Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.4.2.8 Observed Evolutionary Responses to Climate Change ====

<div id="h3-14-siblings" class="h3-siblings"></div>

Previous sections document the tendency of species to retain their climate envelopes by some combination of range shift and phenological change ''(very high confidence)'' . However, this tracking of climate change can be incomplete, causing species or populations to experience hotter conditions than those to which they are adapted, and thereby incur ‘climate debts’ (section 2.4.2.3.1) ( [[#Devictor--2012|Devictor et al., 2012]] ). The importance of population-level debt is illustrated by a study in which the estimated debt values were correlated with population dynamic trends in a North American migratory songbird, the yellow warbler, ''Setophaga petechia.'' Populations that were genetic outliers for their local climate space had larger population declines (greater debt) than those with genotypes closer to the average values for that particular climate space. Debt values were estimated from genomic analyses independent of the population trends, and were distributed across the species’ range in a mosaic, not simply concentrated at range margins, rendering the results robust to being confounded by broad-scale geographical trends ( [[#Bay--2018|Bay et al., 2018]] ). Soroye et al. (2020) found similar results for 66 species of bumble-bees across Europe and North America, with declines in abundances spread throughout species’ ranges, but being greatest where populations already near their climate limits were being pushed beyond their climatic tolerances with climate change {2.4.2.3.1} .

In the absence of evolutionary constraints, climate debts can be cancelled by genetically based increases in thermal tolerance and the ability to perform in high ambient temperatures. In species already showing local adaptation to climate, populations currently living at relatively cool sites should be able to evolve to adopt the traits of populations currently at warmer sites as their local experience of climate changes ( [[#Singer--2017|Singer, 2017]] ; [[#Socolar--2017|Socolar et al., 2017]] ).

An increasing number of studies document evolutionary responses to climate change in populations not at their warm range limits ( [[#Franks--2012|Franks and Hoffmann, 2012]] ). Organisms with short generation times should have a higher capacity to genetically track climate change than species with long generation times, such as mammals ( [[#Boutin--2014|Boutin and Lane, 2014]] ). Indeed, observed evolutionary impacts have been mainly documented in insects, especially at expanding range margins ( [[#Chuang--2016|Chuang and Peterson, 2016]] ) where evolutionary changes have increased dispersal ability ( [[#Thomas--2001|Thomas et al., 2001]] ) and decreased host specialisation ( [[#Bridle--2014|Bridle et al., 2014]] ; [[#Lancaster--2020|Lancaster, 2020]] ) ''(medium evidence, medium agreement)'' .

Away from range margins, individual populations experiencing regional warming have evolved diverse traits related to climate adaptation. For example, pitcher-plant mosquitoes ( ''Wyeomyia smithii)'' in Pacific Northwest America have evolved to wait for shorter days before initiating diapause. This adaptation to lengthening summers enables them to delay overwintering until later and add an extra generation each year ( [[#Bradshaw--2001|Bradshaw and Holzapfel, 2001]] ). Among 26 populations of ''Drosophila subobscura'' studied on three continents, 22 experienced climate warming across two or more decades, and 21 of these 22 showed increasing frequency of the chromosome inversion characteristic of populations adapted to hot climates ''(robust evidence, high agreement)'' ( [[#Balanya--2006|Balanya et al., 2006]] ).

However, for populations already at their warm range limits, their ability to track climate change ''in situ'' would require evolving to survive and reproduce outside their species’ historical climate envelope: abilities of wild species to do this is not supported by experimental or observational evidence ( ''medium evidence'' , ''high agreement'' ) ( [[#Singer--2017|Singer, 2017]] ). Whether or not they can depends on the level of ‘niche conservatism’ operating at the species level ( [[#Lavergne--2010|Lavergne et al., 2010]] ). If a species whose range limits are determined by climate finds itself completely outside of its traditional climate envelope, extinction is expected in the absence of ‘evolutionary rescue’ ( [[#Bell--2009|Bell and Gonzalez, 2009]] ; [[#Bell--2019|Bell et al., 2019]] ).

To investigate the evolutionary potential of a species to survive in a novel climate entirely outside its traditional climate envelope, experiments have been carried out on ectotherms testing thermal performance, thermal tolerance and their evolvabilities ( [[#Castaneda--2019|Castaneda et al., 2019]] ; [[#Xue--2019|Xue et al., 2019]] ). Tests of thermal performance have been complicated, as both long-term acclimation and trans-generational effects occur ( [[#Sgro--2016|Sgro et al., 2016]] ). However, the results to date have been consistent: despite widespread local adaptation to climate across species’ ranges, substantial constraints exist regarding the evolution of greater stress tolerance (e.g., high temperatures and drought) at warm range limits ( ''medium evidence'' , ''high agreement'' ) ( [[#Hoffmann--2011|Hoffmann and Sgro, 2011]] ; [[#MacLean--2019b|MacLean et al., 2019b]] ).

For example, as temperature was experimentally increased, the amount of genetic variance in the fitness of ''Drosophila melanogaster'' decreased; in hot environments, flies had low evolvability ( [[#Kristensen--2015|Kristensen et al., 2015]] ). The hypothesis that heat-stress tolerance is evolutionarily constrained is further supported by experiments in which 22 ''Drosophila'' spp. drawn from tropical and temperate climes were subjected to extremes of heat and cold. They differed, as expected, in cold tolerances, but not in heat tolerances nor in temperatures at which optimal performances were observed ( [[#MacLean--2019b|MacLean et al., 2019b]] ).

Plasticity (flexibility) in acclimating to thermal regimes helps organisms adapt to environmental change. The form and extent of plasticity can vary among populations experiencing different climates ( [[#Kelly--2019|Kelly, 2019]] ) and generate phenotypic values outside the prior range for the species, but plasticity itself has not yet been observed to evolve in response to climate change ( [[#Kelly--2019|Kelly, 2019]] ).

Relevant genetic changes in nature (e.g., affecting heat tolerance) have not yet been shown to alter the boundaries of existing genetic variation for any species. Further, a recent global analysis of 91 species found, on average, a 5.4–6.5% decline in genetic diversity within populations since the start of the Industrial Revolution, with much larger declines for island species (27.6–30.9% reductions) ( [[#Leigh--2019|Leigh et al., 2019]] ). In [[#Leigh--2019|Leigh et al. (2019)]] , genetic declines were documented in both common and already endangered species of fish, mammals, birds, insects, amphibians and reptiles. These declines in genetic diversity, though not caused by climate change, decrease the abilities of wild species to adapt to climate change via evolutionary responses. Evolutionary rescue of entire species has not yet been observed in nature, nor is it expected, based on experimental and theoretical studies ( ''medium evidence'' , ''high agreement'' ).

Hybridisation between closely related species has increased in recent decades as one species shifts its range boundaries and positions itself more closely to the other. Hybrids between polar bears and brown bears have been documented in northern Canada ( [[#Kelly--2010|Kelly et al., 2010]] ). In North American rivers, hybridisation between invasive rainbow trout and native cutthroat trout has increased in frequency as the rainbow trout has expanded into warming waters ( [[#Muhlfeld--2014|Muhlfeld et al., 2014]] ). Whether climate-changed induced hybridisations can generate novel climate adaptations remains to be seen.

In summary, with our present knowledge, evolution is not expected to be sufficient to prevent the extinction of whole species if a species’ climate space disappears within the region they inhabit ( ''high confidence'' ).

<div id="FAQ 2.1" class="h2-container"></div>

<span id="faq-2.1-will-species-become-extinct-with-climate-change-and-is-there-anything-we-can-do-to-prevent-this"></span>