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It is now widely acknowledged that a high proportion of the world's species are threatened with global extinction in the near future. Conserving all endangered species is impractical due to limited financial resources and manpower. Therefore, there is a real need to understand the mechanisms behind the extinction process in order to develop a proactive rather than reactive conservation strategy. Furthermore, given these limitations there is a need to be selective, prioritising species or areas not just on the basis of extinction risk or the number of species present, but with consideration for their role within the ecosystem. In this thesis, I investigate three aspects of current global extinction: What causes species to become threatened, how we should prioritise those that are, and how such information can be translated into area-based conservation priorities. Firstly, I developed models of plant extinction risk in relation to environmental (anthropogenic, climatic, and physical) variables and used them to show that non- randomness in extinction risk is highly influenced by interactions between the plants traits and their environment. Incorporating such interactions, my models were able to explain ~30% of the variation in plant extinction risk. Results from this large-scale comparative study suggest the biological traits that increase species' susceptibility are dependent upon local environmental conditions. Secondly, I looked at how much unique evolutionary history, a proxy for species value, will be lost under current extinction patterns, whilst investigating how effective current conservation methods are at prioritising species based on their evolutionary value. The results are not promising; species threatened with extinction had disproportionately high quantities of unique evolutionary history. Furthermore, the IUCN Red List makes no allowance for this, for example, species considered Critically Endangered, the highest threat status under the IUCN Red List, may not necessarily be the most evolutionary unique. Finally, I looked at the relationships among species richness, versity (FD) and phylogenetic diversity (PD) as measures of biodiversity and therefore the conservation value of an area. It was found species richness is generally a good surrogate for FD and PD. However, agricultural development and climatic variation bring out discrepancies between species richness, FD and PD, challenging the claim of interchangeability of different diversity measures, with potential consequences for conservation planning.

Considerable variation exists both within and between species in nuclear DNA content. Despite there being no obvious functional role for much of this DNA, many studies have reported phenotypic correlations with genome size at various taxonomic levels. This suggests that DNA plays a functional role beyond the traditionally understood mechanisms. One such example of a phenotypic correlation with DNA content is present in the genus Silene, where a negative correlation between DNA content and flower size exists within and between species. This relationship is consistent with the direction of sexual dimorphism in DNA content (caused by heteromorphic sex-chromosomes) and flower size in the most studied species in the genus: S. latifolia. This thesis takes a comparative approach between two closely related species in the genus (S. latifolia and S. marizii), which differ markedly in their nuclear DNA content, in order to investigate the nature and phenotypic impacts of variation in DNA content. A phenotypic survey from a number of S. marizii populations reveals that the pattern of DNA content variation in this species is very different to that in S. latifolia. In particular, phenotypic correlations with DNA content appear be much weaker, whilst sexual dimorphism in DNA content, when present, appears to occur in either direction. A survey of interspecific hybrids suggests that this may be due to an enlarged S. marizii X-chromosome and that DNA content in hybrids may be biased with regard to their parents. Repetitive elements may be significant constituents of plant genomes. A study of Ty1-copia class retrotransposons in the two species reveals that they are present as a large and highly heterogeneous population. Phylogenetic analysis of these elements suggests a substantial degree of genetic isolation between the two species. Finally, an assessment of the flow-cytometric method, used to estimate DNA content, reveals substantial error associated with the method, but only limited evidence for stoichiometric effects.