Promises and limitations of hitchhiking mapping.

Building the connection between genetic and phenotypic variation is an important 'work in progress', and one that will enable proactive diagnosis and treatment in medicine, promote development of environment-targeted varieties in agriculture, and clarify the limits of species adaptation to changing environments in conservation. Quantitative trait loci (QTL) mapping and genome wide association (GWA) studies have recently been allied to an additional focus on 'hitchhiking' (HH) mapping--using changes in allele frequency due to artificial or natural selection. This older technique has been popularized by the falling costs of high throughput sequencing. Initial HH-resequensing experiments seem to have found many thousands of polymorphisms responding to selection. We argue that this interpretation appears too optimistic, and that the data might in fact be more consistent with dozens, rather than thousands, of loci under selection. We propose several developments required for sensible data analyses that will fully realize the great power of the HH technique, and outline ways of moving forward.