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Molecular phylogeography, intraspecific variation and the conservation of tree species

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Abstract

Tree species are becoming the focus of increasing conservation concern, with some 9000 species now threatened globally. Studies of intraspecific variation can contribute to the development of conservation strategies, by identifying appropriate units for conservation. The recent application of molecular techniques to a variety of tree species has highlighted a far higher degree of population differentiation than indicated by previous isozyme analyses, a result consistent with theoretical predictions. Analysis of the geographic distribution of cpDNA lineages has also enabled current patterns of population differentiation to be related to postglacial migration routes from different forest refugia. Such results highlight the importance of refugial areas for conservation of intraspecific variation in tree species.

Section snippets

Intraspecific variation within temperate trees

Molecular phylogeographic studies in tree species have been greatly facilitated by use of universal primers to amplify specific cpDNA loci, using an approach referred to as CAPS (cleaved amplified polymorphic sequences) or PCR-RFLP (PCR-restriction fragment length polymorphism) (Table 1). This technique has now been applied to a number of European tree species with broad distributions, including oaks (Quercus)6, 7, 8, 9, beech (Fagus)10 and alder (Alnus)11.

Initial studies with oak indicated a

Intraspecific variation within tropical trees

Relatively few studies of intraspecific variation using DNA techniques have been undertaken in tropical tree species. Those investigations that have been completed have again highlighted the existence of pronounced genetic structure within several species. Spanish cedar (Cedrela odorata) is a high-value timber species native to the American tropics and is now considered to be threatened in many parts of its range. Analysis of ten populations from Costa Rica using RAPDs indicated that 55% of the

Implications for conservation

In the wake of the Convention on Biological Biodiversity (Rio de Janeiro, Brazil, 1992), intraspecific variation has increasingly been accepted as a focus for conservation, an approach consistent with the general aim of maintaining the evolutionary potential of species. Molecular approaches could be of value to conservation efforts by providing a tool for measuring and managing genetic diversity and for investigating the processes that influence it25. As existing taxonomies might not fully

A forward look

Recent molecular analyses emphasize the overriding importance of evolutionary history in determining patterns of intraspecific variation in tree species. Historical events, such as range expansion, range fragmentation and population bottlenecks, appear to have had a much greater influence on patterns of genetic differentiation than has been accounted for in traditional population genetic models5. Results indicate that migration from glacial forest refugia still has a detectable impact on the

Acknowledgements

We are grateful to three anonymous reviewers for helpful comments on the manuscript. TA and ACMG are supported by European Community grants, ERBIC-18CT97-0146 (SUCRE project) and FAIR5-CT97-3795 respectively. Peter Hollingsworth, Rafael del Castillo, Andrea Premoli, Toby Pennington and Quentin Cronk are thanked for helpful discussions. Andrew King kindly provided draft manuscripts before publication.

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