| Species ranges often span large geographic regions covering a variety of different environments. Studies have shown that populations within the range of a species have adapted to (i.e. evolve in) each environment in order to survive and reproduce there. But why have populations been able to successfully adapt to environments within the current range, but not to environments at the edge of the range and beyond? To address this question, I studied range limitation in Cocklebur, a native North American annual weed, along its northern border in Michigan. I identified range-limiting traits—traits that would need to evolve for range expansion to occur—and investigated why these range-limiting traits were not evolving.; I used two methods to identify range-limiting traits. First, I planted seedlings from source populations within the current range into gardens corresponding to each collection location to identify traits that had already evolved within this range. Then I established a garden beyond the current species' border and manipulated traits that had evolved to determine whether further evolution of these traits would be sufficient to allow plants to survive and reproduce beyond the range. I found that flowering time was a range-limiting trait. It had evolved within the current range and manipulating plants to flower earlier allowed them to successfully reproduce beyond the range.; One hypothesis for why evolution of range-limiting traits is not occurring in edge populations is that there is less genetic variation for that trait in edge populations compared to internal populations. Therefore, I measured genetic variation for flowering time in edge and internal populations and found that there was no genetic variation for flowering time in populations from the northern edge of Cocklebur's range. |
