Plant species that expand their geographic range typically adapt to new, local environmental factors via natural selection, becoming genetically and phenotypically differentiated from their source populations.
Latitude and elevation are common environmental gradients present in many species’ ranges. Phenological traits, such as flowering time, which rely on temperature and growing season length as biological cues, are predicted to undergo strong selection at either ends of their latitudinal or elevational range.
Selection on phenological traits should favour early-season emergence and rapid reproduction under shorter growing seasons (higher latitudes and elevations), versus later-season emergence and prolonged reproduction expected under longer growing seasons (lower latitudes and elevation). Few studies have investigated whether selection on phenology acts as expected along growing season gradients in natural habitats. However, previous results from these studies have found that selection consistently favours early flowering regardless of season length.
In a recently published paper, former Queen’s Biology PhD Student David Ensing, former MSc Student Dylan Sora, and Professor Dr. Christopher Eckert estimate phenotypic selection on naturally occurring and transplanted individuals of the montane annual plant, Rhinanthus minor, across a 1000 m elevational gradient of growing season length in the Canadian Rocky Mountains. They quantify phenotypic selection on five phenological traits over three consecutive generations to test the hypothesis that selection on these traits varies across a gradient of growing season length. They also address common limitations and biases that may contribute to findings of consistent selection on flowering time, which are contrary to general expectations.
Ensing et al. find that phenotypic selection does not act on most of the phenological traits measured (e.g., time between first flower and first mature fruit). However, selection favours early flowering time across the elevational gradient of growing season length, consistent with previous results. The authors conclude that general expectations of flowering time differentiation based on growing season length may not apply to elevational gradients, in comparison to latitudinal gradients. Determining how selection on phenology differs across elevational gradients can be used as a model for understanding how the force of natural selection might change with anthropogenic effects on the climate. By addressing common sources of bias associated with studies that find selection for early flowering (such as a lack of multiple generations, and missing phenotypic variation), the authors highlight new avenues for further research on the agents and targets of this apparent selection.
To learn more, read their article in Evolution.
