Phenotypic changes and reduced genetic diversity have accompanied the rapid decline of the garden tiger moth (Arctia caja) in the U.K.

1. Previous studies have quantified the recent decline of numerous Lepidopteran species in the U.K., including the garden tiger moth (Arctia caja), in C which abundance has decreased by 85% over the past 30 years. At the same time that overall numbers have been falling, the distribution of abundance of this species has been moving northwards. In this study, morphological and genetic data were used to C, investigate the possibility that these changes in abundance and distribution have been accompanied by microevolutionary changes. 2. A comparison of wing size and shape in current and historical moth samples revealed that wing shape has altered significantly over the past century, resulting in longer, narrower hindwings and narrower forewings for a given forewing length. Habitat fragmentation and increased suitability of northerly sites provide a plausible explanation for the selection of increasingly dispersive individuals. 3. Mitochondrial DNA revealed no phylogeographic structuring either before or after the population decline. However, a comparison of mtDNA haplotypes from current and museum specimens indicated that the recent population decline across the U.K. has been accompanied by a significant loss of genetic diversity. 4. The changes in wing shape suggest recent adaptation to environmental change, whereas a loss of genetic diversity may limit the ability of garden tiger moths to adapt to future environmental change.