INTRODUCTION
Rapid adaptive evolution is considered a potential pathway for species
to cope with ongoing climate change. Principles of rapid evolution have
been studied abundantly by artificial selection under controlled
conditions with short-lived, often single-cell organisms (reviewed in
Barrett & Schluter 2007; Hoffmann & Sgró 2011). However, translating
these findings to real life is difficult:
multiple interacting factors may
substantially impede rapid adaptive evolution under natural conditions,
including e.g. environmental fluctuations, various biotic interactions,
low genetic variation and trait heritability, genetic drift, or
trade-offs between selected traits (Hoffmann & Sgró 2011; Shaw &
Etterson 2012). Thus, there is an urgent need for studying adaptive
evolution within the ‘multivariate space’ of natural conditions
(Hoffmann & Sgró 2011) to assess its relevance for ongoing climate
change (Merilä & Henry 2014; Franks et al. 2014, 2018).
In plants, the limited number of tests under near-natural conditions for
rapid evolution under changing climate focused on two approaches. One is
the resurrection approach, where stored seeds collected years before
within a plant community are revived and compared to plants from
recently sampled seeds from the same location (reviewed in Frankset al. 2018, including eight climate-related cases). Yet,
observed changes can be influenced by seed storage (Weis 2018; Frankset al. 2019) and attributing trait changes unambiguously to
climate as the causal factor is challenging (Franks et al. 2018).
Another approach encompasses multi-year climate manipulations imposed on
natural communities in the field. When plants are subsequently screened
for divergent evolution, changes can be causally attributed directly to
contrasting climatic treatments - a key asset of such experiments. Only
few studies followed this highly demanding approach, reporting
evolutionary change in some genetic markers (Jump et al . 2008,
Ravenscroft et al . 2015) or certain phenotypic traits (Grossman
& Rice 2014, Ravenscroft et al . 2014, Nguyen et al .
2016).
With either approach it has remained a great challenge though to judge
whether observed changes are adaptive. This judgement is often donea posteriori based on ‘intuition’ or ‘common sense’ (Merilä &
Hendry 2014, Franks et al . 2014). Yet, since species can adapt
via different sets of traits to the same climatic challenge (e.g. Biltonet al . 2016, Bergholz et al . 2017, Lampei et al .
2017 for aridity), the traits expected to evolve can likewise differ
among species. Moreover, Sandel et al. (2010) cautioned that
initial trait responses to altered environments may differ from those
for long-term adaptation. Such complexities may render common-sense
interpretations misleading, i.e. we need much clearer justifications to
conclude adaptive responses.
Here, we addressed this challenge using several independent lines of
evidence simultaneously. Firstly, we imposed replicated in situclimate manipulations on entire plant communities to directly control
the causal factor, and we did so in two independent sites. Consistent
evolutionary responses in both sites would then strongly argue against
random drift effects and in favor of adaptivity. Secondly, we combined
these climate manipulations with a corresponding natural climatic
gradient in a ‘space-for-time’ approach: Many species show clinal trait
divergence along natural gradients when grown under common garden
conditions (e.g. Kigel et al . 2011, Petrů et al . 2006,
Lampei et al . 2017). Such clines likely reflect locally adapted
ecotypes, i.e. the species-specific long-term adaptive strategy towards
the corresponding climatic factor (Kawecki & Ebert 2004), and hence
provide clear a priori predictions for directional trait
evolution under climate manipulations. Thirdly, we based our selection
of study traits on evolutionary theory, i.e. attested evidence for
theoretical fitness advantages under differential climatic conditions.
Lastly, we additionally estimated selection (Lande & Arnold 1983) on
target plants grown in the greenhouse, yet under a set of contrasting
abiotic conditions that mirrored our in situ climate
manipulations. If the covariance between trait values and fitness
changes with climatic condition, differential trait values should be
advantageous contingent on climate. By combining these multiple
approaches, we gained unprecedented strong evidence for whether
potential evolutionary changes are adaptive.
Another recent debate addresses the role of phenotypic plasticity in
climate change adaptation (Merilä & Henry 2014; Fox et al . 2019;
Scheiner et al . 2020). High plasticity in adaptive traits, as an
immediate response to altered environments, may help genotypes to better
match their phenotype to these novel conditions (Crispo 2007; Lande
2009; Kelly 2019). It was therefore hypothesized that climate change can
target plasticity itself for evolution and favor more plastic genotypes,
even if this is a transient response that is merely ‘buying time’ until
the occurrence of new genotypes with specific adaptations to the new
conditions (Lande 2009; Fox et al . 2019; Scheiner et al .
2020). However, empirical tests in natural populations for evolution of
increased plasticity in response to changing climate are scarce and
yielded equivocal results (Franks et al. 2018; Arnold et
al. 2019; Kelly 2019). This knowledge gap for evolving plasticity is
unfortunate because moreover, plasticity may also interact with genetic
adaptation by buffering selection and hence slowing down evolution and
genetic adaption (Merilä & Hendry 2014, Kelly 2019, Fox et al .
2019), i.e. rapid evolution may be confined to traits with low
plasticity. Yet, studies are missing that systematically compared
rapidly evolving traits with their degree of plasticity.
To address these gaps, we conducted a uniquely comprehensive test for
rapid evolution in ten target traits and their plasticity in a
large-scale, multi-site climate change experiment (Tielbörger et
al . 2014). Experimental rainfall manipulations (+30%, control, -30%)
where imposed for ten years in two sites on entire resident plant
communities in the Eastern Mediterranean, and combined with a natural
rainfall gradient. Rainfall is the key abiotic factor in these
ecosystems, with a projected -20% decline until 2050 (Smiatek et
al . 2011; Samuels et al . 2013). Annual species dominate these
communities, which allows for potentially rapid evolutionary responses
(Tielbörger et al . 2014). Since migration of most species is
limited (Siewert & Tielbörger 2010), in situ evolution appears
crucial for climate change adaptation. We tested for evolutionary
divergence in traits and plasticity in a naturally occurring annual
plant species after ten years of climate manipulations, and used a
comprehensive combination of lines of evidence to judge adaptivity. We
hypothesized that climate manipulation had caused adaptive evolutionary
trait divergence and selection for higher plasticity, and that evolution
is less probable in highly plastic traits.