Genetic structure and shell shape variation within a rocky shore whelk suggests both diverging and constraining selection with gene flow

Variation in snail shell shape has provided evolutionary biologists with excellent material for the study of local adaptation to local environments. However, assuming shell shape variation is evidence of distinct lineages (species) may have led to taxonomic inflation within some gastropod lineages. Here we compare shell shape variation and genetic structure of two independent lineages of New Zealand rocky shore whelks in order to understand the process that lead to an unusual disjunct distribution. We examined the Buccinulum vittatum complex (three subspecies plus B. colensoi) using mitochondrial DNA sequences, 849 single nucleotide polymorphisms, and geometric morphometric data on shell shape. Specimens within the range of Buccinulum colensoi shared nuclear markers and mtDNA haplotypes with the southern subspecies B. vittatum littorinoides, while the northern samples (B. v. vittatum) had distinct genotypes. We infer that gene flow between B. colensoi and B. v. littorinoides is greater...