Hybridisation and genetic structure of woodland specialist ants in fragmented habitat

High genetic diversity within populations allows them the potential to adapt to environmental changes. Hybridisation and gene flow between species is common in nature, has a role in a variety of evolutionary processes, and can influence genetic diversity. Moundbuilding red wood ants (Formica rufa group) are ecologically dominant, keystone species within woodlands across Eurasia. They are known to hybridise extensively where their species ranges overlap, making their evolutionary relationships highly reticulate and of great interest to biologists. As woodland specialists and poor dispersers they are particularly susceptible to the effects of habitat fragmentation, which causes isolation and reduction in gene flow between populations. This decreases the adaptive potential of a population and increases the risk of local extinction. Three red wood ant species currently occur in the British Isles: F. rufa, F. aquilonia, F. lugubris. European wood ant populations are well studied, in contrast there is a paucity of broad scale data for their British conspecifics. To address this substantial knowledge gap, I characterised morphological and genetic variation in populations across Britain. Introgression of mitochondrial haplotypes into F. lugubris morphospecies suggested gene flow between species where they co-occur. However, genomic data from 135 nests indicated a picture of more sporadic hybridisation events followed by backcrossing into parental species. British forests are highly fragmented after millennia of human activity. However, I found little evidence of habitat fragmentation affecting genetic diversity. This may reflect a resilience to such habitat change, or a lack of statistical power and future work will be able to address this question. The data presented in this thesis support a picture of largely robust species boundaries with rare hybridisation events between species pairs, and initial modelling of fragmentation effects suggests no immediate threat to British wood ants.