Nuclear DNA content variation over orders of magnitude across species has been attributed to ‘junk’ repetitive DNA with limited adaptive significance. By contrast, our previous work on Silene latifolia showed that DNA content is negatively correlated with flower size, a character of clear adaptive relevance. The present paper explores this relationship in a broader phylogenetic context to investigate the long–term evolutionary impacts of DNA content variation. The relationship between nuclear DNA content and phenotype variation was determined for four closely related species of Silene section Elisanthe (caryophyllaceae). In addition to a consistent sexual dimorphism in DNA content across all of the species, we found DNA content variation among populations within, as well as among, species. We also found a general trend towards a negative correlation between DNA content and flower and leaf size over all four species, within males and females as well as overall. These results indicate that repetitive DNA may play a role in long–term phenotypic evolution.