Phylogenetic relationships among species of Besseya (Scrophulariaceae)
were examined using cladistic analysis of morphological characters. A
single most parsimonious tree was discovered on which B. bullii, loca
ted in central North America, was placed as the sister species of the
rest of the genus, making up taxa of the Rocky Mountains and the Pacif
ic Northwest. Among the species in the Rocky Mountain and Pacific Nort
hwest group are B. rubra and B. wyomingensis, both of which have flowe
rs that are characterized by extreme corolla diminution. Besseya rubra
and B. wyomingensis did not form a monophyletic group on the most par
simonious cladogram. Topologies on which B. rubra and B. wyomingensis
were placed as a monophyletic group required at least two character st
ate changes more than the most parsimonious cladogram. Alternative sce
narios for corolla evolution were possible based on the most parsimoni
ous cladogram. One scenario indicated that the extreme corolla diminut
ion characteristic of B. rubra and B. wyomingensis occurred once and t
hat an evolutionary reversal reestablished the full-size corolla of so
uthern Rocky Mountain species. A second scenario, for which the hypoth
esis of an evolutionary reversal was not required to explain the full-
size corolla of the southern Rocky Mountain species, indicated that ex
treme corolla diminution occurred separately in the evolution of both
B. rubra and B. wyomingensis. Sympetaly, generally considered a highly
conserved attribute among angiosperms, was diminished within Besseya
through the loss of a corolla tube in the southern Rocky Mountain spec
ies. In the evolution of Besseya, especially in association with the o
rigin of the monophyletic group consisting of B. oblongifolia, B. plan
taginea, and B. ritteriana, leaf lamina length/width ratios greatly in
creased, lamina bases became less cordate, and basal secondary vein di
ameter/midvein diameter ratios decreased.