Data from: Two decades of evolutionary changes in Brassica rapa in response to fluctuations in precipitation and severe drought

As climate changes at unprecedented rates, understanding population
responses is a major challenge. Resurrection studies can provide crucial
insights into the contemporary evolution of species to climate change. We
used a seed collection of two Californian populations of the annual plant
Brassica rapa made over two decades of dramatic precipitation
fluctuations, including increasingly severe droughts. We compared
flowering phenology, other drought response traits, and seed production
among four generations, grown under drought and control conditions, to
test for evolutionary change and to characterize the strength and
direction of selection. Post-drought generations flowered earlier, with a
reduced stem diameter, and lower water-use efficiency (WUE), while
intervening wet seasons reversed these adaptations. There was selection
for earlier flowering, which was adaptive, but delayed flowering after wet
years resulted in reduced total seed mass, indicating a maladaptive
response caused by brief wet periods. Furthermore, evolutionary changes
and plastic responses often differed in magnitude between populations and
drought periods, suggesting independent adaptive pathways. While B. rapa
rapidly evolved a drought escape strategy, plant fitness was reduced in
contemporary generations, suggesting that rapid shifts in flowering time
may no longer keep up with the increasing severity of drought periods,
especially when drought adaptation is slowed by occasional wet seasons.