The decline of bumblebee (Bombus spp.) populations in the UK and worldwide has been well reported. It has been generally assumed that such declines result in the genetic impoverishment of some species, potentially leading to reduced fitness and increased extinction risk. This study tested the fundamental assumption linking population fragmentation with fitness, in a model system of two Bombus species native to the UK. Bombus monticola has declined significantly in range across the UK in recent years and occupies fragmented upland areas, while Bombus pratorum has remained abundant and widespread over many habitat types. The effects of genetic diversity on fitness have been addressed in wild Bombus species, but this is the first study to explicitly compare data from species of differing levels of population connectivity and hence test the assumptions of traditional population genetic theory. As genetic diversity has often been linked with immunocompetence, aspects of the innate immune response were quantified, together with parasite load. These empirical measures of fitness showed lower than expected variability between the two study species, and no evidence was found to support the theory of lower fitness in fragmented populations. However, the considerable variability between sample sites in both species for all parameters measured raised interesting questions as to the underlying evolutionary processes; it is postulated that B. monticola populations may maintain a higher than expected Ne, despite their fragmented distribution. This study also provided methodological developments. An alternative method for the quantification of wing wear as a proxy for age was proposed, which could be easily applied to other Bombus species and possibly adapted for use in other flying insects. In addition possible sources of error in AFLP analysis were highlighted which have not been adequately discussed in the current literature, namely the effects of sample storage. Given the utility of AFLPs for non-model species, this is an important avenue for future research, and would be applicable to studies in other systems. Overall, the data presented here emphasise the challenges of studying fitness in wild populations, and underline the requirement for research into the fundamental principles underlying many assumptions made by conservation genetic theory.

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