Flower colour polymorphism in plants had been used as a classic model for understanding the importance of neutral processes versus natural selection in population differentiation. Iris lutescens (Iridaceae) is a widespread species in the northern Mediterranean basin, which shows a stable and striking purple–yellow flower colour polymorphism.
Using an extensive sampling over the distribution range of the species, Wang et al. provide evidence that genetic drift contributed to monomorphism in Spain, while gene flow between adjacent populations seems to be an important factor maintaining populations polymorphic in the South of France. Overall, neutral processes contribute to patterns of spatial variation for flower colour in I. lutescens.