5. Adaptive Radiation
75
be traceable to the small target area of the Hawaiian chain, and the much greater
proximity of the Hawaiian Islands to each other than to other moist, montane areas
that could support lobeliads in the tropics.
Third, the exclusion of Centropogon, Burmeistera, and Pratia as close relatives
of the three Hawaiian genera that share with them fleshy fruits and a woody habit
(Fig. 3B,C) illustrates the danger of assessing phylogeny based on characters that
are subject to strong selection pressures. Fleshy fruits arose at least twice in the
woody lobeliads (in Cyanea-Clermontia-Delissea and Burmeistera-Centropogon),
each time in moist tropical forests. Fleshy fruits were lost once, in Brighamia endemic to dry sea cliffs. Comparative data on the occurrence of different fruit types
along ecological gradients, as well as functional arguments, suggest that fleshy
fruits are adaptive under moist forest conditions (Givnish 1998). The woody habit
evolved long before lobeliads colonized the Pacific, and appears to have first arisen
in moist or wet tropical forests (Fig. 3C). Similarly, bird-pollinated flowers appear
to have evolved long before lobeliads arrived in Hawaii (Fig. 3D), involving hummingbirds in North and South America, sunbirds in Africa and A,>ia, and whiteeyes, honeyeaters, and honeycreepers in the Pacific. The habitats of bird-pollinated
lobcliads are mostly wet and/or cold, in which pollinator thermoregulation would
be favored. Both shifts to hawkmoth pollination (in Brighamia and Isotoma) occurred in dry forest and scrub.
Finally, the habitat occupied by the ancestor of the Hawaiian lobeliads appears
to have been wet subalpine openings or bogs - like those now occupied by Lobelia
section Galeatella and Trematolobelia, and by Lobelia nicotianifolia in southern
Asia. There appears to have been a rapid initial radiation into four main lineages in
the Hawaiian Islands, involving life in bogs and other moist or wet, relatively open
sites (Trematolobelia-Lobelia sect. Galeatella), moist, inland rock walls and crests
(Lobelia sect. Revolutella), dry forests, scrub, and sea cliffs (Brighamia-Delissea)
and moist and wet forest interiors and edges (Cyanea-Clermontia).
Based on the estimated origin of Cyanea at least 8.3 million (M) years ago
(Givnish et al. 1995) and the extent of genetic divergence of Cyanea from the other
Hawaiian lobeliads in this current study, this initial radiation appears to have taken,
at most, a few million years. Adaptive radiation in habitat and general growth-form
thus appears to have occurred soon after the lobeliads arrived in the Hawaiian chain,
involving divergence into four broad adaptive zones. Yet the lobeliads diversified
to become the largest family represented in the endemic flora, with roughly 110
species known historically. What factors drove such extensive speciation long after
the initial radiation in habit and habitat?
2.3 Seed Dispersal and Geographic Speciation
Givnish et al. (1995) argued that limited seed dispersal, associated with the invasion of moist forest interiors, may have been pivotal. More than half the Hawaiian
taxa occur in Cyanea, which differs from its sister genus Clermontia in having
three times as many species (65 vs. 22), a much greater fraction of which are re-
75
be traceable to the small target area of the Hawaiian chain, and the much greater
proximity of the Hawaiian Islands to each other than to other moist, montane areas
that could support lobeliads in the tropics.
Third, the exclusion of Centropogon, Burmeistera, and Pratia as close relatives
of the three Hawaiian genera that share with them fleshy fruits and a woody habit
(Fig. 3B,C) illustrates the danger of assessing phylogeny based on characters that
are subject to strong selection pressures. Fleshy fruits arose at least twice in the
woody lobeliads (in Cyanea-Clermontia-Delissea and Burmeistera-Centropogon),
each time in moist tropical forests. Fleshy fruits were lost once, in Brighamia endemic to dry sea cliffs. Comparative data on the occurrence of different fruit types
along ecological gradients, as well as functional arguments, suggest that fleshy
fruits are adaptive under moist forest conditions (Givnish 1998). The woody habit
evolved long before lobeliads colonized the Pacific, and appears to have first arisen
in moist or wet tropical forests (Fig. 3C). Similarly, bird-pollinated flowers appear
to have evolved long before lobeliads arrived in Hawaii (Fig. 3D), involving hummingbirds in North and South America, sunbirds in Africa and A,>ia, and whiteeyes, honeyeaters, and honeycreepers in the Pacific. The habitats of bird-pollinated
lobcliads are mostly wet and/or cold, in which pollinator thermoregulation would
be favored. Both shifts to hawkmoth pollination (in Brighamia and Isotoma) occurred in dry forest and scrub.
Finally, the habitat occupied by the ancestor of the Hawaiian lobeliads appears
to have been wet subalpine openings or bogs - like those now occupied by Lobelia
section Galeatella and Trematolobelia, and by Lobelia nicotianifolia in southern
Asia. There appears to have been a rapid initial radiation into four main lineages in
the Hawaiian Islands, involving life in bogs and other moist or wet, relatively open
sites (Trematolobelia-Lobelia sect. Galeatella), moist, inland rock walls and crests
(Lobelia sect. Revolutella), dry forests, scrub, and sea cliffs (Brighamia-Delissea)
and moist and wet forest interiors and edges (Cyanea-Clermontia).
Based on the estimated origin of Cyanea at least 8.3 million (M) years ago
(Givnish et al. 1995) and the extent of genetic divergence of Cyanea from the other
Hawaiian lobeliads in this current study, this initial radiation appears to have taken,
at most, a few million years. Adaptive radiation in habitat and general growth-form
thus appears to have occurred soon after the lobeliads arrived in the Hawaiian chain,
involving divergence into four broad adaptive zones. Yet the lobeliads diversified
to become the largest family represented in the endemic flora, with roughly 110
species known historically. What factors drove such extensive speciation long after
the initial radiation in habit and habitat?
2.3 Seed Dispersal and Geographic Speciation
Givnish et al. (1995) argued that limited seed dispersal, associated with the invasion of moist forest interiors, may have been pivotal. More than half the Hawaiian
taxa occur in Cyanea, which differs from its sister genus Clermontia in having
three times as many species (65 vs. 22), a much greater fraction of which are re-