The maintenance of variability in functional genes has attracted much attention from evolutionary biologists, because such a process could help us understand how evolutionary mechanisms, such as natural selection and genetic drift, work in natural populations. Toll-like receptor (TLR) genes are an ancient family of genes responsible for the initial response against pathogens at both innate and acquired immune systems. TLR genes are extremely polymorphic. Yet, we know very little about how such polymorphisms are maintained in a natural population. Particularly, we know very little about how the allele frequency variation in TLR genes in an introduced species compare to an ancestral population, which has been separated from the introduced population over a considerable period of time. Such comparison would be insightful since the immune system of the introduced population would likely face new and different selection pressures to adapt to new environmental conditions, which may potentially be reflected in the TLR diversity. We have been investigating the allele frequencies in several TLRs loci in dunnocks (Prunella modularis) between an ancestral population in United Kingdom and an introduced population (ca. 150 years ago) in New Zealand. We will present our principal results, which have revealed intriguing differences between the two populations