Local adaptation in shell shape traits of a brooding chiton with strong population genomic differentiation

Comparing divergence in quantitative traits and neutral molecular markers, such as QST–FST comparisons, provides a means to distinguish between natural selection and genetic drift as causes of population differentiation in complex polygenic traits. Onithochiton neglectus (Rochebrune, 1881) is a morphologically variable chiton endemic to New Zealand, with populations distributed over a broad latitudinal environmental gradient. In this species, the morphological variants cluster into two geographically separated shell shape groups, and the phenotypic variation in shell shape has been hypothesised to be adaptive. Here, we assessed this hypothesis by comparing neutral genomic differentiation between populations (FST) with an index of phenotypic differentiation (PST). We used 7,562 putatively neutral single nucleotide polymorphisms (SNPs) across 15 populations and three clades of O. neglectus throughout New Zealand to infer FST. PST was calculated from 18 shell shape traits and gave highly v...