Dodecatheon (Primulaceae) provides an ideal study system for investigating the influence of climate change on the diversity and distribution of plants. One section in the Genus has a complex taxonomic history, but currently recognized species fall into two categories relevant to climate. Four rare species that are endemic to patchy, cool habitats are considered glacial relicts. The remaining two species are widespread, polymorphic and grow in habitats with contrasting microclimates. The characteristics of the first category suggest that among these species, climate niche evolution is conservative: traits that confer fitness to local climate evolve slowly and populations respond to rapid climate change by migrating. The second category suggests climate niche lability: species adapt quickly to local climate. As such, this taxonomy suggests one transition in the rate of adaptation to climate in the group.
However, complex evolutionary relationships among taxa raise the possibility that rare taxa are really convergent ecotypes of widespread species. This is especially apparent in the eastern United States where widespread and rare species sometimes segregate along local microclimate gradients. For instance, in the unglaciated Midwest, populations of the rare French’s Shooting Star (Dodecatheon frenchii) grow in sheltered sandstone rockhouses while populations of the widespread Mead’s Shooting Star (Dodecatheon meadia) often inhabit the margins of exposed cliff-top glades directly above. When grown in a common garden, an adaptive trait which distinguishes these species in nature (leaf thickness) does not change, suggesting it is genetically controlled rather than environmentally induced. Also, cytological data indicate that D. meadia has twice as many chromosomes as D. frenchii, and ploidy differences often prevent hybridization.
Together, the results of the common garden experiment and the cytological data, support the designation of both taxa as species under most species concepts, and suggest that they are adapted to the microclimates in their respective habitats. However, two alternative scenarios that differ with respect to the tempo and mode of adaptation to climate can account for their parapatric distribution and the evolution of their traits.
The first scenario is consistent with climate niche conservativism—that traits change more slowly than recent climate change. In this region, rockhouse habitats often harbor disjunct southern populations of northern species of clubmosses and ferns, while glades harbor eastern populations of cactus and rock pink. The microclimate contrast and community associations suggest that the current parapatric distribution of these closely related Dodecatheon has resulted from secondary contact following climate-forced migration.
The second scenario is consistent with climate niche lability—that traits change more quickly than climate changes. Changes in chromosome number (polyploidy) have been associated with rapid evolution of traits, including those which distinguish these species. Depending on how often plants with chromosome numbers that differ from their parents successfully establish, and in which direction chromosome counts change, D. meadia may have evolved as tetraploid from diploid D. frenchii or D. frenchii may have evolved from a recently autotetraploid D. meadia. Despite the fact that an evolutionary halving of chromosome number, or polyhaploidy, is considered extremely unlikely, several lines of evidence suggest that may be occurring in Eastern Dodecatheon. Experimentally induced polyhaploids often exhibit disrupted meiosis, thin leaves and poor competitive ability. All of these characteristics distinguish D. frenchii from D. meadia. Moreover, repeated polyhaploidy can account for the disjunct distribution of D. frenchii and its restriction to marginal habitat.
My dissertation research uses phylogenetic and phylogeographic approaches to identify the genetic signature associated with the alternative responses to climate change in this Genus. This research will clarify the complicated taxonomy of the group and suggest how it may respond to predicted global warming. These outcomes have applied relevance for the 6 currently recognized rare taxa that have conservation status in 23 US States and 2 Canadian Provinces.
This research has been funded by the National Science Foundation and Sigma Xi.
