A seven-year study of flower-color polymorphism in a Mediterranean annual plant

Flower-color polymorphisms are well-documented, but how color morph frequencies change over time is usually unknown. We studied this question using the milk thistle, Silybum marianum (Asteraceae). Purple flowers commonly occur in Israel, but an infrequent white morph has been recorded since the 1920s, at first from the south of the country. Over seven years, we recorded the frequencies of the two color morphs in 17 populations along a 156 km south-north transect with a steep rainfall gradient. The proportion of white inflorescences per population consistently decreased from south to north, and their mean proportion decreased from 24% to 12% over the study period. Four of the populations went extinct, but two were recolonized, and two additional populations decreased to