Abstract
A population faced with a new selection pressure can only adapt if appropriate genetic variation is available. This genetic variation might come from new mutations or from gene exchange with other populations or species, or it might already segregate in the population as standing genetic variation (which might itself have arisen from either mutation or gene flow). Understanding the relative importance of these sources of adaptive variation is a fundamental issue in evolutionary genetics (Orr & Betancourt ; Barrett & Schluter ; Gladyshev et al . ) and has practical implications for conservation, plant and animal breeding, biological control and infectious disease prevention (e.g. Robertson ; Soulé & Wilcox ; Prentis et al . ; Pennings ). In this issue of Molecular Ecology , Roesti et al . ( ) make an important contribution to this longstanding debate.