UBC Theses and Dissertations
An examination of the evolutionary basis of range limitation Slooten, Greta
The thesis objective - to test the importance of gene flow in regulating species range expansion - is accomplished through two studies. The first compares the predicted and actual density distributions of two species, Cakile maritima, an outcrosser, and C. edentula, an inbreeder. Both are coastal, succulent, annual Brassicaceae whose distributions cover the Pacific coast of North America. The second study examines variation in seed oil content along this latitudinal gradient. Lipids are the primary reserve material in most seeds and are composed of triacylglycerides containing various proportions of saturated and unsaturated fatty acids. Previous work has documented variation in seed oil responding to germination temperature. These changes are presumed to represent a response to selection for optimal germination at different temperatures. It has been shown that plants occurring at higher latitudes have higher percentages of unsaturated fatty acids allowing seed oils with lower melting points to reach germination temperatures faster. Plants at lower latitudes have lower percentages of unsaturated fatty acids as they provide less energy to the plant embryo (Linder, 2000). The theory above predicts that the genes of the lower latitude plants, will overwhelm the adaptive response of border populations to the cold climate in the north, forcing these populations away from their optimum; thus, limiting adaptation and causing range limitation. However, results provide no evidence of an effect of gene flow on density distribution or seed oil adaptation at the periphery in either species. Many other factors are known to affect adaptation at the border of these species ranges and further work investigating the effect of gene flow in natural populations is needed to clarify the contribution, if any, of gene flow on range limitation.
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