG-168 isolate and denbinobin isolated from stem extracts of D. nobile have been shown to have potential anti tumor activities (Wang et al. 2010; Yang et al. 2010; Yoon et al. 2011). Thus, immense ethnobotanical, ornamental and medicinal use of D. nobile, it has subjected it to severe anthropogenic pressure for centuries leading to the populations becoming threatened and endangered in natural habitats (Miyazawa et al. 1997; Yang et al. 2006). In order to design an effective conservation based utilization strategy for this endangered orchid species, therefore it is important to have a clear view about the availability and...
