Phylogenetics, the study of the evolutionary history of a group of organisms, provides another perspective on genetic diversity, one that I think is particularly relevant to Echinacea. Given the degree of morphological similarity between the species, the relatively young evolutionary age of this genus—as compared to its "cousin" genus Helianthus at 30 to 60 million years (Rieseberg, 2001)—the hybridization between extant populations and its possible role in speciation, what information can be gleaned from the study of molecular variation in an evolutionary context?
A primary goal of molecular systematics is the accurate reconstruction of phylogenetic relationships. Several systematic studies of the coneflower genera have included a limited sample of Echinacea species. Both chloroplast restriction site variation (Urbatsch and Jansen, 1995) and sequence divergence of the nuclear ITS-1, ITS-2, and intervening 5.8S regions (Urbatsch et al., 2000) have revealed relatively low divergence among a number of Echinacea species. These studies were not intended to fully explore Echinacea molecular systematics, but give clues to the challenges ahead. Low molecular variation is the bane of the molecular systematist, but low divergence at the molecular level can also indicate recent divergence of taxa given the appropriate analytical context.
Phylogeography utilizes the genealogy of alleles and the geographic patterns of their lineages to identify evolutionary processes underlying the distribution of species (Avise, 2000). It is an analytical tool that has been widely utilized in animal systems and is gaining ground in plant evolutionary and biogeographic studies (Schaal et al., 1998). The power of phylogeography lies in its link to both micro- and macroevolutionary disciplines; common ancestry, patterns of divergence and genetic exchange, when mapped on contemporary distributions, can illuminate the biogeogra-phy of species (Avise, 2000). I have initiated a broad phylogeographic study of Echinacea using nucleotide sequence variation in the chloroplast genome. Preliminary data indicate a shallow divergence among the taxa, consistent with the chloroplast restriction site data of Urbatsch and Jansen (1995), and a broad bipartite geographical partitioning of gene lineages on either side of the Ozark Mountains that is independent of species identity. The pattern is suggestive of a rapid range expansion (Avise, 2000) that might be consistent with the spread of prairie forbs during the Hypsithermal Interval of the Holocene (Baskin et al., 1997). I must emphasize that these are preliminary data; the full analysis will be published elsewhere. Additionally, the Ozarks have been earmarked as the center of diversity and possible origin of Echinacea as based on morphological variation (Baskin et al., 1997; Binns et al., 2002; McGregor, 1997; McKeown, 1999); the phylo-geographic data have potential to support this common observation. Upon completion, the genuslevel phylogeography will hopefully contribute to the broad evolutionary story of Echinacea in North America.
Was this article helpful?