Radiation of Plants Adaptive Radiation

Radiation of Plants

Adaptive Radiation in the Genus Aeonium

Tropical islands are frequently distinct habitats for the expansive growth and proliferation of a diversity of flora and fauna. As the article by Jorgensen & Olesen (2001) indicates, much of this is generally attributed to the phenomenon of adaptive radiation whereby species derived from a common genetic strand take on differing morphological and phenotypic characteristic according to the survival needs precipitated by the specifics of their respective environmental surroundings. However, Jorgensen & Olesen suggest that this assessment is based largely on indirect evidence with more direct evidence of the phenomenon still somewhat scarce. Engaging an examination of certain species of flora found in the Hawaiian and Canary Islands, the article considered here points out that in light of the largely indirect nature of existing evidence, it may be reasonable to deduce that some level of hybridization is also responsible for the speciation of different plants derived from a common lineage.

The study centers particularly on the genus Aeonium, which the researchers identify as the largest plant radiation found in the Canary Island system. (p. 29) According to Jorgensen & Olesen, there is a particular interest in the way that such a genus might proliferate and differentiate within the confines of a contained expanse such as an island. Accordingly, the researchers note that "within these restricted areas, numerous plant genera contain a considerable number of endemic species. Such island radiations are of particular interest because, unlike many continental plant groups, they have radiated within a very restricted geographic area and their relatively recent rapid radiation can be demonstrated through the geological history of the islands." (p. 29)

The article goes on to observe that certain common traits have proliferated through the Aeonium species and provide us with a basis for observing their shared evolutionary origins. Among these traits, the article refers to woodliness, which is a feature specific to the island contexts studied. Much of the vegetation also carries the characteristic of being monocarpic, which means that the species is unbranched. As the article indicates, this is a feature which has been altered at least four different times since the Aeonium first arrived in the island regions examined. Still, it retains the basic quality of monocarpy, which the article indicates is most likely an attribute indicative of its short lifespan and high reproductive capacity. This helps the species to grow upon the barren landscapes of lava fields and other such geologically unique expanses. Additionally, it helps to reinforce, although still somewhat indirectly, the notion that plant species are prone to this sort of adaptive radiation with greater evidentiary likelihood in tropical island contexts. Other traits that the study assess in order to make these observations include herbivore defences and sexual dimorphism, both of which relate directly to the evolutionary viability and reproductive character of the genus. The variations by which these traits occur further suggests that the Aeonium genus has achieved its proliferation and variation by way of an adaptive radiation with specific relevance to the simultaneous diversity and quality of containment which define the island chains here considered.