This interesting paper that calls into question the common interpretation of clinal variation as a result of natural selection.

Clinal variation, that is the gradual change in genetic or phenotypic traits exhibited by populations over a geographical area, is often interpreted as the result of natural selection. In this work, we demonstrated that demography can also shape variation genome-wide generating patterns that are similar to the ones we often attributed to natural selection. You can read these results at bioRxiv and you can discuss them at Haldane´s sieve.  

Reference Article: Secondary contact and local adaptation contribute to genome-wide patterns of clinal variation in Drosophila melanogaster. 2014. Alan O. Bergland, Ray Tobler,  Josefa González, Paul Schmidt, Dmitri Petrov. bioRxiv doi: http://dx.doi.org/10.1101/009084