ON THE EVOLUTIONARY ORIGINS OF THE CACTI

Understanding evolutionary responses of plants to desert environments depends upon phylogenetic knowledge of desert plants. The diverse Amer ican desert family Cactaceae has been presumed, on the basis of distin ctiveness, to be phylogenetically isolated and relatively ancient (> 6 5 million years old). Using maximum likelihood and parsimony analyses of the rapidly evolving internal transcribed spacer (ITS) sequences of nuclear ribosomal DNA (nrDNA), we show that the cacti are phylogeneti cally nested among other aridity-adapted lineages of the angiosperm fa mily Portulacaceae. The ITS divergence between pereskioid cacti and th e genus Talinum (Portulacaceae) is less than that between many Portula caceae genera. Synthesis of the ITS data with morphological and chloro plast DNA evidence suggests an origin of cacti in mid-Tertiary, c. 30 million years ago, and a later Tertiary diversification coincident wit h development of the American desert. This, in turn, implies that the diversification rate in cacti was much higher than in their nearest re latives. The present results illustrate the central role of phylogenet ic reconstruction in ecological and evolutionary theory.