Ecological genetics of local adaptation: Climatic races in Arabidopsis thaliana and Arabis fecunda

Local adaptation, or the presence of genetically based trait differences among populations that result from natural selection, appears to be quite common in plant species. However, any two independent populations may diverge in phenotypes, even in the absence of differential selective pressures. The degree of population divergence that results from natural selection is therefore of fundamental interest to evolution as well as the applied sub-discipline of conservation genetics. In chapter 1 I review a body of theory which develops an expectation of both marker and trait divergence in the absence of natural selection. Based on this theory and data from 29 species I compare expected divergence at neutral markers and neutral quantitative traits. Results show that quantitative trait divergence among populations is greater than expected from neutral divergence, providing support for the ubiquity of local adaptation. Chapter 2 describes a case study of Arabis fecunda populations at high and low elevation sites that differ in water availability. Common garden screenings show that several specific quantitative traits likely to be important for drought adaptation have diverged among populations, despite a general lack variation or structure at neutral markers.; Ultimately, evolutionary constraints and the rate of adaptive evolution will depend on the genetic basis of adaptive traits. Drought (water availability) is thought to be one of the major climatic factors to which populations locally adapt, and a number of traits have been identified which may contribute to drought adaptation. Understanding the genetic basis of drought tolerance is an important goal in both ecology and agriculture, and provides an ideal framework to understand potential genetic constraints and correlated responses. In chapter 3 I examine natural variation in drought tolerance traits in Arabidopsis thaliana, a model system whose genome has been completely sequenced. A screening of natural accessions reveals a large positive genetic correlation between drought tolerance and time to flowering. I demonstrate that allelic variation at gene for flowering time (Frigida) is also responsible for variation in physiological and drought tolerance traits, suggesting that natural selection has caused the fixation of a pleiotropic gene.