Morphological variation within and between taxa of the Santolina rosmarinifolia L. aggregate were studied. This work demonstrates that polyploidy and hybridization may be effective evolutionary mechanisms of speciation, promoting the persistence and survival of new species, and ultimately increasing the diversity of plant species. The patterns of morphological variation of the S. rosmarinifolia aggregate indicate a recent diversification process for these taxa; as a consequence they are poorly differentiated. The intriguing taxonomic complexity of the taxa of the S. rosmarinifolia aggregate can probably be explained to a large degree by recurrent hybridization and subsequent interbreeding of the resulting genotypes, and by the absence of karyotypic divergence and of spatial isolation (except for S. impressa) between diploid taxa. Quantitative and qualitative data support two evolutionary lines that are not yet strongly differentiated in this aggregate. On one hand are the diploid and tetraploid cytotypes of S. pectinata and S. ageratifolia, and on the other the remaining taxa. Two new subspecies, S. rosmarinifolia subsp. castellana and S. pectinata subsp. montiberica, are described from the Iberian Peninsula.