Parapatric evolution at the population-species interface and the adaptive significance of genetic clines

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Parapatric diversification assumes primary, ecological adaptation along an environmental gradient in a continuous landscape. The process results in genetic clines where the width of each cline is determined by a balance between selection and gene flow. In the face of homogenising gene flow, parapatric diversification leading to speciation is controversial but must include secondary selection involving positive assortative mating. The interaction between primary and secondary selection should therefore determine the fate of specific divergence. The present study combines field data, laboratory experiments and genetic analyses in an integrative approach to study the evolution and adaptive significance of genetic clines between two contiguous populations of the tephritid fly Urophora cardui. The study addresses where the two populations are found in the continuum between population and species, and it analyses the relative importance of primary and secondary selection with gene flow in the parapatric diversification process. The study introduces genotyping-by-sequencing to test genetic divergence across the genome, quantifying the distribution of outlier loci. Outlier loci are generally interpreted as being affected by divergent selection and/or causing reproductive isolation but this has rarely been directly tested, which is a major aim of the present study.