Chronic selection for early reproductive phenology in an annual plant across a steep, elevational gradient of growing season length

Colonisation along ubiquitous gradients of growing season length should require adaptation of phenological traits, driven by natural selection. While phenology often varies with season length and genetic differentiation in phenological traits sometimes seems adaptive, few studies test whether natural selection is responsible for these patterns. The annual plant Rhinanthus minor is genetically differentiated for phenology across a 1000-m elevational gradient of growing season length in the Canadian Rocky Mountains. We estimated phenotypic selection on five phenological traits for three generations of naturally occurring individuals at 12 sites (n = 10112), and two generations of genetically and phenotypically more variable transplanted populations at nine of these sites (n = 24611). Selection was weak for most traits, but consistently favoured early flowering across the gradient rather than only under short seasons. There was no evidence that apparent selection favouring early reproduction arose from failure to consider all components of fitness, or variation in other correlated phenological traits. Instead, selection for earlier flowering may be balanced by selection for strong cogradient phenological plasticity which indirectly favours later flowering. However, this probably does not explain the consistency of selection on flowering time across this steep elevational gradient of growing season length.

Methods
Full data collection details are available in the related manuscript. The data herein were distilled from raw repeat visits to individual plants across each growing season.