5
Adaptive Radiation, Dispersal,
and Diversification of the Hawaiian
Lobeliads
THOMAS J. GIVNISH
Department of Botany, University of Wisconsin, Madison, WI 53706, USA
Abstract
Molecular data provide several key insights into the origin and diversification of
the Hawaiian lobeliads, which comprise one-ninth of the flora of the most isolated
archipelago on earth. Soon after colonizing the Hawaiian chain from elsewhere in
the Southern hemisphere, this group radiated into four major clades, adapted to
bogs and other open, wet high-elevation habitats, to inland outcrops and rock walls,
to sea cliffs and dry forest, and to rain- and cloud-forest edges and interiors. Woodiness and bird pollination arose long before the lobeliads arrived in Hawaii; fleshy
fruits evolved autochthonously in moist to wet forests, and were lost on sea cliffs.
Limited seed dispersal (associated with fleshy fruits in forest-interior plants) appears to have triggered substantial speciation in the genus Cyanea, and helped
generate convergent radiations in flower length, elevational distribution, and plant
height on each of the four major islands. Some of the processes that accelerated
speciation in Cyanea - such ~s limited seed dispersal, narrow ranges, and highly
specialized flowers -- also appear to have increased the I ikelihood of extinction. The
roles of adaptive radiation vis-a-vis limited dispersal and sexual selection in promoting speciation in other groups (African rift-lake cichlids, coral reef fish, lilies
and their relatives) are also discussed, with special reference to the phenomena of
convergent adaptive radiations and of visual selection. Natural enemies may exert
stronger density-dependent mortality on their hosts under rainier, more humid,
warmer, and less seasonal conditions, and help create strong ecological gradients
in plant species diversity within the tropics. Adaptive radiation may thus have interacted synergistically with several processes to generate the taxonomic richness
and ecological diversity of the largest clade in the Hawaiian flora.
Key words. convergent adaptive radiation, molecular systematics, geographic speciation,
Hawaii, insular evolution, Lobeliaceae, fleshy fruits, species richness, ecological determinants of speciation and extinction, rift-lake cichlids, coral-reef fish, tropical forest diversity,
natural enemies, density-dependent mortality, whole-plant compensation point
67
Adaptive Radiation, Dispersal,
and Diversification of the Hawaiian
Lobeliads
THOMAS J. GIVNISH
Department of Botany, University of Wisconsin, Madison, WI 53706, USA
Abstract
Molecular data provide several key insights into the origin and diversification of
the Hawaiian lobeliads, which comprise one-ninth of the flora of the most isolated
archipelago on earth. Soon after colonizing the Hawaiian chain from elsewhere in
the Southern hemisphere, this group radiated into four major clades, adapted to
bogs and other open, wet high-elevation habitats, to inland outcrops and rock walls,
to sea cliffs and dry forest, and to rain- and cloud-forest edges and interiors. Woodiness and bird pollination arose long before the lobeliads arrived in Hawaii; fleshy
fruits evolved autochthonously in moist to wet forests, and were lost on sea cliffs.
Limited seed dispersal (associated with fleshy fruits in forest-interior plants) appears to have triggered substantial speciation in the genus Cyanea, and helped
generate convergent radiations in flower length, elevational distribution, and plant
height on each of the four major islands. Some of the processes that accelerated
speciation in Cyanea - such ~s limited seed dispersal, narrow ranges, and highly
specialized flowers -- also appear to have increased the I ikelihood of extinction. The
roles of adaptive radiation vis-a-vis limited dispersal and sexual selection in promoting speciation in other groups (African rift-lake cichlids, coral reef fish, lilies
and their relatives) are also discussed, with special reference to the phenomena of
convergent adaptive radiations and of visual selection. Natural enemies may exert
stronger density-dependent mortality on their hosts under rainier, more humid,
warmer, and less seasonal conditions, and help create strong ecological gradients
in plant species diversity within the tropics. Adaptive radiation may thus have interacted synergistically with several processes to generate the taxonomic richness
and ecological diversity of the largest clade in the Hawaiian flora.
Key words. convergent adaptive radiation, molecular systematics, geographic speciation,
Hawaii, insular evolution, Lobeliaceae, fleshy fruits, species richness, ecological determinants of speciation and extinction, rift-lake cichlids, coral-reef fish, tropical forest diversity,
natural enemies, density-dependent mortality, whole-plant compensation point
67