Impacts of climate warming on terrestrial ectotherms
Curtis A. Deutsch*†‡, Joshua J. Tewksbury†§, Raymond B. Huey§, Kimberly S. Sheldon§, Cameron K. Ghalambor¶,
and David C. Haak§, and Paul R. Martin§?
*Program on Climate Change and Department of Oceanography and§Department of Biology, University of Washington, Seattle, WA 98195;
and¶Department of Biology and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO 80523
Edited by David B. Wake, University of California, Berkeley, CA, and approved March 3, 2008 (received for review October 4, 2007)
The impact of anthropogenic climate change on terrestrial organ-
rate of warming. Yet the biological impact of rising temperatures
also depends on the physiological sensitivity of organisms to
temperature change. We integrate empirical fitness curves describ-
ing the thermal tolerance of terrestrial insects from around the
for the next century to estimate the direct impact of warming on
insect fitness across latitude. The results show that warming in the
tropics, although relatively small in magnitude, is likely to have the
most deleterious consequences because tropical insects are rela-
close to their optimal temperature. In contrast, species at higher
latitudes have broader thermal tolerance and are living in climates
that are currently cooler than their physiological optima, so that
warming may even enhance their fitness. Available thermal toler-
ance data for several vertebrate taxa exhibit similar patterns,
Our analyses imply that, in the absence of ameliorating factors
such as migration and adaptation, the greatest extinction risks
from global warming may be in the tropics, where biological
diversity is also greatest.
biodiversity ? fitness ? global warming ? physiology ? tropical
impact on species is likely to vary geographically (2–4), but a
mechanistic framework to predict its magnitude and global
distribution has not yet been developed (5). One important
determinant of biological responses to climate change will be the
degree of warming itself, which will continue to be greater at
high latitudes (6). Also relevant, however, is the physiological
sensitivity of organisms to changes in the temperature of their
environment (7, 8). The thermal tolerance of many organisms
has been shown to be proportional to the magnitude of temper-
ature variation they experience (9–11), a characteristic of cli-
mate that also increases with latitude. Evaluating the impacts of
rapidly changing climates on population fitness and survival thus
requires linking geographic patterns of the magnitude of tem-
perature change with the physiological sensitivity of organisms
to that change (12).
Ectotherms constitute the vast majority of terrestrial biodi-
versity (13) and are especially likely to be vulnerable to climate
warming because their basic physiological functions such as
environmental temperature. The ability of ectotherms to per-
form such functions at different temperatures is described by a
thermal performance curve (14), which rises gradually with
temperature from a minimum critical temperature, CTmin, to an
optimum temperature, Topt, and then drops rapidly to a critical
thermal maximum, CTmax(Fig. 1). Critical temperatures CTmin
and CTmax, operationally defined by the limits of organism
performance, have been measured for diverse ectotherms (15–
18) and usually covary with latitude, reflecting at least partial
lobal warming in this century may be the largest anthropo-
genic disturbance ever placed on natural systems (1, 2). Its
adaptation of ectotherms to their climate (9–11). Thermal
performance curves index the direct effect of temperature on
organism fitness (14–15), and thus provide a physiological
framework for elucidating a fundamental component of the
Author contributions: C.A.D. and J.J.T. contributed equally to this work; C.A.D. and J.J.T.
designed research; C.A.D. and J.J.T. performed research; C.A.D., J.J.T., R.B.H., K.S.S., C.K.G.,
D.C.H., and P.R.M. analyzed data; and C.A.D., J.J.T., and R.B.H. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
†To whom correspondence may be addressed. E-mail: firstname.lastname@example.org or
‡Present address: Department of Atmospheric and Oceanic Sciences, University of Califor-
nia, Los Angeles, CA 90095.
?Present address: Department of Biology, Queen’s University, Kingston, ON, Canada
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2008 by The National Academy of Sciences of the USA
Change in fitness, 2100
60S 40S 20S020N 40N 60N
-10 0 10
for all insect species studied, as a function of latitude. (A and B) Fitness curves
are derived from measured intrinsic population growth rates versus temper-
ature for 38 species, including Acyrthosiphon pisum (Hemiptera), from 52°N
(England) (A), and the same for Clavigralla shadabi (Hemiptera) from 6°N
(Benin) (B). CTmin, Thab, Topt, and CTmaxare indicated on each curve. Climato-
logical mean annual temperature from 1950–1990 (Thab, drop lines from each
curve), its seasonal and diurnal variation (gray histogram), and its projected
increase because of warming in the next century (?T, arrows) are shown for
the collection location of each species. For each of 38 species, fitness is
integrated over both seasonal and diurnal temperature cycles for both the
observed climate of the late 20th century (1950–1990) and for a model-
simulated climate of the late 21st century (2070–2100) (23). (C) Predicted
change in fitness of insects versus latitude is a measure of the impact of 21st
century climate warming on population growth rates. Negative values indi-
cate decreased rates of population growth in 2100 AD and are found mainly
in the tropics. Positive values are found in mid- and high-latitudes. Line is a
spline-fit with a span of 0.9.
Fitness curves for representative insect taxa from temperate (A) and
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impact of global climate change in a spatially explicit and
empirically constrained way.
Estimating Impact on Insects. Insects are the largest group of
terrestrial organisms. The impact of temperature on intrinsic
rates of population growth (r), a direct measure of Darwinian
fitness, has been quantified experimentally for numerous insect
species from around the globe (15). We use these data to
construct fitness curves for each species to calculate the frac-
tional change in population growth rate from the observed
climate of the late 20th century (1950–1990) (19) to a model-
simulated climate (20) of the late 21st century (2070–2100),
where climate data were taken at the source site of each species
(see supporting information (SI) Methods and Figs. S1 and S2).
ambient air temperature, and fitness is averaged through diurnal
and monthly temperature variations, implicitly accounting for
potential shifts in organisms’ preferred activity times (see SI
Methods). The difference between population growth rates
under current versus projected climates quantifies the direct
thermal impact (positive or negative) of future warming on the
fitness of organisms.
After a century of warming, population growth rates of insects
change dramatically and exhibit a conspicuous latitudinal trend
(Fig. 1). At mid- to high-latitudes, population growth rates are
predicted to increase, indicating enhanced population fitness
because of warming. In the tropics, however, intrinsic rates of
population growth are expected to decrease by up to 20%,
implying that warming will substantially reduce fitness. This
latitudinal trend is robust: it is insensitive to how fitness is
averaged through time, or to potential differences between
environmental and body temperatures (see SI Methods and Figs.
S3 and S4). Instead, the pattern of global warming’s impact on
insect fitness derives from fundamental geographic relationships
between climate and physiological performance that can be
distilled into two simple heuristic indicators, and that are
observed among several other taxa.
Warming Tolerance and Thermal Safety Margin. The impact of
warming on insect fitness at each location depends on several
factors, including the breadth of each performance curve, its
position relative to the mean climate, and the spectrum of local
temperature variability. However, the basic latitudinal trend in
impact can be qualitatively elucidated by using two simple
metrics that characterize the geographic covariations of fitness
curves and climate. In addition to providing a heuristic expla-
nation for the impact of warming across latitude, these two
metrics allow impacts diagnosed for insects at point locations to
be extrapolated globally and generalized to other ectotherm
In the context of long-term climate warming, a key charac-
teristic of an ectotherm’s performance curve is the difference
between its critical thermal maximum and the current climato-
logical temperature of the organism’s habitat, Thab, here taken
to be mean annual surface air temperature. This quantity
approximates the average amount of environmental warming an
ectotherm can tolerate before performance drops to fatal levels,
and we refer to this difference as an organism’s ‘‘warming
tolerance,’’ (WT ? CTmax ? Thab). Although annual mean
temperature provides a single robust climate statistic for refer-
encing the position of performance curves, its use does not imply
that mean temperature determines an organism’s fitness, which
will vary throughout the year.
For insect species, CTmaxdecreases slightly with latitude (11),
but not as rapidly as does surface air temperature, either on an
annual basis or during the hottest months (Fig. S1). The warming
tolerance of tropical insects is, on average, only one-fifth that of
near-lethal temperatures much faster than will insects from
temperate climates, even though the rate of warming in the
tropics is predicted to be half that of high latitudes (6). The
latitudinal trend in warming tolerance alone will tend to increase
A second important characteristic of performance curves is
the difference between an organism’s thermal optimum (Topt)
and its current climate temperature, Thab. We call this difference
the ‘‘thermal safety margin’’ (TSM ? Topt? Thab). Species living
in environments that are already close to their physiological
optimum have small thermal safety margins, and thus even small
amounts of warming will likely decrease performance. In con-
trast, species with large thermal safety margins are living in
environments that are on average cooler than optimal, and
should experience initial increases in population growth rates
and performance as temperatures rise.
For insect species, thermal safety margins increase sharply
with latitude (Fig. 2B). Tropical insects are already living at
environmental temperatures very close to their physiological
optimum (TSM ? 0; see Acyrthosiphon pisum in Fig. 2), whereas
higher-latitude insects are experiencing environmental temper-
atures significantly below Topt, even during summer (TSM ? 0,
see Clavigralla shadabi in Fig. 2). The latitudinal trend in the
impact of climate change inferred from warming tolerance alone
is thus accentuated by the poleward increase in thermal safety
margins, because it allows insect performance to increase in
colder climates, at least initially. Taken together, warming
tolerance and thermal safety margin thus provide simple but
u t i t a
Thermal Safety margin
safety margin (Topt? Thab; B), the primary heuristic indicators of the impact of
warming on the thermal performance of insects. The projected magnitude
tropical insects and raises the temperature of their habitats above their
thermal optima for much of the year, resulting in decreased thermal perfor-
mance, whereas temperatures will remain below the thermal optimum for
using annual mean temperature (Thab), the calculated impact of warming
on performance takes into account both seasonal and diurnal temperature
Deutsch et al.PNAS ?
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useful indicators for understanding the latitudinal trend in the
impact of warming calculated from detailed climate data and
individual insect performance curves (Fig. 1).
Global and Taxonomic Extrapolation. We now combine these met-
rics into a simple conceptual model that we use first to extrap-
olate results for insects to the global scale, and next to estimate
impacts of warming on other taxonomic groups for which only
limited performance data are available. In this model, warming
tolerance and thermal safety margins are assumed to capture the
most salient geographic variations in the performance curves of
ectotherms. Patterns of warming tolerance and thermal safety
margins are estimated from their empirical relationships to the
magnitude of the seasonal temperature cycle. Climate variability
has long been considered a mechanistic driver of differences in
thermal performance across latitude (10); high latitudes have
greater climatic variability, which should favor organisms with a
broad thermal tolerance. Indeed, seasonality is a strong predic-
tor of both warming tolerance and thermal safety margin of
insects (Fig. S5 and Table S1 ). We use these relationships to
interpolate insect fitness curves from specific locations to the
global scale. We then compute the impact of warming on fitness
worldwide, accounting for both seasonal and diurnal tempera-
ture cycles (see SI Methods).
The global impact of 21st century warming on insects in this
simplified model reproduces the qualitative pattern diagnosed
above using actual fitness curves of individual insect species (Fig.
3A). This concordance confirms that the most important geo-
graphic variations in fitness curves are indeed captured by the
heuristic indicators, warming tolerance, and thermal safety
margin. The most deleterious impacts of warming on population
growth rates are again predicted to occur in the tropics (Fig. 3B).
This result depends only on the latitudinal increase in warming
tolerance and is independent of the latitudinal increase in
thermal safety margins (see SI Methods and Fig. S6). In contrast,
the predicted increase in population growth rates at higher
latitudes is due entirely to the relatively large thermal safety
margins observed in cold climates and thus depends strongly on
the poleward trend in thermal safety margins (see SI Methods
and Fig. S6).
To examine the generality of these results beyond insects, we
and turtles), using published studies in which critical thermal
limits were experimentally determined for at least 12 popula-
tions of closely related taxa across large climate gradients
[(16–18), Table S1]. In each taxonomic group, warming toler-
ance of an organism increases strongly with the seasonal tem-
perature variability of its habitat (Fig. S5). This consistent
pattern indicates that warming will cause tropical vertebrate
ectotherms to approach their critical maximum temperatures
proportionately faster than similar high-latitude species, despite
lower absolute rates of tropical warming. Consequently, tropical
representatives of all four taxonomic groups will likely experi-
ence the most deleterious changes in thermal performance
during warming (Fig. 3C).
In contrast, many mid- to high-latitude vertebrates should
experience enhanced thermal performance because of warming,
because they tend to inhabit climates that are currently cooler
than optimal. Some high-latitude ectotherms may have narrower
thermal safety margins than do insects, and such species would
show more modest increases in performance than we have
predicted (Fig. 3C). In addition, if global temperatures continue
to rise beyond the 21st century, as projected under most climate
scenarios (6), even high-latitude species will begin to experience
o i t c i dne r p
1 . 0 -2 . 0 1 . 0
0 . 0 2 . 0 -
s t c
s n I
z i L
e l t r u
d r a
1 . 0 -1 . 00 . 0 2 . 0 -3 . 0 -
fit to intrinsic population growth rates measured for each species (black circles, from Fig. 1) and for a global model (red line) in which performance curves at
from the simplified conceptual model are shown globally for insects (B) for which performance data are most complete, and versus latitude for three additional
taxa of terrestrial ectotherms: frogs and toads, lizards, and turtles (C), for which only warming tolerance was available. On the basis of patterns in warming
tolerance, climate change is predicted to be most deleterious for tropical representatives of all four taxonomic groups. Performance is predicted to increase in
mid- and high-latitudes because of the thermal safety margins observed there for insects, and provisionally attributed to other taxa.
www.pnas.org?cgi?doi?10.1073?pnas.0709472105Deutsch et al.
decreased performance, as temperatures exceed their optimum
temperatures. Expanded performance datasets are needed both
to constrain the future fitness trajectories of high-latitude ecto-
therms and to further test the generality of the predicted impact
patterns across taxa. Critical thermal limits of marine species
may be especially illuminating (21–23), because the latitudinal
profile of seasonal temperature variability in surface ocean
waters exhibits minima in polar as well as tropical latitudes and
may lead to strong thermal sensitivity and thus reduced future
fitness in both climate extremes (21).
The impact of increasing atmospheric temperature on the ther-
mal performance of organisms is an important and direct
biological consequence of climate change and can be evaluated
on a global scale (Fig. 3). Individual organisms will react to
changes in their performance, whether positive or negative, in a
variety of ways including acclimation (10, 16, 24), adaptation
(25–27), and dispersal and behavioral modification (3, 4). These
responses and the resulting changes in ecosystem community
structure will modulate the global patterns of impact predicted
here. Evaluating these effects will require more fine-grained,
species-specific models (28, 29), as well as expanded datasets.
Even so, these processes seem unlikely to reverse our qualitative
conclusion that tropical ectotherms are most at risk in a green-
house future (11, 27, 30, 31).
Acclimation, adaptation, dispersal, and behavioral plasticity will
all help mitigate the adverse impacts of climate change but are
unlikely to completely offset the decreased fitness predicted here
for tropical organisms. The capacity of ectotherms to acclimate
their physiological performance curves is generally low in tropical
ameliorate the impacts of warming, but we currently lack a strong
empirical or theoretical foundation upon which to assess the
potential for evolutionary responses among tropical ectotherms to
the rapid rates of projected warming (32, 33). Changes in behavior,
including shifts in the timing of activity, and changes in range
boundaries have been documented particularly at high latitudes (3,
4, 34, 35), but such phenological shifts may be caused by either
be constrained by both physiological and ecological tradeoffs (5)
and, in the case of migration, by increasing habitat fragmentation.
Indeed, one of the best documented cases of climate-induced
extinctions, that of high-elevation tropical frogs (36, 37), illustrates
the complexity of impacts and the potential limitations in all of
Ultimately, organisms with the greatest risk of species extinc-
tion from rapid climate change are those with a low tolerance for
warming, limited acclimation ability, and reduced dispersal.
Most terrestrial organisms having these characteristics are trop-
ical and many of these organisms are occupying disappearing
climate regimes (38). This conclusion is troubling because it
places the greatest biological risks of climate change in the
tropics where biodiversity is greatest.
Climate Data. Baseline climate conditions for the late 20th century (1950–
2000) were obtained from the Climatic Research Unit CL 2.0 high-resolution
of warming using climate model projections from the fourth assessment
emissions scenario, and physical climate model. The latitudinal gradient of
all of these: a poleward intensification of warming emerges in every model,
anomalies (relative to a control) from 2070–2100 in a simulation from the
A2 greenhouse gas emissions scenario. Climate warming in each month was
added to the observed temperature distribution to eliminate any biases from
temperature range was assumed to retain its observed seasonal and geo-
graphic pattern into the next century. The spatial resolution of our results is
limited by that of the global climate model but is appropriate for present
Site-Specific Performance. To predict physiological consequences of climate
change, we must first estimate a thermal performance curve, P(T), at each
location for each taxonomic group. For insects, we also use the term fitness
curve because the ordinate is derived from intrinsic population growth rates,
a direct measure of Darwinian fitness. For all taxa, these curves are asymmet-
ric, and we use a Gaussian function to describe the rise in performance up to
CTmaxand higher temperatures (Fig. 1) (14):
T ? Topt
T ? Topt
for T ? Topt
for T ? Topt
This formulation of performance conforms to both observed and theoretical
considerations (14, 26). Because an organism’s critical thermal minimum
(CTmin) does not enter directly into this formulation (the Gaussian perfor-
mance at cold temperatures never strictly reaches zero), we adopt an opera-
tional definition of CTminas the temperature 4?Pbelow Topt, (i.e., CTmin?
Topt? 4?P), where performance reaches a low value (P ? 1.8%). We then
define the asymmetry, ?, of a performance curve as the ratio of its width on
the warm and cold sides of Topt,so that ? ? (CTmax? Topt)/(Topt? CTmin) ?
The fitness curves of insects are derived by fitting data on observed
intrinsic rates of population growth at multiple temperatures to the above
equation (14), yielding least-squares estimates of all three parameters [?p,
Topt, and CTmax(the temperature at which populations are unable to grow);
see Dataset S1 for data). To ensure physiologically realistic fitness curves,
the cost function for the least squares fit additionally penalized solutions
whose asymmetry deviated from a specified value. Because the true asym-
metry is unknown, we used a range of target values of ? from 2–8, thus
obtaining a family of plausible-fit curves. For each species, we took the
average of those solutions that satisfied two objective criteria: the rms
error in growth rates was ?10% of the mean value, and the asymmetry
remained between 1 and 10. Most of the insect species (38 of 46) yielded at
least one fitness curve meeting these criteria, and of the 8 species that did
not, only one would have yielded a thermal impact outside the main
latitudinal trend (Fig. 1).
Global Performance. To extrapolate the set of insect performance curves from
individual locations to the global scale, we use empirical relationships be-
tween the estimated thermal performance parameters obtained above, and
the seasonal climate variability where each species was studied. Seasonality is
chosen as the predictor variable because its global distribution is well known
(Fig. S2) and it has long been considered a mechanistic driver of variations in
the thermal performance of organisms (9, 10).
At the collection site of each insect species, mean habitat temperature
(Thab) and seasonality (?t) were extracted from high-resolution climate data
(see Climate Data) as the mean and standard deviation of monthly surface air
temperatures, respectively (25). Consistent with theoretical expectations (9,
Table S1) and also of thermal safety margins (TSM ? Topt? Thab; Table S1).
These correlations simply reference the position of performance curves to
local climate conditions, and are used with the same global climatology (Fig.
S2) to compute empirically interpolated maps of both CTmaxand Topt. The
remaining free parameter, ?p, is assigned by noting that the asymmetry of
insect performance curves exhibits no large-scale pattern. We therefore use a
constant value of ? ? 3 that is typical of insects. A range of performance
asymmetry values and deviations from constancy was also investigated, but
the quantitative influence on the predicted impact was found to be small and
restricted to high latitudes, where current climate temperatures are much
cooler than Toptfor most species (see SI Methods). The similarity between
point estimates of impact based on actual performance curves and the global
maps derived in this model (see Fig. 3A) confirms that the patterns of WT and
captured by linear relationships to seasonality.
Deutsch et al.PNAS ?
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Theglobalmodeldevelopedforinsectscannowbeextendedtothreeother Download full-text
taxonomic groups: frogs, lizards, and turtles. For these vertebrate taxa, mea-
sured critical thermal limits have been published for multiple related species
at locations spanning large climate gradients (see Fig. S5 and Table S1), but
empirical thermal performance curves are generally unavailable. Using the
same procedure as for insects, we compute warming tolerance and its linear
are highly consistent across all taxa (Fig. S5).
Because estimates of Topt were not available for these taxa, we have
provisionally ascribed the relationship between thermal safety margin and
seasonality for insects to these taxa as well. Although this assumption intro-
duces some uncertainty into our prediction of increasing high-latitude per-
formance, it does not affect our conclusion that the most deleterious impacts
will be in the tropics. The strong latitudinal gradient in warming tolerance
permits no realistic pattern of thermal safety margins that could reverse the
overall latitudinal trend in impact (see SI Methods and Fig. S6).
ACKNOWLEDGMENTS. This manuscript was much improved by the sugges-
tions of three anonymous reviewers. This work was supported by a Program
on Climate Change fellowship (C.A.D.), by National Science Foundation (NSF)
Grant IOB-0416843 (to R.B.H.), and by NSF predoctoral fellowships (K.S.S. and
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