Changes in phenology can alter patterns of natural selection: the joint evolution of germination time and post-germination traits.

The timing of a developmental transition (phenology) can influence the environment experienced by subsequent life stages. When phenology causes an organism to occupy a particular habitat as a consequence of the developmental cues used, it can act as a form of habitat tracking. Evolutionary theory predicts that habitat tracking can alter the strength, direction, and mode of natural selection on subsequently expressed traits. To test if germination phenology altered natural selection on post-germination traits, we manipulated germination time by planting seedlings in seven germination cohorts spanning two years. We measured selection on post-germination traits relating to drought-, freezing- and heat-tolerance using a diverse combination of Arabidopsis thaliana mutants and naturally occurring ecotypes. Germination cohorts experienced variable selection: when dry, cold, and hot environments were experienced by seedlings, selection was intensified for drought-, freezing- and heat-tolerance, respectively. Reciprocally, post-germination traits modified the optimal germination time; genotypes had maximum fitness after germinating in environments that matched their physiological tolerances. Our results support the theoretical predictions of feedbacks between habitat tracking and traits expressed after habitat selection. In natural populations, whether phenological shifts alter selection on subsequently expressed traits will depend on the effectiveness of habitat tracking through phenology.