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Evolution (Terrestrial Plants: Drought-Induced) -- Summary
Evolution is generally thought of as acting over long periods of time. So is there anything it can do to help plants cope with the rapid climate changes that the IPCC predicts will be caused by the ongoing rise in the air's CO2 content? In what follows, this question is carefully considered as it applies to the daunting environmental challenge of droughts, which climate alarmists contend will become more intense and occur more frequently throughout many parts of the world in the years and decades to come.

Working with the field mustard (Brassica rapa) plant, Franks et al. (2007) compared "phenotypic and fitness values of ancestral, descendant, and ancestral x descendant hybrid genotypes grown simultaneously under conditions that mimicked those before and after a real-world switch from above-average to below-average precipitation in southern California (USA), which led to abbreviated growing seasons from 2000 to 2004. Fortuitously, as they described it, they had "collected B. rapa seed in 1997, before the drought, and then again in 2004 from two populations," a dry site and a wet site. Hence, they could grow - at the same time and under the same circumstances, in a new set of experiments - plants that had experienced extended drought conditions (descendants) and plants that had not experienced such conditions (ancestors), as well as hybrids of the two; and they could see if flowering times (FT) differed as would be expected from life history theory, which "predicts that the optimal FT in annual plants will be shorter with shorter growing seasons," such as those that were imposed by the extended drought that occurred between the two times of their seed collecting.

This work revealed, in the three researchers' words, that as predicted, "the abbreviated growing seasons caused by drought led to the evolution of earlier onset of flowering," such that plant descendants "bloomed earlier than ancestors, advancing first flowering by 1.9 days in one study population and 8.6 days in another." Moreover, they found that "the intermediate flowering time of ancestor x descendant hybrids supports an additive genetic basis for divergence." And in consequence of these several observations, they concluded that "natural selection for drought escape thus appears to have caused adaptive evolution in just a few generations," while further noting, in almost the same words, that "abundant evidence has accumulated over the past several decades showing that natural selection can cause evolutionary change in just a few generations (Kinnison and Hendry, 2001; Reznick and Ghalambor, 2001)."

In discussing the significance of these findings, Franks et al. wrote that they "provide evidence for a rapid, adaptive evolutionary shift in flowering phenology after a climatic fluctuation," which "adds to the growing evidence that evolution is not always a slow, gradual process but can occur on contemporary time scales in natural populations," and in response to real-world climatic changes.

In a follow-up study conducted with the same plant material, Franks and Weis (2008) found that several life-history traits "differed between the ancestral genotypes collected before and descendant genotypes collected after the natural drought," stating that "this shows directly that an evolutionary change in the life-history traits has occurred during a 5-year drought." They also reported that "the evolutionary changes in trait levels following the drought are consistent with predictions from life-history theory," since "the drought selected for individuals that flowered earlier, continued to flower for longer given sufficient resources, and produced a more consistent, evenly distributed pattern of flowering over time," which suite of changes, in their words, constitutes "a true genetically based evolutionary change rather than an expression of phenotypic plasticity."

Working concomitantly in Spain's Garraf Natural Park with Fumana thymifolia (a small shrub that occurs around the Mediterranean Basin), Jump et al. (2008) sought to determine whether reduced seedling establishment observed as a consequence of climate manipulation is a random or selective process, in an attempt to answer what they called "the key question" - Does climate change provoke evolutionary change within natural populations?

The seven scientists' study had, as an unaltered control treatment, a drought treatment that employed automatically-activated transparent plastic shields that covered a third of the plots in response to rainfall and retreated when rainfall stopped (which decreased soil moisture by about 20%), and a warming treatment that employed reflective covers that reduced nighttime re-radiation of energy received from the sun during the prior daylight hours from another third of the plots (which increased temperature by about 1°C).

As a result of these environmental interventions, Jump et al. found that over the 7-year period 1999-2005, mean yearly seedling density per treatment was significantly reduced in the drought and warming treatments compared with the control treatment, and that "when compared against control samples, high single-locus genetic divergence occurred in drought and warming treatment samples, with genetic differentiation up to 37 times higher than background (mean neutral locus) genetic differentiation."

In discussing their discoveries, the researchers said they suggest that the significant reduction in seedling survival they observed in the drought and warming treatments "results from an episode of selection for individuals tolerant of the modified climatic conditions and is not due simply to a random reduction in plant establishment," which implication, in their words, "reinforces results reported by other authors that show that genetic variability for climate-related traits exists within natural plant populations (Hamrick and Holden, 1979; Cobb et al., 1994; Kelly et al., 2003; Mitton and Duran, 2004; Franks et al., 2007)."

Jump et al. thus concluded that contemporary climate change "is driving changes in gene frequency within natural plant populations," and that these changes "are occurring on the same time scale as current climatic changes, based on preexisting genetic variability within populations," additionally citing, in this regard, the supportive findings of Jump and Penuelas (2005), Thomas (2005), Jump et al. (2006) and Reusch and Wood (2007). What is more, they stated that this ability to rapidly adapt to rapid climate change may increase the persistence of species "beyond that predicted under a species-based climate envelope approach," such as is typically used by climate alarmists to justify their predictions of impending extinctions of huge numbers of species.

Therefore, and in a conclusion that clearly repudiates the catastrophic extinction scenario, Jump et al. wrote that their results actually demonstrate "that rapid evolution in response to climate change may be widespread in natural populations, based on genetic variation already present within the population," which likelihood is becoming ever more evident with each new study that investigates the subject. Hence, it is reasonable to expect that earth's plants are likely to be much more resilient to rising temperatures and reductions in precipitation than the people of the world have long been led to believe.

One year later, Franks and Weis (2009) wrote that "one rigorous way to determine if evolution has occurred in a natural population is to collect propagules before and after an environmental change and raise them under common conditions," stating that "this approach was used previously to show that Brassica rapa [a self-incompatible weedy winter annual] evolved drought escape through earlier flowering following a series of recent dry years in Southern California, and that early flowering results in higher fitness under drought conditions (Franks et al., 2007)," while noting that "a related study showed that multiple phenological traits and their interactions evolved in response to the drought (Franks and Weis, 2008)."

Working with the same pre- and post-drought collection lines from the Franks et al. (2007) experiment, the two researchers went on to estimate the amount of assortative mating within, and the degree of phenological isolation between, two B. rapa populations. And in doing so, they determined that "climate change can alter plant phenology, which can change patterns of assortative mating within populations," and that "this assortative mating can directly change genotype frequencies and can also increase the rate of evolution by interacting with selection." In addition, they demonstrated that "climatically driven changes in phenology can potentially influence gene flow among populations due to changes in overlap in flowering schedules," and that "these changes in gene flow can also influence both the rate and pattern of evolutionary change."

In the end, therefore, the two scientists concluded that "the high degree of interdependence of flowering time, assortative mating, selection and gene flow make predicting evolutionary responses to changes in climate particularly complex and challenging." On the positive side, however, this great degree of complexity suggests that among the multiplicity of outcomes, there is a good chance that one or more of them will be just what plants need to successfully respond to the climate change that elicited the many outcomes.

With the passing of two more years, Vigouroux et al. (2001) wrote that "one important phenomenon that is often overlooked and is poorly documented is the ability of agro-systems to rapidly adapt to environmental variations," noting that such adaptations can occur by either the adoption of new varieties or by the adaptation of existent varieties to a changing environment. Thus working, as they were, in "one of the driest agro-ecosystems in Africa, the Sahel," they "analyzed samples of pearl millet landraces collected in the same villages in 1976 and 2003 throughout the entire cultivated area of Niger," in order to see how the agro-system had responded to recurrent drought over that time interval, which they did by studying "phenological and morphological differences in the 1976 and 2003 collections by comparing them over three cropping seasons in a common garden experiment."

This work revealed, in the words of the fifteen researchers, that "compared to the 1976 samples, samples collected in 2003 displayed a shorter lifecycle and a reduction in plant and spike size." They also found that an early flowering allele "increased in frequency between 1976 and 2003," and they stated that this increase "exceeded the effect of drift and sampling, suggesting a direct effect of selection for earliness on this gene." And so it was that Vigoruoux et al. concluded that "recurrent drought can lead to selection for earlier flowering in a major Sahelian crop," thereby reinforcing the similar earlier findings of Franks et al. (2007), Franks and Weis (2008, 2009) and Jump et al. (2008), while adding to the likelihood that other important food crops, as well as the native vegetation of natural ecosystems, can respond to environmental changes in an analogous fashion.

Jumping ahead a little over a decade in time, Pluess and Weber (2012) introduced their study by writing that "with increasing temperatures and dryer summers [as predicted by various climate models], areas nowadays covered by beech forests are expected to shrink tremendously," but they opined that "if individuals at the dry distribution limits [of the species: Fagus sylvatica L.] are adapted to lower moisture availability, F. sylvatica might contain the genetic variation for the continuation of beech forests under climate change," even in areas that are predicted to become devoid of the trees.

In an investigation into the strength of this hypothesis, Pluess and Weber employed an AFLP (Amplified Fragment Length Polymorphism) genome scan approach that was designed to explore the "neutral and potentially adaptive genetic variation in Fagus sylvatica in three regions [within the lowland forests of Switzerland] containing a dry and mesic site each," after which they "linked this dataset with dendrochronological growth measures and local moisture availabilities based on precipitation and soil characteristics."

The two Swiss scientists reported that this approach revealed that a "potential for adaptation to water availability" was reflected in observed outlier alleles that "indicated micro-evolutionary changes between mesic and dry stands." And they noted, in this regard, that "while Rose et al. (2009) found adaptation to drought in a common garden experiment with seedlings originating from provenances which were more than 1000 km apart," they found genetic differentiation in relation to water availability in neighboring stands. And in light of this set of real-world observations, Pluess and Weber concluded that "dispersal across large distances is thereby not needed for the spread of 'preadapted' genes in F. sylvatica," for the trees apparently do indeed contain the genetic material needed for "the continuation of beech forests under climate change," even in areas that have been predicted to become too dry for F. sylvatica trees to survive.

Last of all in this particular subject area, Zhang et al. (2012) introduced their study by writing that "a key question in ecology and evolution is to what degree variation in ecologically important traits is heritable, because heritability determines the potential for evolutionary change of traits (Fisher, 1930; Falconer and MacKay, 1996)," which phenomenon significantly enhances the ability of a species "to adapt to changing environments (Visser, 2008; Hoffmann and Sgro, 2011)," such as the changes climate alarmists are predicting for the entire planet over the next several decades or more. And in an attempt to answer this "key question," Zhang et al. conducted a glasshouse experiment in which they tested the response of a large number of epigenetic recombinant inbred lines or epiRILs (i.e., lines that are nearly isogenic but highly variable at the level of DNA methylation, which latter phenomenon can stably alter the gene expression pattern in cells) of Arabidopsis thaliana to drought and increased nutrient conditions.

This work, in the words of the four researchers, led to their finding "significant heritable variation among epiRILs both in the means of several ecologically important plant traits and in their plasticities to drought and nutrients." Thus, they indicated that the significant selection gradients of the several mean traits and plasticities they discovered "suggest that selection could act on this epigenetically based phenotypic variation." And as a result, they felt confident in stating, in the concluding sentence of their paper's abstract, that their study "provides evidence that variation in DNA methylation can cause substantial heritable variation of ecologically important plant traits, including root allocation, drought tolerance and nutrient plasticity, and that rapid evolution based on epigenetic variation alone should thus be possible."

And in interpreting these several findings, one can only conclude that they bode well indeed for the future of earth's many terrestrial plants, even if a significant degree of global warming were to begin again ... after it's already near-decade-and-a-half hiatus.

References
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Falconer, D.S. and MacKay, T.F.C. 1996. Introduction to Quantitative Genetics, 4th Edition. Longman, New York, New York, USA.

Fisher, R.A. 1930. The Genetical Theory of Natural Selection. Oxford University Press, Oxford, United Kingdom.

Franks, S.J., Sim, S. and Weis, A.E. 2007. Rapid evolution of flowering time by an annual plant in response to a climate fluctuation. Proceedings of the National Academy of Sciences USA 104: 1278-1282.

Franks, S.J. and Weis, A.E. 2008. A change in climate causes rapid evolution of multiple life-history traits and their interactions in an annual plant. Journal of Evolutionary Biology 21: 1321-1334.

Franks, S.J. and Weis, A.E. 2009. Climate change alters reproductive isolation and potential gene flow in an annual plant. Evolutionary Applications 2: 481-488.

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Last updated 2 April 2014