76
T.J. Givnish
stricted to single islands (91% vs. 59%). Ecologically, Cyanea differs from
Clermontia in being native to forest interiors rather than forest edges, having smaller
fruits, and being generally unbranched. Givnish et al. (1995) and Givnish (1998,
1999) argued that fleshy fruits dispersed by forest-interior birds - which are notoriously loathe to cross water- and habitat-barriers elsewhere in the tropics - should
be associated with relatively poor dispersal ability. Low dispersal rates should tend
to increase genetic divergence between populations, accelerate speciation, and result in most taxa having narrow ecological and geographic ranges. Fleshy fruits
dispersed by forest-edge or scrub birds - whose dependence on successional or
disturbed vegetation would require frequent movement - should have greater dispersal ability, resulting in lower rates of genetic differentiation and speciation, and
broader ranges. Finally, groups with dust-like, wind-dispersed seeds that inhabit
open, windswept, high-elevation habitats are likely to disperse over large distances,
at least within the Hawaiian archipelago. High dispersal rates would tend to minimize rates of genetic divergence and speciation.
Indeed, high-elevation lobeliad lineages with dust-like seeds have 4.8 ± 3.0
species each; Clermontia and Delissea, with fleshy fruits in partly closed habitats,
have 15.5 ± 9.2 species each; and Cyanea, with fleshy fruits in mainly closed forests, has 65 species. Within Cyanea itself, the expected pattern in species numbers
also seems to hold. The orange-fruited clade (found almost exclusively in closed
forest interiors) has 54 species, while the purple-fruited clade [found mainly in
forest edges and in more open mesic forests (Givnish et al. 1995)] has only 12
species, despite both groups showing comparable ranges in both flower length and
plant height. Thus, the invasion of moist forest interiors appears to have triggered
substantially higher rates of speciation, arguably by favoring the evolution of fleshy
fruits, decreased dispersability due to reliance on sedentary forest-interior birds,
and increased rates of genetic differentiation within species, and leading to increased rates of geographic speciation and narrow species distributions.
2.4 Limited Dispersal and Repeated Small-Scale
Radiations
Limited dispersal can trigger extensive speciation and narrow endemism without
adaptive differentiation (Givnish 1997), as has apparently happened in many groups
of relatively sedentary land snails, fossorial rodents, and marine invertebrates with
poorly dispersing larvae (e.g., Gittenberger 1991; Cameron et al. 1996; Poulin and
Feral 1996; Cook and Lessa 1998). Moreover, limited dispersal can also act synergistically with selection for ecological divergence to produce multiple, parallel adaptive radiations.
Cyanea provides several apparent examples of this phenomenon. On each of
the four major Hawaiian Islands, species have evolved roughly the same range of
corolla lengths (Givnish et al. 1995): 17 to 65 mm on Kaua'i, 24 to 75 mm on
O'ahu, 18 to 75 mm on Maui, and 22 to 75 mm on Hawai'i (calculations based on
assigning each species a corolla length midway between the upper and lower limits
T.J. Givnish
stricted to single islands (91% vs. 59%). Ecologically, Cyanea differs from
Clermontia in being native to forest interiors rather than forest edges, having smaller
fruits, and being generally unbranched. Givnish et al. (1995) and Givnish (1998,
1999) argued that fleshy fruits dispersed by forest-interior birds - which are notoriously loathe to cross water- and habitat-barriers elsewhere in the tropics - should
be associated with relatively poor dispersal ability. Low dispersal rates should tend
to increase genetic divergence between populations, accelerate speciation, and result in most taxa having narrow ecological and geographic ranges. Fleshy fruits
dispersed by forest-edge or scrub birds - whose dependence on successional or
disturbed vegetation would require frequent movement - should have greater dispersal ability, resulting in lower rates of genetic differentiation and speciation, and
broader ranges. Finally, groups with dust-like, wind-dispersed seeds that inhabit
open, windswept, high-elevation habitats are likely to disperse over large distances,
at least within the Hawaiian archipelago. High dispersal rates would tend to minimize rates of genetic divergence and speciation.
Indeed, high-elevation lobeliad lineages with dust-like seeds have 4.8 ± 3.0
species each; Clermontia and Delissea, with fleshy fruits in partly closed habitats,
have 15.5 ± 9.2 species each; and Cyanea, with fleshy fruits in mainly closed forests, has 65 species. Within Cyanea itself, the expected pattern in species numbers
also seems to hold. The orange-fruited clade (found almost exclusively in closed
forest interiors) has 54 species, while the purple-fruited clade [found mainly in
forest edges and in more open mesic forests (Givnish et al. 1995)] has only 12
species, despite both groups showing comparable ranges in both flower length and
plant height. Thus, the invasion of moist forest interiors appears to have triggered
substantially higher rates of speciation, arguably by favoring the evolution of fleshy
fruits, decreased dispersability due to reliance on sedentary forest-interior birds,
and increased rates of genetic differentiation within species, and leading to increased rates of geographic speciation and narrow species distributions.
2.4 Limited Dispersal and Repeated Small-Scale
Radiations
Limited dispersal can trigger extensive speciation and narrow endemism without
adaptive differentiation (Givnish 1997), as has apparently happened in many groups
of relatively sedentary land snails, fossorial rodents, and marine invertebrates with
poorly dispersing larvae (e.g., Gittenberger 1991; Cameron et al. 1996; Poulin and
Feral 1996; Cook and Lessa 1998). Moreover, limited dispersal can also act synergistically with selection for ecological divergence to produce multiple, parallel adaptive radiations.
Cyanea provides several apparent examples of this phenomenon. On each of
the four major Hawaiian Islands, species have evolved roughly the same range of
corolla lengths (Givnish et al. 1995): 17 to 65 mm on Kaua'i, 24 to 75 mm on
O'ahu, 18 to 75 mm on Maui, and 22 to 75 mm on Hawai'i (calculations based on
assigning each species a corolla length midway between the upper and lower limits