INTRODUCTION

The study of intraspecific genetic variation has demonstrated a vast potential to reconstruct phylogeographic patterns, infer historical demographic processes and define levels of gene flow of conservation relevance (Avise 2004). Evolutionary and demographic studies, along with evidence of current genetic and ecological diversity can, in fact, describe levels of population distinctiveness and direct management initiatives of importance to the retention of intraspecific genetic variability and the long-term fitness of endangered species (Fraser and Bernatchez 2001).

Population divergence and taxonomy

Molecular genetics is a particularly valuable tool for the study of island systems where different selective pressures and dispersal ability of endemic species can hamper clear patterns of morphological and ecological diversification for populations of taxonomic importance. In the Galápagos giant tortoise Geochelone nigra (or G. elephantopus: see Zug 1997), the taxonomy first proposed by Van Denburgh (1914) has been somewhat controversial. Taxon designation was originally based on two main tortoise morphologies and their variants: a large, dome morphotype with rounded carapace and short limbs, and a smaller saddlebacked form with a highly elevated anterior part of the carapace, longer neck and limbs, and thinner shell. Five saddlebacked subspecies were described, on the islands of Española (hoodensis), San Cristóbal (chatamensis), Pinzón (ephippium), Fernandina (phantastica) and Pinta (abingdoni). Domed tortoises were instead reported from Santa Cruz (porteri), Rábida (wallacei) and in Isabela on Volcan Darwin (microphyes), Volcan Alcedo (vandenburghi), Sierra Negra (guntheri) and Cerro Azul (vicina).

Tortoises from Santiago (darwini) are of intermediate morphology. Similarly, heterogeneous morphotypes, assigned to the becki subspecies, were described on Volcan Wolf, in northern Isabela. For the majority of island populations recent genetic analysis validated the proposed taxonomy, while for others new patterns were recovered which were inconsistent with previous morphologically based nomenclature (Caccone et al. 2002; Beheregaray et al. 2003a; Russello et al. 2005; Ciofi et al. 2006).