How does gene flow limit local adaptation at a species range-edge? An artificial selection Drosophila model

Gene flow has the potential to create species range limits by impeding adaptation to selective pressures at the range-edge, but it is unclear whether there is a threshold level of gene flow that causes this effect. This gene swamping hypothesis was tested using laboratory populations of Drosophila melanogasterunder selection for desiccation resistance, and subject to a gradient of migration from unselected populations. Desiccation tolerance was impeded across the entire migration gradient, and populations receiving intermediate levels of migration exhibited no tolerance for desiccation stress, following twelve selection events. Female, but not male, flies increased desiccation tolerance following selection by reducing water loss rates, but not by carrying more water or becoming more tolerant of dehydration. This pattern is likely due to selection for increased female body size. Thus, intermediate levels of gene flow, in particular, have the potential to establish a species range-limit by confounding the response to selection