GIVNISH, THOMAS J.*, TIMOTHY M. EVANS, KENNETH J. SYTSMA, THOMAS B. PATTERSON, AND MICHELLE L. ZJHRA. Department of Botany, University of Wisconsin, Madison, WI 53706. - Molecular evolution, adaptive radiation, and origin of the amphiatlantic distribution of the family Rapateaceae.
The Rapateaceae (17 genera, ca. 90 species) is largely restricted to
the tepuis and sandplains of the ancient Guayana Shield in South
America, with monotypic Maschalocephalus restricted to a region
of West Africa that abutted the Guayana Shield prior to the opening of
the Atlantic. Members of this family have undergone extensive
radiation in flower form, leaf shape, leaf arrangement, and habitat.
To provide a basis for analyzing patterns of geographical and
morphological evolution in this group, we produced a molecular
phylogeny based on a cladistic analysis of cpDNA sequence variation in
the chloroplast-encoded ndhF gene, using several members of the
Bromeliaceae as outgroups. Our strict consensus tree indicates that
the lowland subfamily Rapateoideae is paraphyletic and includes the
largely montane subfamily Saxofridericioideae as a monophyletic
subclade; the tribe Saxofridericeae of the latter is paraphyletic,
while tribe Schoenocephaliae is monophyletic. Spathanthus is
sister to all other genera of Rapateaceae, with the tribes Rapateae
and Monotremae representing successively divergent lineages within
Rapateoideae. Saxofridericia/Stegolepis/ Epidryos/Marahuacaea,
which have yellow, cup-shaped flowers and bee pollination, are sister
to the tribe Schoenocephalieae, which have reddish, tubular flowers,
the only nectaries in the family, and hummingbird pollination. Branch
lengths are consistent with the operation of a molecular clock;
Maschalocephalus is only slightly divergent from other
Monotremae, suggesting that it is the product of a relatively recent,
long-distance dispersal event, NOT continental drift. The
distributions of present-day rapateads are consistent with the origin
of the family in lowland inundated forests, followed by (i)
colonization of lowland Amazonian savannas, (ii) invasion, frequent
speciation, and evolution of hummingbird pollination in highland
habitats atop tepuis (perhaps following uplift of the latter in
association with the opening of the Atlantic), and (iii) reinvasions
of lowland habitats by members of Saxofridericioideae and
Schoenocephaliae.
Key words: Guayana Shield, hummingbird pollination, Rapateaceae, tepuis