• coevolution;
  • colonial seabirds;
  • host specificity;
  • host–parasite interactions;
  • Ixodes;
  • microsatellites;
  • speciation

Due to the close association between parasites and their hosts, many ‘generalist’ parasites have a high potential to become specialized on different host species. We investigated this hypothesis for a common ectoparasite of seabirds, the tick Ixodes uriae that is often found in mixed host sites. We examined patterns of neutral genetic variation between ticks collected from Black-legged kittiwakes (Rissa tridactyla) and Atlantic puffins (Fratercula arctica) in sympatry. To control for a potential distance effect, values were compared to differences among ticks from the same host in nearby monospecific sites. As predicted, there was higher genetic differentiation between ticks from different sympatric host species than between ticks from nearby allopatric populations of the same host species. Patterns suggesting isolation by distance were found among tick populations of each host group, but no such patterns existed between tick populations of different hosts. Overall, results suggest that host-related selection pressures have led to the specialization of I. uriae and that host race formation may be an important diversifying mechanism in parasites.