Adaptive Radiation of the Hawaiian Silversword Alliance (Compositae- Madiinae): Ecological, Morphological, and Physiological Diversity

Abstract

Tlif ecolof^ical, morphological, and physiological diversity of species in the Hawaiian silversword alliance is exceptional.The 28 species, which belong to the endemic genera Argyroxipluiutu Duhautia, and Wilkesia.have a wide variety of geographical distributions and elevational ranges within the archipelago.They grow in habitats as varied as dry scrub and woodland, wet scrub and forest, cinder and lava, and bog.Ecological diversity is also evident among sympatric species.At a site of sympatry on the island of Hawaii, for example, i).ciliolata and D. scahra are restricted to diflerent lava flows, even though individuals of the two species may grow within a meter of one another.The 28 species have growth forms as varied as rosette shrubs, shrubs, trees, ami lianas.They have a wide range of leaf sizes and shaj)es, with the Dubautia species exhibiting significant variation in leaf turgor maintenance capacities.Morphological and physiological diversity is also evident among sympatric species.At a site of sympatry on the island of Maui, for example, A.sandwiiense and D. menziesii exhibit different suites of morphological and physiological traits enabling them to cope with the severe environmental conditions.The patterns of diversity and the genomic relationships among the 28 species suggest that a variety of factors may have played important roles in their adaptive radiation.Tlie Hawaiian silversword alliance is a premier versification probably trace to a single colonizing example of adaptive radiation in plants (Carlquist, ancestor (Baldwin et a!., 1988; Carr et al., 1989 Carr et al., ).1980;; Carr ct al., 1989).The alliance includes 28 Our objective in this review is to provide insight species in three endemic genera: Argyroxiphium, into the ecological, morphological, and physiolog- Duhautid, and WUkesia {Carr, 1985).The species ical diversity of species in the silversword alliance, grow in a wide range of habitats and have a wide With respect to ecological diversity, we compare variety of growth forms.They are also closely the geographical distributions, habitats, and elerelated, as evidenced by the high frequency of vational ranges of the 28 species, then analyze the spontaneous interspecific and intergeneric hybrids local distributions of several sympatric Duhaulia in nature, coupled with the ease of production of species.With respect to morphological and physartificial hybrids in the laboratory (Carr & Kyhos, iological diversity, we compare the growth forms, 1981, 198C).The detailed analysis of the hybrids leaf sizes, and leaf shapes of the 28 species, then and parental taxa using cytogenetic, electropho-examine the turgor maintenance capacities of the retic, and molecular approaches has provided com-Dubautia species and the water and temperature pelling evidence that the silversword alliance is a balances of two sympatric /ir^jro-rZ/^/n/i/ri and Aigenclically cohesive group whose origin and di-hautia species.Our primary theme is that the '