Adaptive variation of plant species is best evaluated under environmental gradients. Silybum marianum is a native to the Mediterranean basin, distributed continuously along an aridity gradient from northern Israel to the edge of the Negev desert. To elucidate the adaptive significance of traits associated with proximity to the desert and with increasing levels of aridity, we compared northern populations from the mesic Mediterranean end of the aridity gradient with southern, adjacent to the Negev desert populations, from the arid end. The F1 self-progeny of all populations were evaluated under open field conditions. Plants originated from southern populations grew taller and narrower, completed their life cycle earlier, and produced more abundant, smaller achenes, with a higher content of polyphenols, which grew into smaller seedlings. Correlative analysis revealed a latitudinal cline towards the desert, of a longer life cycle, and fewer, heavier, better germinating achenes, which grew into larger seedlings. We concluded that the proximity to the desert was reflected in the appearance of genotypes with improved chances of survival under arid conditions and with higher contents of polyphenols.