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Hershkovitz, M. A. & Zimmer, E. A.: On
the evolutionary origins of the cacti. - Taxon 46: 217-232. 1997.
- ISSN 0040-0262.
Understanding evolutionary responses of plants to desert
environments depends upon phylogenetic knowledge of desert
plants. The diverse American desert family Cactaceae has
been presumed, on the basis of distinctiveness, to be
phylogenetically isolated and relatively ancient (> 65 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
phylogenetically nested among other aridity-adapted lineages of
the angiosperm family Portulacaceae. The ITS divergence
between pereskioid cacti and the genus Talinum
(Portulacaceae) is less than that between many Portulacaceae
genera. Synthesis of the ITS data with morphological and
chloroplast DNA evidence suggests an origin of cacti in
mid-Tertiary, c. 30 million years ago, and a later Tertiary
diversification coincident with development of the American
desert. This, in turn, implies that the diversification rate in
cacti was much higher than in their nearest relatives. The
present results illustrate the central role of phylogenetic
reconstruction in ecological and evolutionary theory.
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Effective publication date: 15 May 1997