Natural Selection on Phenology Across an Elevational Gradient in Seasonality

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Sora, Dylan
Natural Selection , Phenology , Adaptation , Plant Biology , Trade-Offs
Many populations are locally adapted to their environment, but across an environmental gradient, trade-offs among phenotypic traits may result in a failure to adapt. Species exist across a seasonal gradient, which can create selection for phenology, the timing of key life history events. Flowering phenology is important because flowering late exposes the plants to abiotic risks, like frost damage, whereas flowering early can lower fecundity if there is a trade-off between time to and size at flowering. This trade-off should result in different patterns of selection across a seasonal gradient, with stabilizing selection favouring intermediate flowering in long growing seasons, and selection favouring early flowering in short growing seasons. While this theory is well established, few studies have measured selection on flowering phenology across a seasonal gradient in natural populations. In 2015 and 2016, I measured selection on the time to and size at flowering from 12 populations (24 site × year combinations) of the annual herb Rhinanthus minor (yellow rattle), across an elevational gradient of 900m, in the Rocky Mountains of Alberta, Canada. For each site × year I quantified the growing season length measured using cumulative growing degree days (CGDD). I predicted that 1) CGDD would vary linearly with elevation, 2) There would be selection favouring early flowering in sites with low CGDD, 3) There would be stabilizing selection for intermediate flowering in sites with moderate and high CGDD, 4) There would be a trade-off between time to and size at flowering. Contrary to my predictions I found that 1) CGDD varied quadratically with elevation, as low and mid elevational sites had similar CGDD, 2) Selection favoured early flowering plants across the gradient in season length, 3) There was no stabilizing selection for flowering time, 4) There was no evidence of a trade-off between time to and size at flowering. There was no evidence for a trade-off, therefore earlier flowering plants were able to flower both early and at a larger size, resulting in higher fitness. My results challenge commonly held life history assumptions and demonstrate the importance of foundational research in natural systems.
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