Image by Aarón Blanco Tejedor

CLIMATE CHANGE AS
AN EVOLUTIONARY EVENT

thomas whitham

After so many records in heat, drought, flooding, fire, declining air and water quality, pathogen outbreaks, invasive species, rising sea levels, melting glaciers and the list goes on, it seems that the public is starting to recognize that climate change and other stressors are real problems.  They affect their loved ones, lifestyles, insurance rates, travel plans, and places they hold dear like national parks.  With all of these changes in the environment, whether it is climate or an invasive species (the two often go hand-in-hand), we need to recognize that these events are not just ecological events as they are currently being treated, but also evolutionary events.  

 

As an ecological event alone, they have a higher probability of returning to their previous state if climate change reverses.  If they are also evolutionary events in which the genetics of individual species and whole communities are altered, then the changes are likely to be long-term and the trajectories of surviving species and communities could go in new directions that are hard to predict.  These changes are expected to be associated with a loss of biodiversity as some to many species will be more constrained than others and won’t shift fast enough to match the environmental changes.  

 

There are three basic options: move, adapt (evolution), or die out.  With rapid environmental changes as we are now experiencing, many expect local extinction to be common, but with slower environmental changes, moving to more favorable environments and adapting to ongoing changes could be more common.

We tend to think of evolution as something that has occurred only in the past as the result of millions of years of selection and that somehow the present has escaped this fundamental process.  However, times of great environmental change as we are currently witnessing have great potential to become evolutionary events with profound implications.  If you are surprised that this might be possible, rapid evolution is also a relatively new concept to many scientists.  How can this be so, what does it mean and what does it take to present a convincing case that it is commonly occurring over short time spans — like right now?  For example, the rapid evolution of diseases like COVID-19 and the newly emerged omicron variant illustrates how new diseases can emerge, rapidly expand into naïve populations that have little immunity, and impact the entire planet, at least from our human-centric perspective.  Such rapid evolutionary events have happened throughout the history of our planet and will continue to effect the evolutionary trajectories of all life on earth. 

With the above example in mind, we can then ask what it takes for an ecological event such as the transfer of a disease from a remote jungle to a densely populated city, or a record drought and extreme temperatures the southwest is currently experiencing to become an evolutionary event?  While evolution happens all the time, environmental change can jumpstart the process.  How so?  Change creates inequalities in which some individuals, populations, or whole communities do better than others.  For example, individual trees growing in a dry, nutrient-poor soil may be more stressed by drought and high temperatures than other individuals of the same species living in wetter, more nutrient rich soils, which are associated with differences in plant growth, survival and reproduction.  

 

By itself, these differences in performance may not have evolutionary consequences. However, if these individuals, populations or communities differ genetically by chance or previous evolutionary events, then those that perform best will have an advantage over those whose performance is reduced.  And, if these differences in performance persist such that those with the “right” genetic makeup (i.e., traits associated with drought tolerance) increase in frequency over time relative to those with the “wrong” genetic makeup, then evolution has occurred.  

Pinyon pine (Pinus edulis) provides an example of such rapid evolution in response to drought that has been documented over the past 40 years of research from faculty and students with the Center for Adaptable Western Landscapes at Northern Arizona University.  Prior to 1996, this dominant tree of the American Southwest grew under relatively favorable conditions, but in subsequent years the climate shifted and beginning around 2000 the region suffered from a megadrought that continues 21 years later and is expected to continue for the foreseeable future.  As a result of the 2002 record drought, trees started to die at the landscape level in which mortality of some stands was negligible while others approached 100%.  Several striking mortality patterns were observed. 

 

First, trees growing on stressful south-facing slopes died more than those on more protected north-facing slopes.  Second, trees growing at low elevations where it is hotter and drier, died more than those at cooler and wetter high elevations.  Third, those growing in stressful poorer soils died more than those in more favorable soils.  Fourth, those growing at high competitor densities where resources are more limiting died at higher levels than those growing more spaced out at lower densities.  Fifth, small trees growing in association with shrubs that reduced soil temperatures and rodent herbivory survived better than those growing in the open.  And, sixth, those with a certain community of beneficial fungi on the roots that help in water and nutrient uptake survived better than those with another community of fungi that were less beneficial.  

 

In combination, these stress gradients involving both climate and associations with beneficial shrubs and microbes resulted from an ongoing climate change event that created the conditions for rapid evolution to occur.  However, for drought to result in an evolutionary event, there must be evidence that crucial plant traits such as tree tolerance to drought are genetically based and associated with one or more of the above gradients.  Numerous studies have now shown that drought tolerance is genetically based in which drought intolerant trees suffered 68% mortality in just one record drought year, whereas drought tolerant trees suffered only 21% mortality.  Thus the patterns of mortality were not random increasing the probability that drought was an evolutionary event.

While these findings show that climate change is associated with the differential survival of drought tolerant and intolerant trees, to be convincing, research must also demonstrate that drought tolerance is a heritable plant trait that can be passed on to future pinyon generations.  By collecting seeds from both drought tolerant and intolerant tree types and growing them side-by-side in a common garden, NAU researchers found that seedlings carried the same traits as their mothers.  In other words, drought tolerance is heritable such that drought tolerant mothers produced drought tolerant offspring while those from drought intolerant mothers produced drought intolerant offspring.  Thus, all the pieces fell into place showing that the record drought was an evolutionary event that changed the genetic structure of the tree population.

Because pinyon pine is a dominant tree on the landscape that supports over 1,000 other species, evolutionary changes in the tree are likely to cascade to affect many other species and interactions with these species, may in turn feedback to affect the tree.  For example, many studies show that individual trees differ in numerous traits (e.g., chemistry, phenology, morphology) resulting in their supporting different communities of associated organisms including fungi, bacteria, insects, lichens, pathogens, birds and mammals.  Thus, as the tree evolves in response to climate change, these associated organisms may also evolve in response to changes in the tree, which in turn may positively or negatively affect the tree.  

 

Although little research has been conducted in this area, recent studies show that insect herbivores have rapidly evolved to live on individual trees that have a specific suite of traits and that the presence of specific communities of beneficial fungi can enhance tree survival in the face of climate change.  This raises the exciting possibility that the genetic changes in one organism in response to climate change can cascade to affect the evolution of entire communities.  This also means that the effects of climate change could be much greater than previously thought and as humans are a part of a much larger ecosystem, we must be especially cognizant of how we impact the landscape in ways that will come back to affect us. Climate change is already having profound effects on human food production, diseases, migration, fire, and a myriad of ecosystem services that we often take for granted such as clean air and water. 

While we don’t understand the full implications of global climate change on the planet, we now know enough to surmise that climate change will have both ecological and evolutionary effects that will ripple throughout the ecosystems of the world.  It is also highly likely that climate change will result in the extinction of many species as the changes will be so fast that evolutionary process can’t keep up and many species will simply go extinct.  Because of this new reality, we need to reverse ongoing climate change impacts and learn the sciences associated with restoration on a global scale.  

 

A current bill pending in the U.S. House of Representatives, known as Bill 5145, with support from representatives in Arizona, California, and Washington, is aimed at understanding these evolutionary processes and learning how to use genetic solutions to mitigate the impacts of climate change on riparian forest ecosystems in the U.S.  This research into ecosystem restoration may serve as a model for similar programs that could be replicated around the globe and include restoration of coral reefs that are suffering from heat stress from rising water temperatures.  To learn more about this bill and to support its goals, please sign the petition and contact Tom Whitham if you have questions.

 

Short list of reading materials that develop conceptual themes presented in this article:

Gehring, C.A., C.M. Sthultz, L.H. Flores-Rentería, A.V. Whipple, and T.G. Whitham.  2017.  Tree genetics defines fungal partner communities that may confer drought tolerance.  Proceedings of the National Academy of Sciences 114:11169-11174


Whitham, T.G., G.J. Allan, H.F. Cooper, and S.M. Shuster.  2020.  Intraspecific genetic variation and species interactions contribute to community evolution.  Annual Review of Ecology, Evolution, and Systematics 51:587-612.

Additional reading materials that focus on specific examples:

Gehring, C.A., D. Flores-Rentería, C.M. Sthultz, T.M. Leonard, L. Flores-Rentería, A.V. Whipple, and T.G. Whitham.  2014.  Plant genetics and interspecific competitive interactions determine ectomycorrhizal fungal community responses to climate change.  Molecular Ecology 23:1379–1391.


Gehring, C.A., R.C. Mueller, K.E. Haskins, T.K. Rubow, and T.G. Whitham.  2014.  Convergence in mycorrhizal fungal communities due to drought, plant competition, parasitism and susceptibility to herbivory:  Consequences for fungi and host plants.  Frontiers in Microbiology https://doi.org/10.3389/fmicb.2014.00306


Kuske, C.R., L.O. Ticknor, J.D. Busch, C.A. Gehring, and T.G. Whitham.  2003.  The pinyon rhizosphere, plant stress, and herbivory affect the abundance of microbial decomposers in soils.  Microbial Ecology 45:340–352.


Mueller, R.C., C.M. Scudder, M.E. Porter, R.T. Trotter, C.A. Gehring, and T.G. Whitham.  2005.  Differential tree mortality in response to severe drought: evidence for long-term vegetation shifts.  Journal of Ecology 93:1085–1093.


Naesborg, R.R., M.K. Lau, R. Michalet, C.B. Williams, and T.G. Whitham.  2022.  Tree genes affect rock lichens and understory plants: Examples of trophic-independent interactions. Ecology e03589.


Smith, D.S., M.K. Lau, R. Jacobs, J.A. Monroy, S.M. Shuster, and T.G. Whitham.  2015.  Rapid plant evolution in the presence of an introduced species alters community composition.  Oecologia 179:563–572


Sthultz, C.M, C.A. Gehring, and T.G. Whitham.  2007.  Shifts from competition to facilitation between a foundation tree and a pioneer shrub across spatial and temporal scales in a semi-arid woodland.  New Phytologist 173:135–145.


Sthultz, C.M., C.A. Gehring, and T.G. Whitham.  2009.  Deadly combination of genes and drought: Increased mortality of herbivore-resistant trees in a foundation species.  Global Change Biology 15:1949–1961.


Stone, A.C., C.A. Gehring, N.S. Cobb, and T.G. Whitham.  2018.  Genetic-based susceptibility of a foundation tree to herbivory interacts with climate to influence arthropod community composition, diversity and resilience.  Frontiers in Plant Science 9:1831. doi: 10.3389/fpls.2018.01831.


Whitham, T.G, J.K. Bailey, J.A. Schweitzer, S.M. Shuster, R.K. Bangert, C.J. LeRoy, E.V. Lonsdorf, G.J. Allan, S.P. DiFazio, B.M. Potts, D.G. Fischer, C.A. Gehring, R.L. Lindroth, J.C. Marks, S.C. Hart, G.M. Wimp, and S.C. Wooley.  2006.  A framework for community and ecosystem genetics:  From genes to ecosystems.  Nature Reviews Genetics 7:510–523.


Whitham, T.G., W.P. Young, G.D. Martinsen, C.A. Gehring, J.A. Schweitzer, S.M. Shuster, G.M. Wimp, D.G. Fischer, J.K. Bailey, R.L. Lindroth, S. Woolbright, and C.R. Kuske.  2003.  Community and ecosystem genetics:  A consequence of the extended phenotype.  Ecology 84:559–573.


Williams, A.P., E.R. Cook, J.E. Smerdon, B.I. Cook, J.T. Abatzoglou, K. Bolles, S.H. Baek, A.M. Badger, and B. Livneh.  2020.  Large contribution from anthropogenic warming to an emerging North American megadrought. Science 368:314–318. https://doi.org/10.1126/scien ce.aaz9600


Wooley, S.C., D.S. Smith, E.V. Lonsdorf, S.C. Brown, T.G. Whitham, S.M. Shuster, and R.L. Lindroth.  2020.  Local adaptation and rapid evolution of aphids in response to genetic interactions with their cottonwood hosts.  Ecology & Evolution 10:10532-10542 https://doi.org/10.1002/ece3.6709
 

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thomas whitham

is a core member of the Center for Adaptable Western Landscapes and a Regents’ Professor, Department of Biological Sciences. A major emphasis of his research is the use of a genes-to-ecosystem approach to restore threatened poplar riparian communities and pinyon woodlands in the western United States. Tom served as the executive director of NAU’s Merriam-Powell Center for Environmental Research from 1999 until 2020. He has authored or co-authored 273 peer-reviewed publications that have appeared top-ranked journals, including Science and Nature. Additionally, he has mentored 32 successful Ph.D candidates and 29 master’s students. Tom has received numerous awards and honors, including the Eminent Ecologist Award presented by the Ecological Society of America. In 2020, he was honored with a Life Time Achievement Award, Department of Biological Sciences, Northern Arizona University.

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