OOS 38-4
A resurrection experiment reveals rapid evolution in an experimentally introduced population of Brassica rapa
When a founder population is introduced to a novel environment, the mismatch between the traits of the organism and the environmental conditions can result in strong selective pressures and rapid evolution. While there are several examples of evolution in introduced environments, this study is novel in investigating selection and evolution as it occurs. In May 2011, two thousand seeds from a Southern California population of the annual plant Brassica rapa were experimentally introduced to ten plots at Louis Calder Biological Field Station in Armonk, NY. The two sites differ in climate, soils, and other factors. This study examines selection and evolution in the introduced population of B. rapa over three years since introduction. Using a resurrection approach, five hundred seeds from both the original California population (ancestors) and the introduced population (descendants) were grown in a common garden in the introduced environment. A suite of morphological, phenological, and physiological phenotypic traits, as well as fitness proxies, were measured throughout the study.
Results/Conclusions
Directional selection for increased size, earlier flowering time, and longer duration of flowering was observed during the first three years after introduction. The common garden experiment demonstrated a significant difference between the ancestors and descendants in several phenotypic traits, indicating very rapid evolution. The descendant plants were, on average, 3.66 cm shorter, had leaves that were 1.65 cm shorter and 0.84 cm narrower, stems 0.65 cm thinner, 4.76 fewer leaves, and flowered 1.24 days earlier and for 7.51 fewer days. Surprisingly, while the descendant plants were smaller and flowered for a shorter period of time, there was no significant difference in the number of seed pods produced by ancestors and descendants. This suggests that the descendant population was able to maintain fitness while evolving decreased size and rapid reproduction. The direction of morphological evolution was in the opposite direction from observed selection. This indicates important limitations of using selection measured in the field to predict evolution, and demonstrates the utility of the resurrection approach for detecting evolutionary change.