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

Abstract

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. Postdrought 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.

Evolutionary and plastic changes in a native annual plant after a historic drought

“…However, only the wettest population (Domino) showed evolution toward earlier flowering, although all three populations showed plasticity toward earlier flowering in response to the experimental drought. These results contrast with those of a resurrection study of Brassica rapa, which demonstrated evolution, but no plasticity in flowering time in response to natural and experimental drought (Franks, 2011; Franks et al, 2007; Hamann et al, 2018). We also observed trade‐offs associated with earlier flowering, but again, these results differed across populations.…”

“…The degree to which an escape versus. avoidance strategy is favored within a population varies with its moisture availability and the time at which drought begins, indicating local adaptation to drought (Edwards et al, 2012; Hamann, Weiss, & Franks, 2018; Heschel & Rignios, 2005; Kooyers, Greenlee, Colicchio, Oh, & Blackman, 2015; Monroe et al, 2018; Sherrard & Maherali, 2006).…”

“…Rapid evolution in response to conditions linked to climate change has been detected for numerous taxa in as few as a couple of generations (reviewed in Parmesan, 2006; Franks, Hamann, & Weis, 2018). One approach used to detect this rapid evolution in plants is the resurrection study, in which seeds collected at one point in time (ancestral) are grown and compared with those collected later (descendant; Etterson et al, 2016; Franks, 2011; Franks et al, 2018; Franks, Sim, & Weis, 2007; Gómez, Méndez‐Vigo, Marcer, Alonso‐Blanco, & Picó, 2018; Hamann et al, 2018; Sultan, Horgan‐Kobelski, Nichols, Riggs, & Waples, 2012; Thomann, Imbert, Engstrand, & Cheptou, 2015; Vigouroux et al, 2011). For plants, this approach is dependent on the collection and storage of seeds over a period in which an environmental change occurs.…”

Environmental variability and the evolution of bet-hedging strategies

“…Most species in nature contend with variable, uncertain conditions in some portion of their geographic range (Vasseur and Yodzis 2004; Coumou and Rahmstorf 2012) posing a challenge to adaptive evolution (Bell 2010). In variable environments, an adaptive response to conditions at a given point in time might be maladaptive on longer timescales, for example, if the direction of selection fluctuates inter-annually (Hamann et al 2018). Although phenotypic plasticity, i.e., the ability of a single genotype to produce different phenotypes in different environments, might evolve in response to frequent and predictable variation in environmental conditions, it may be selected against if fluctuating conditions are unpredictable (Simons 2011), such as when fluctuations are infrequent and extreme (Chevin and Hoffmann 2017) or sample a great range of multidimensional environmental space.…”

A new phenological metric for use in pheno‐climatic models: A case study using herbarium specimens of Streptanthus tortuosus

“…Because the majority of the S. tortuosus records analyzed here were collected from localities that experience a Mediterranean climate, their growth or reproduction in the spring and summer may be strongly influenced by winter water availability. Where winter precipitation is relatively low, soils dry out more quickly during the following spring, and this may select for earlier flowering genotypes or induce earlier flowering as a plastic response (Franks, ; Hamann et al., ). Consequently, our second prediction is that flowering date will be positively correlated with total winter precipitation.…”