Cardamine pratensis early/late ecotype transects Dibbinsdale Nature Reserve 2012-2014

Phenological escape, whereby species alter the timing of life-history
events to avoid seasonal antagonists, is usually analyzed either as a
potential evolutionary outcome given current selection coefficients or as
a realized outcome in response to known enemies. We here gain mechanistic
insights into the evolutionary trajectory of phenological escape in the
brassicaceous herb Cardamine pratensis, by comparing the flowering
schedules of two sympatric ecotypes in different stages of a disruptive
response to egg-laying pressure imposed by the pierid butterfly
Anthocharis cardamines, whose larvae are pre-dispersal seed predators
(reducing realized fecundity by ~70%). When the focal point of highest
intensity selection (peak egg-laying) occurs early in the flowering
schedule, selection for late flowering dependent on reduced egg-laying
combined with selection for early flowering dependent on reduced predator
survival results in a symmetrical bimodal flowering curve; when the focal
point occurs late, an asymmetrical flowering curve results with a large
early flowering mode due to selection for reduced egg-laying augmented by
selection for infested plants to outrun larval development and dehisce
before seed-pod consumption. Unequal selection pressures on high and low
fecundity ramets, due to asynchronous flowering and morphologically
targeted (size-dependent) egg-laying, constrain phenological escape, with
bimodal flowering evolving primarily in response to disruptive selection
on high fecundity phenotypes. These results emphasize the importance of
analyzing variation in selection coefficients among morphological
phenotypes over the entire flowering schedule to predict how populations
will evolve in response to altered phenologies resulting from climate
change.