The impact of molecular systematics, especially DNA sequences, on plant taxonomy and biosystematics has been fundamental and era-splitting. It has led to reactions by researchers ranging from advocating that it is now the only kind of research worth pursuing to considering molecular data as merely another body of evidence to be utilised equally with all the others. Can DNA "lie"? What reasons might there be to question the veracity of what it appears to tell us about plant evolution? This paper is an attempted discussion on the extent to which we can afford to rely solely upon DNA sequences to unravel the evolution of and relationships between plants, and on the principles underlying our utilisation of DNA data in making taxonomic decisions. The main topics visited are: interpretation of chloroplast DNA and rDNA ITS sequences in polyploids in the light of maternal inheritance and concerted evolution; importance of "traditional" characters such as crossability and chromosomal homology as revealed by DNA evidence; desirability of insisting on a monophyletic classification; effects of sample-size on phylogenetic analyses; and the suitability for evolutionary studies of a molecule composed of only four different elements in terms of likely levels of homoplasy. Special reference is made to recent phylogenetic analyses of festucoid grasses, especially the genus Vulpia, where it is proposed that the obvious "misplacement" of certain polyploid species can actually be used to infer their parentage.
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