Vol. 4/1, pp. 29–42 Perspectives © Urban & Fischer Verlag, 2001 in Plant Ecology, http://www.urbanfischer.de/journals/ppees Evolution and Systematics Adaptive radiation of island plants: evidence from Aeonium (Crassulaceae) of the Canary Islands Tove H. Jorgensen & Jens M. Olesen Department of Ecology and Genetics, University of Aarhus, Ny Munkegade Block 540, 8000 Aarhus C, Denmark; e-mail: [email protected] Abstract The presence of diverse and species-rich plant lineages on oceanic islands is most often associated with adaptive radiation. Here we discuss the possible adaptive sig- nificance of some of the most prominent traits in island plants, including woodiness, monocarpy and sexual dimorphisms. Indirect evidence that such traits have been ac- quired through convergent evolution on islands comes from molecular phylogenies; however, direct evidence of their selective value rarely is obtained. The importance of hybridization in the evolution of island plants is also considered as part of a more gen- eral discussion of the mechanisms governing radiations on islands. Most examples are from the Hawaiian and Canarian floras, and in particular from studies on the mor- phological, ecological and molecular diversification of the genus Aeonium, the largest plant radiation of the Canarian Islands. Key words: Hawaiian Islands, heterophylly, hybridization, monocarpy, pollen-ovule ratios, sexual dimorphisms Introduction Why are some plant lineages more species- tion occurs at an accelerated rate, as was rich than others? The question arises repeat- also done by Sanderson (1998) and Bateman edly when studying the floras of isolated (1999), though some authors avoid this more oceanic islands. Within these restricted restricted view of radiation (e.g. Barrett & areas, numerous plant genera contain a con- Graham 1997; Givinsh 1997). An adaptive ra- siderable number of endemic species. Such diation is defined as the diversification of a island radiations are of particular interest be- lineage into species that exploit a variety of cause, unlike many continental plant groups, different resource types and that differ in the they have radiated within a very restricted morphological or physiological traits used to geographic area and their relatively recent, exploit those resources (Schluter 1996, but rapid radiation can be demonstrated through see Givnish 1997 for other definitions). It is the geological history of the islands. generally assumed that most cases of island A radiation is defined as a process leading radiation, including the examples presented to character diversification and/or ecological here, are adaptive in this sense. diversification and eventually may lead to lin- Whether plant radiations are a more fre- eage diversification (Sanderson 1998). In this quent phenomenon on islands and island-like paper the focus will be on radiations leading habitats than on continents still needs to be to lineage diversification. We restrict the con- investigated. It is possible, though, to list a cept of radiation to cases in which diversifica- number of deterministic and stochastic forces 1433-8319/01/4/01-29 $ 15.00/0 30 T. H. Jorgensen & J. M. Olesen that generally promote speciation and that position) between lineages. Table 1 lists the may be particularly prominent on oceanic is- genera in the Hawaiian and Canarian lands. Determining the relative importance of archipelagos comprising more than ten en- these forces is a difficult task, especially since demic species. It is apparent that genera they may vary between lineages and islands. within the Asteraceae have been particularly The rich diversity of habitats on many oceanic successful in diverging within both archipela- islands, created primarily by geological and gos, and that the family Campanulaceae has climatic conditions, offers a wide array of eco- several species-rich genera in the Hawaiian logical opportunities to colonizers. In addition, archipelago. This pattern of several indepen- due to the remoteness of some islands, the dent radiations within the same families sug- number of organisms capable of colonizing gests a phylogenetic component to island ra- from the nearest continent is limited, and com- diations. However, it may also reflect factors petition and predation from well-established such as the presence within these families of and specialized plants and animals may be a large and diverse continental species pool low (McArthur & Wilson 1967). Under these or advanced dispersal abilities that ensured circumstances, selection may favour charac- more frequent colonizations of the islands. ters which offer the opportunity to exploit new Soon after establishment on an island the dis- resources and which eliminate competition persal capacity may be lost rapidly (Cody & with close relatives. Sometimes the acquisi- Overton 1996). tion of a particular trait or traits predisposes a Species richness and the presence of mor- taxon to rapid speciation, thus giving access phological and ecological diversity in a lin- to a new range of resources. For example, the eage are often taken as evidence for adaptive evolution of the spur in continental Aquilegia radiation in island plants, in particular where (Ranunculaceae) appears to be such a key in- this is combined with low genetic differentia- novation which led to rapid radiation within the tion (e.g. Baldwin et al. 1990; Okada et al. genus. Other circumstances may also favour 1997). In such rapid, adaptive radiations, neu- radiation: in his flush-crash-founder model tral characters are expected to exhibit little di- Carson (1975) suggests that episodic relax- versity while features related to fitness will ation from selection may cause disruption of vary considerably. Few studies, though, actu- old adaptive gene complexes and allow selec- ally seek to identify the significant traits in tion to operate on new genetic systems. these suggested adaptive radiations. In this Random evolutionary processes are also paper, we assess this problem by discussing invoked to explain island radiations (Crawford the possible adaptive significance of some of et al. 1987; Grant 1998a). With habitat the most prominent traits in island plants. The turnover due to volcanic activity, and the op- importance of hybridization in the evolution of portunity for repeated cycles of inter-island island plants is also considered as part of a dispersal and isolation, population bottle- more general discussion of the mechanisms necks, founder events and geographic sepa- governing radiations on islands. Most exam- ration of conspecifics may have occurred re- ples will be taken from the Hawaiian and Ca- peatedly through the evolution of island floras. narian floras and in particular from our studies These factors will promote evolution by drift; on the morphological, ecological and molecu- this could occur, for example, through the ran- lar diversification of the Canarian genus Aeo- dom fixation of different advantageous genes nium. Our choice of examples reflects both in separate populations experiencing similar the geographic bias in the literature of island selection pressures (Schluter 1996, and listed plant radiations and the fact that the flora of references). these two archipelagos has been very well Not all lineages on an island or archipelago recorded. Both archipelagos are of volcanic are equally successful, even when they have origin but differ in age and degree of remote- the same ecological opportunities and are ex- ness. The Hawaiian Islands are situated posed to the same chance events. This may some 4000 km off the Californian coast and be a simple question of colonization order, or comprise eight major islands varying in age may reflect chance events which lead to ex- from 0.5 to 7.5 Mya (MacDonald et al. 1983; tinction. Variation in the success of lineages Wagner et al. 1990). The seven major islands may also be due to differences in the evolu- of the Canarian archipelago lie some 100 km tionary potential (or genetic variability) and from the African coast and range in age from the phylogenetic constraints (or genetic com- 0.8 to 21 Mya (Carracedo 1978). Adaptive radiation of island plants 31 Table 1. Genera of vascular plants comprising more than ten endemic species in the Hawaiian and Ca- narian archipelagos. Data were obtained from Wagner et al. (1990) (Hawaiian Islands), Hansen & Sunding (1993) and Bramwell & Bramwell (1990) (Canarian Islands) supplemented with the records of newly de- scribed species of Aeonium (Bramwell 1982; Liu 1989; Bañares Baudet 1992, 1999; Hernández & Bañares 1996). Canarian species include those also occurring on other Macaronesian islands. The percentage of di- chotomies that include vegetative, floral or fruit traits are taken from taxonomic keys to the genera as are the percentage of species producing secondary wood. The “numbers of hybrids” describe intergenic hy- brids involving both endemic, indigenous and naturalized parent species. Dichotomies (%) ––––––––––––––––––––– Endemics Indiginous and naturalized Vegetative Floral Fruit spp. (%) Woody Hybrids Hawaiian Islands Cyrtandra (Gesneriaceae) 53 0 36 61 3 100 67 Cyanea (Campanulaceae) 52 0 25 53 6 100 3 Pelea (Rutaceae) 47 0 29 44 27 100 0 Phyllostegia (Lamiaceae) 27 0 42 58 0 37 3 Peperomia (Piperaceae) 23 3 72 25 2 0 0 Bidens (Asteraceae) 19 4 20 50 30 11 11 Chamaesyce (Euphorbiceae) 15 7 57 24 19 73 3 Clermontia (Campanulaceae) 22 0 5 92 3 100 6 Hedyotis (Rubiaceae) 20 2 43 44 13 80 3 Schiedea (Caryophyllaceae) 22 0 41 48 11 91 2 Dubautia