The basic floral ground plan is remarkably constant across Brassicaceae. Ho
wever, within Lepidium (ca. 175 species), deviations from this ground plan
are common, with over half of Lepidium species having only two stamens rath
er than the usual six and a further eighth of the species having only four
stamens. Furthermore, petals are reduced in size in a majority of Lepidium
species. In order to determine the frequency and direction of changes in fl
oral structure within Lepidium, we have inferred the phylogeny within the g
enus from sequences of the internal transcribed spacers of the nuclear ribo
somal DNA. On the basis of this inferred phylogeny, we conclude that floral
structure within Lepidium is relatively fluid. In order to account for the
phylogenetic distributions of the different floral ground plans, at least
two independent reductions to the two-stamen condition and at least one rev
ersal to flowers with increased organ numbers are likely to have occurred.
To account for the frequency of morphological evolution observed within the
genus, we propose that some clades within Lepidium may be predisposed to c
hanges in floral structure, in addition, several transoceanic dispersals ar
e needed to explain the geographic distributions of the clades inferred fro
m the phylogeny.