The discovery that nuclear DNA content varies widely among species, and even within species, was unexpected because it was thought that the number of genes required for an organism should be common across taxa. We now know that the bulk of nuclear DNA content variation is caused by repetitive DNA sequences characterized according to the nature of repeat (tandem vs dispersed) or chromosomal location/mechanism of replication (pericentromeric, telomeric or subtelomeric, microsatellites, minisatellites, satellites, transposable elements, retroelements). Variation in repetitive DNA, manifested as variation in nuclear DNA content, has been shown to have broad ecological and life-history consequences. For example, large genome size appears to limit fitness in extreme environmental conditions. Within species, variation in DNA content has been coupled to growth and development, such as maturation time in crop species. In Silene latifolia, DNA content is negatively correlated with flower size, a character that, in turn, has well documented ecological significance. These intraspecific studies suggest a connection between repetitive DNA and quantitative genetic determination of continuous characters. Novel insights into mechanisms by which repetitive DNA influences phenotype will lead to models of evolutionary change that extend well beyond the conventional view of evolution by allelic substitution.