Data from: Drift, not selection, shapes toll-like receptor variation among oceanic island populations

Understanding the relative role of different evolutionary forces in
shaping the level and distribution of functional genetic diversity among
natural populations is a key issue in evolutionary and conservation
biology. To do so accurately genetic data must be analyzed in conjunction
with an unambiguous understanding of the historical processes that have
acted upon the populations. Here we focused on diversity at toll-like
receptor (TLR) loci, which play a key role in the vertebrate innate immune
system and, therefore, are expected to be under pathogen-mediated
selection. We assessed TLR variation within and among 13 island
populations (grouped into three archipelagos) of Berthelot's pipit,
Anthus berthelotii, for which detailed population history has previously
been ascertained. We also compared the variation observed with that found
in its widespread sister species, the tawny pipit, Anthus campestris. We
found strong evidence for positive selection at specific codons in TLR1LA,
TLR3 and TLR4. Despite this, we found that at the allele frequency level,
demographic history has played the major role in shaping patterns of TLR
variation in Berthelot's pipit. Levels of diversity and
differentiation within and across archipelagos at all TLR loci
corresponded very closely with neutral microsatellite variation, and with
the severity of the bottlenecks that occurred during colonization. Our
study shows that despite the importance of TLRs in combating pathogens,
demography can be the main driver of immune gene variation within and
across populations, resulting in patterns of functional variation that can
persist over evolutionary timescales.