Eco-Evo Evo-Eco: Climate change and evolution

The world is changing at an unprecedented rate. This change renders populations (and sometimes entire species) maladapted for the new conditions. This maladaptation can decrease fitness, which can cause population declines, extirpation, and extinction. The world is going to hell in a hand-basket.

Populations can arrest their declines and thereby prevent extirpation through changes that better match their phenotypes for the new conditions. These changes might be most easily achieved by moving to a better location: if it is getting too warm, simply move to a colder place. Indeed, changes in geographic ranges are a common feature of species’ responses to climate change in Earth’s past and present. In many instances, however, such movement is not possible, such as when barriers hinder movement between locations or when organisms have restricted movements, such as in plants with limited seed dispersal.

When organisms cannot move to better locations, they must alter their phenotypes in situ to better suit the new conditions. Perhaps the easiest way to do so is through phenotypic plasticity, where changing environmental conditions induce new adaptive phenotypes without evolution. Indeed, many instances are known of induced changes in traits, such as breeding time, that better suit organisms for altered conditions. Plasticity is not a panacea, however, because it has limits and costs. Sustainability in the face of environmental change will therefore often also require evolutionary change.

Populations and species currently found in different environments clearly have traits that are well suited for (adapted to) those different conditions – as extremes, think of Emperor penguins in the Antarctic (and Galapagos!) and camels in the Sahara (and Mongolia!). Evolution can thus accomplish a remarkable diversity of adaptations to different climates. These existing differences, however, usually evolved over very long time frames, whereas current climate change is occurring much faster. For this reason, the classic perception has been that evolution will not have a noteworthy ability to rescue populations that are otherwise likely to be extirpated under rapid climate change.

This classic expectation of the separation of ecological and evolutionary time began to wane with the publication of a number of instances where organisms were seen to have evolved on very short time scales, variously called “contemporary evolution,” “evolution on ecological time scales,” or “rapid evolution.” I was involved in the early stages of this realization through my own empirical work on introduced salmon and through several review papers that I wrote with Mike Kinnison and others (1999, 2001, 2003, 2008). Tantalizingly, a number of the emerging examples of contemporary evolution seemed related to climate change. And thus, a solution to climate change seemed to have emerged (evolution to the rescue!), with a large number of review papers beginning to tout this possibility. Moreover, many examples of phenotypic change were appearing in the literature and evolution was often advanced as a likely cause. Problem solved.

A compilation of images of some of the species shown to have evolved on short time scales in response to environmental change.

Or not. Examining the published work, Juha Merilä (and others) came to the conclusion that very few studies had employed methodologies sufficient for revealing evolutionary response – and, of the few studies that had used robust methods, most found no evolutionary change. Annoyed by the seemingly lax standards in the literature, Juha wrote several papers (2008, 2012) arguing that evolution in response to climate change was rare – or at best, still largely undocumented.

At an Evolutionary Applications editorial board meeting at the European Society for Evolutionary Biology in 2011, Juha and I got into a spirited argument about climate change and evolution. Juha argued that it had not been demonstrated. I argued that, while this might well be so, evolution is almost certainly occurring given many examples of contemporary evolution in response to local environmental change. What we could both agree on was that a serious appraisal of the methodology and literature had not been attempted and was sorely needed. Given the venue for our argument, the solution was right at hand: Juha and I should write a perspective piece on the problem for Evolutionary Applications and use this article as a springboard for a special issue – which has just been published.

In the perspective that opens the special issue, Juha and I critically evaluate the different methods for inferring (1) evolution versus plastic responses to environmental change, (2) whether any such changes are adaptive or not, and (3) whether changes are specifically the result of climate change (as opposed to some other environmental factor). After drafting the perspective, we sent it to experts on the topic in particular taxa (plants, fish, birds, mammals, amphibians and reptiles, terrestrial invertebrates, freshwater invertebrates, marine animals and plants, marine phytoplankton) and encouraged them to write review papers evaluating published examples of phenotypic change for the strength of evidence relating to the above three topics. In addition, we asked other experts for a review of Bergmann’s Rule (larger body size at higher latitudes) in the context of climate change and of theoretical models of evolution (and plasticity) under environmental change.

Although the individual papers (see below) should be consulted for details, a few key points can be summarized here:

1. A diversity of methods exist for assessing evolutionary and plastic responses to climate change. These methods vary in the strength of inferences they provide. Some of the best methods include “animal models,” common-garden experiments, and reciprocal transplants.

2. Many studies have documented phenotypic changes associated with climate change. In the vast majority of such cases, strong inferential methods have not been employed, and so the relative contributions of evolution and plasticity are unknown. In such cases, neither plasticity nor evolution should be considered a “null” model – that is, both remain in play until appropriate methods are implemented.

3. Studies of at least 26 species have employed strong inferential methods to test for evolutionary responses to climate change. Studies of birds and mammals – often using “animal model” approaches – usually do not find evolutionary responses. Studies of plants and insects – often using common-garden approaches – usually do find evolutionary responses.

4. Evolution clearly occurs in response to climate change. Plasticity also plays a role in many instances. For the vast majority of cases, however, inferential methods are as yet insufficient to conclude the relative contributions of these two types of response.

Red squirrels – a mammal that has evolved in response to climate change (although even this example is controversial). Photo: Andrew P. Hendry.

“To sum up, this perspective and the accompanying eleven articles have focused on methods and quality of evidence for genetic and phenotypic changes in response to climate change in nature. While the current picture emerging from all of this work might not seem particularly encouraging, it should provide guidelines, avenues, and inspiration for research to come. Identification of the challenges and knowledge gaps can be viewed as a first step toward progress in improving our understanding of the relative roles of genetic change and plasticity in mediating adaptive organismal responses to changing climatic conditions.” (From the concluding paragraph.)