Genetic Variation in Echinacea angustifolia Along a Climatic Gradient

I have long been fascinated by biological examples of clines. These are linear distributions of species in which almost imperceptible variation between neighbouring populations can lead to marked differences between those at the two ends of the range. I first became of aware of genetic clines in certain bird species, but in any organism with a long-range distribution there is the possibility of gradually changing genetic make-up, especially if the distribution lies along an environmental gradient. Just such a situation has been investigated by Still et al., (pp. 467�477). In North America, Echinacea angustifolia occurs from the plains of in the north to in the south. From within that range, the authors sampled ten populations from to , a 1500 km north�south range embodying a very marked temperature gradient. Genetic variation within and between populations was assessed by amplified fragment length polymorphism (AFLP) in which different pairs of primers were used in polymerase chain reactions to amplify a range of DNA fragments within the total genome. A total of 1290 fragments were scored for presence or absence in a number of individuals from each population. The data clearly showed that the smallest genetic distance between any pair of populations was nearly twice the average genetic distance between individuals within a population. When the data were used to construct a phenogram (effectively an intra-specific phylogenetic tree) there was clear evidence of four major groupings of populations that fell along the north�south gradient, correlating with temperature. One key feature of AFLP analysis is that it does not focus on DNA sequences under selective pressure in respect of, in this instance, temperature. It thus gives a good overall picture of microevolution. Accordingly, this paper provides good evidence for restricted gene flow between separated populations along a climatic gradient�a beautiful example of a cline.