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SEE HOW THEY CHANGEóRAPID EVOLUTION IN INVASIVE PLANTS

For some people, evolution conjures up images of fossil records and phylogenies, but the speed with which evolutionary processes can operate is only just beginning to be understood. Invasive species, in particular, are ideal candidates for exhibiting rapid adaptive evolutionary change. In a recent paper in the journal Trends in Plant Science, C·I·B researchers collaborated with Australian partners to review the potential for rapid adaptation in plants.

For an introduced plant species to invade a new region (i.e., establish and proliferate across a range of environmental conditions) two basic options (not mutually exclusive) are available. Either the plant must possess sufficiently high levels of physiological tolerance and plasticity, or it must undergo genetic differentiation to achieve required levels of fitness.

Phenotypic plasticity (the ability of an organism with a given genotype to change its phenotype in response to changes in the environment) is undoubtedly important for many invasive species from many taxonomic groups and in diverse habitats. Many invasive species, however, display evidence of rapid adaptation. Geographical clines in life-history traits have been found in many invasive plants, providing compelling evidence of local adaptation to environmental conditions. An example of a species where this has been well studied is the California poppy (Eschscholzia californica).

By definition, introduced species are present in biogeographic regions where they did not evolve and to which they might be poorly adapted. In such conditions they have a high chance of encountering a suite of novel stresses and selection pressures. Consequently, introduced populations have considerable potential for rapid adaptation, and it has been proposed that this adaptation can explain why some introduced species establish, proliferate, and become invasive in new environments.

Recent genetic studies show that adaptation to new environments can occur within 20 generations or less. Consequently, evolutionary processes can clearly influence the invasiveness of plants introduced to new environments. However, the source of genetic or epigenetic variation underlying these changes in invasive species has not been studied.

The study examined the potential for rapid evolution in invasive species provided by standing genetic variation compared to that from new mutations, and looked at four types of evolutionary change that that could potentially promote or constrain rapid adaptation during invasion: genetic bottlenecks, polyploidy (genome doubling), admixture and hybridization (evolution via gene flow); and evolution induced by environmental stress. Each of these mechanisms is particularly likely to occur at different points in the invasion process.

Understanding the source of variation that contributes to adaptive evolution in invasive plants is important for predicting future invasion scenarios, identifying candidate genes involved in invasiveness, and, more generally, how populations can evolve rapidly in response to novel and changing environments. Research is underway to elucidate these issues for important invasive species in South Africa. This research has much potential for explaining the differential success among groups of introduced plants and will contribute to improved management strategies.

Prentis, P.J., Wilson, J.R.U., Dormontt, E.E., Richardson, D.M. & Lowe, A.J. 2008. Adaptive evolution in invasive species. Trends in Plant Science 13, 288-294.

Fig. 1. Adaptive evolution during the stages of invasion. Key stages in plant invasions can be depicted with reference to a series of barriers that the species must negotiate (top panel), or with regard to changes in range size over time (centre panel). Barriers to invasion predispose plants to certain types of evolutionary change at different points in the invasion process (bottom panel). Links between these different ways of portraying invasions are poorly understood, and elucidation of these dynamics is crucial for advancing our understanding of the role of adaptive evolution in biological invasions. Whereas rapid adaptation from standing genetic variation is likely at all stages of invasion, other evolutionary processes are important at particular stages. Bottlenecks are likely to be particularly prevalent during the introduction and establishment stages because of founder effects and population isolation after dispersal involving long-distance dispersal.

Fig 2. California poppies (Eschscholzia californica; shown here in Central Chile). Genetic shifts in traits have occurred in invasive populations of this species in Chile, with invasive individuals from Chile being better at maximizing growth and reproduction in open environments than those from the native range in California. (Photo: Dave Richardson)