Download Evolution in response to climate change

Prospects & Overviews
Juha Merilä
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Climate change is imposing intensified and novel selection pressures on organisms by altering abiotic and
biotic environmental conditions on Earth, but studies
demonstrating genetic adaptation to climate change
mediated selection are still scarce. Evidence is accumulating to indicate that both genetic and ecological constrains may often limit populations’ abilities to adapt to
large scale effects of climate warming. These constraints
may predispose many organisms to respond to climate
change with range shifts and phenotypic plasticity,
rather than through evolutionary adaptation. In general,
broad conclusions about role of evolutionary adaptation
in mitigating climate change induced fitness loss in the
wild are as yet difficult to make.
.
Keywords:
adaptation; climate change; evolution; genetic variation
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Introduction
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Climate on Earth is changing rapidly: the global surface temperature has increased by 0.2 8C per decade over the last 30 years,
corresponding to what is often referred to as an unprecedented
0.8 8C over the past 100 years [1]. Unsurprisingly, ecological
responses to the ongoing change in climate are widespread
and strong: meta-analyses show strong shifts in species
phenology and range distributions [2–5]. There is also evidence
for adaptive, genetically-based responses to climate-mediated
DOI 10.1002/bies.201200054
Ecological Genetics Research Unit, Department of Biosciences, University
of Helsinki, Helsinki, Finland
Corresponding author:
Juha Merilä
E-mail: [email protected]
Supporting information online
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
selection (Table 1), but as compared to massive evidence for
ecological responses (e.g. [6, 7]), solid evidence for evolutionary
responses to ongoing climate change from the wild is far
thinner [8].
There are a number of possible explanations as to why
so few studies from the wild have reported evolutionary
(i.e. genetic; Box 1) responses to climate-mediated selection.
Some of these emphasize methodological (or technical) issues,
whereas others represent real biological phenomena. The
distinction between the two classes of explanations is somewhat arbitrary, as methodological concerns may overshadow,
and even hide, interesting biological phenomena. There are
also explanations which are more conceptual or philosophical
in nature, and as I will argue below, these may suggest some
degree of perception bias in respect to our expectations: while
extrapolation from synchronic (i.e. indirect, spatial) studies
provide good reasons to believe the power of natural selection
may result in local adaptation, and hence, also for adaptation
to climate change, we tend to forget that synchronic studies
exclude an unknown fraction of populations which have
failed to adapt and become extinct. In contrast, allochronic
(i.e. direct, longitudinal) studies – such as those examining
evolution in response to climate change in time-series data –
are likely to capture a mixture of populations on their way
towards adaptation and extinction, respectively. Viewed
in this perspective, and especially in combination with the
theoretical considerations suggesting that the current rate of
climate change may be far too fast for many populations to
track it [9–11], the relative lack of evidence for evolutionary
adaptation to climate change mediated selection might
actually not be as surprising and puzzling as it first appears.
In the following, I will briefly review and discuss the merits
and shortcomings of the ideas put forward explain the paucity
of evidence for climate change mediated adaptation. I will do
this by following and building upon the critical review of
ref. [8], with particular focus on the identification of problems,
and progress made in resolving them, as emerging from the
recent literature. Acknowledging the existence of interesting
experimental evolution work done in climate change context
(e.g. [12, 13]), I will focus in the empirical evidence from the
wild. Although several other reviews have recently focused on
climate change adaptation (Fig. 1), many of the perspectives
and issues covered here have not been articulated in these
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Problems & Paradigms
Evolution in response to climate change:
In pursuit of the missing evidence
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Problems & Paradigms
Table 1. Studies demonstrating evolutionary response to climate change mediated selection
Species
Fruitfly (Drosophila subobscura)
Fruitfly (Drosophila melanogaster)
Fruitfly (Drosophila robusta)
Pitcher-plant mosquito (Wyeomyia smithii)
Red squirrel (Tamiasciurus hudsonicus)
Blackcap (Sylvia atricapilla)
Field mustard (Brassica napa)
Tawny owl (Strix aluco)
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Response
Inversion polymorphism shifts
Adh allele frequency shifts
Inversion polymorphism shifts
Change in critical photoperiod length
Advanced parturition date
Change in migratory behaviour
Change in flowering time
Plumage polymorphism change
Selection
Inferred
Inferred
Inferred
Inferred
Measured
Inferred
Measured
Measured
Reference
[85]
[86]
[87]
[88]
[89]
[90]
[91]
[92]
13% of the response genetic, 62% environmental (but see [23]).
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earlier treatments. And while this review cannot provide clearcut resolution and answers to all issues raised, it should
facilitate progress towards a better understanding of the incidence and pace of adaptation to climate change by highlighting issues and areas in need of further attention.
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Methodological and technical
considerations
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Limited research effort?
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Genetic
evidence
Yes
Yes
Yes
Yes
Yesa
Yes
Yes
Yes
Lack of interest and modest research effort on documenting
genetic responses to climate-mediated selection would provide a straightforward and trivial explanation for the paucity
of evidence. While it is true that ecological effects such as
range shifts and phenological changes are easier to document
than evolutionary responses – there is an enormous and
increasing interest directed towards evolutionary con-
sequences of climate change (Fig. 1). The number of review
and perspective articles dealing with the evolutionary consequences of climate change mediated selection has been
increasing almost exponentially during the past years, and
their number already exceeds that of case studies finding
evidence for climate change mediated evolution by a factor
of four (Fig. 1). Furthermore, there are also a large number of
studies which have sought to document climate change mediated evolution, but failed to find any (reviewed in refs. [8, 14]).
Hence, lack of interest and research effort alone is unlikely to
explain the low incidence of case studies providing evidence
for climate change mediated evolution. This is especially true
in view of the intensive research efforts invested in particular
model systems (e.g. Drosophila, passerine birds), as well as
possible – if not likely – publication bias in favour of studies
finding evidence for adaptation to climate change.
Box 1
Evidence for climate change driven evolution
requires genetic proof
Evolution is refers to a genetic change in a population
over generations. Genetic changes can be caused by
migration, mutation, genetic drift or selection, of which
only the latter can be considered as adaptive, in situ
evolution. Hence, in order to demonstrate a climate
change driven adaptive evolution in a given trait, a
genetic shift in the mean trait value needs to be demonstrated. This can be done either by utilizing quantitative genetic [39] or molecular/population genetic (e.g.
[15, 16]) methods. Genetic evidence is critical because
traits important for climate change adaptation are often
phenotypically plastic, and plastic responses can be
similar to genetic responses. For instance, birds can
advance their breeding time in response warming springs
either by plastic or genetic responses. Ideally, also the
selection pressure causing the trait shift should be
measured, or at least inferred, to establish the causality
between the observed trait shift and climate change [8].
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Figure 1. Cumulative number of review (including perspectives; solid
line) and primary research articles (dotted line) focused on evolution
in response to climate change as a function of their publication year.
Primary research articles include only the studies (listed in Table 1)
considered to have provided evidence for evolution in response to
climate change (Box 1). References behind each of the depicted
lines are given in Table 1 (primary research articles) and Supporting
information (reviews).
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Box 2
Counter-gradient variation refers to phenomenon in
which genetic and environmental influences on trait
oppose each other across populations along an environmental gradient [20]. Depending on the relative strengths
of the two effects, phenotypic differentiation across the
gradient may be uncorrelated with genetic differentiation, or correlated either positively or negatively with
it. Hence, in the counter-gradient variation situation –
known to occur commonly in wide variety of organisms
[20, 25] – environmental variation can effectively uncouple phenotypic and genetic differentiation. This highlights the importance of genetic evidence in studies of
climate change adaptation: changing environmental
condition over time have potential to shift phenotypic
trends without any changes in genotypic trends.
Likewise, changing environmental conditions can hide
evolutionary response (i.e. genetic change) to climate
change if only phenotypic data is analysed [21].
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Detectability problems provide yet another – and perhaps
underrated – explanation for the paucity of evidence.
Namely, while genetic responses to selection may actually
have occurred, the analytical tools and approaches used to
detect these changes might have been too blunt to pick up the
signal from the noise and/or control for confounding factors.
For instance, counter-gradient variation – referring to the
phenomenon in which environmental influences on trait value
oppose those of genetic influences (Box 2) – may conceal
evolutionary responses at the level of the phenotype [20,
21]. The potential importance of this concern is high-lighted
by the fact that several studies that have estimated temporal
changes in breeding values of traits under climate mediated
selection have found discordant patterns of phenotypic and
genetic change (Table 2). However, estimation of breeding
values from data collected from the wild are subject to a
number of technical challenges and even biases [22–24],
and the power of such analyses to detected weak responses
may be low (e.g. [17]). Yet, the growing number of examples of
counter-gradient variation in the wild (e.g. [25, 26]), as well as
discordant patterns of phenotypic and genotypic variation
in long-term studies of individually marked populations
(Table 2), should keep us on alert with respect to the detectability problem.
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Problems & Paradigms
Counter-gradient variation and adaptation to climate
change
Detectability problems?
Lack or poor quality of data?
Lack of suitable, high-resolution data provides another
possible explanation. In order to demonstrate an evolutionary
response to climate change, one has to demonstrate that the
observed phenotypic response is genetically based (Box 1).
This can be done either by demonstrating allele frequency
shifts underlying the phenotypic response (e.g. [15, 16]), or
by inferring those shifts with the aid of quantitative genetic
analyses (e.g. [8, 17, 18]). The analytical tools and data
required for such analyses are not within reach of all researchers and study systems. However, even in the cases where
researchers have had an access to data and methods needed
to demonstrate adaptation, expected responses have not
always been detected (e.g. [17, 19]). Thus, while the relative
rarity of high-quality and high-resolution biological timeseries datasets is limiting our ability to infer evolution in
response to climate change, it seems unlikely that lack of
data alone provides a general explanation for the observed
rarity of examples for evolution mediated by ongoing climate
change.
Biological explanations
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Evolutionary time lags?
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Another possible – and mutually non-exclusive – explanation
for the paucity of evidence for evolutionary responses to
climate change mediated selection is provided by evolutionary
time lags. Although an increasing number of studies support
the conjecture that evolutionary rates can be fast [27] and
ecological and evolutionary times scales are often similar
[28, 29], evolutionary responses can still be slow to occur,
for instance in species with long generation times (e.g. [30–
32]). Furthermore, it is plausible that populations can be
to some extent buffered against evolutionary changes. For
instance, phenotypic plasticity (e.g. [33]) and migration from
neighbouring populations (e.g. [34]) can buffer populations
from responding immediately to selection. However, in the
light of several examples of rapid evolutionary responses to
natural selection (e.g. [27, 35]), evolutionary time lags alone
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Table 2. Phenotypic (P) and genetic (G) trends in mean body mass of vertebrates subject to long-term study using methods allowing
inference about genetic basis of the observed trends. þ ¼ positive trend, ¼ negative trend, 0 ¼ not trend over time
Species
Red-billed gull (Larus novahollandiae)
Soay sheep (Ovis aries)
Yellow-bellied marmot (Marmorata flaviventris)
Great tit (Parus major)
Great tit (Parus major)
Great tit (Parus major)
Siberian jay (Perisoreus infaustus)
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No. of years
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Reference
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are unlikely to provide a general explanation for the observed
paucity of evidence.
Ecological constraints and niche conservatism?
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Ecological constraints provide yet another category of diverse
biological explanations for lack of evolutionary responses to
climate change. Organisms live in complex communities subject to selection pressures stemming from networks of biotic
and abiotic interactions. Hence, it is not difficult to envision
that changes in selection pressures caused by climate change
are directly or indirectly modulated by the ecological interactions the population in question is experiencing (e.g. [36]).
In fact, according to theoretical models, ecological communities are buffered against change through niche conservatism:
species interactions may inhibit evolution and promote
stasis [37].
Yet, however complex these interactions, a heritable trait –
not constrained by any of the genetic complexities listed above
– found to be under net directional selection is expected to
respond to the selective pressures acting upon it. If no
response is observed, ecological constraints are an unlikely
explanation for this. However, this reasoning holds true only if
the net forces of selection are accurately measured – an
assumption which may not always hold true because fitness
and selection in the wild are extremely hard to estimate, and
because spurious correlations between fitness and focal traits
occur (e.g. [38] and references therein).
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Genetic constraints provide yet another class of explanations
as to why populations might not respond to selection. The
most obvious and ultimate constrain is a lack of additive
genetic variance in a trait under selection. While most traits
are commonly thought to be heritable in most populations and
able to evolve in response to selection [39], several recent
studies indicate either total lack or very limited additive
genetic variance in key tolerance traits in the wild [40–42]
(see also [43]). For instance, a study of ten species of tropical
and high-latitude Drosophila species found consistently low –
almost non-existing – additive genetic variance for desiccation
and cold tolerance among the five tropical Drosophila species
studied, whereas the corresponding values for the high-latitude species were high [41]. Likewise, [42] found no or little
additive genetic variance for thermal tolerance in populations
of the copepod, Tigriopus californicus, sampled along the west
coast of North America. Similarly, [19] found that strong
selection for early breeding in Red-billed gulls (Larus novaehollandiae) appears to be constrained by low additive genetic
variance in breeding time.
Even if traits under climate-mediated selection have
additive genetic variance, selection responses may be constrained if that variance is not expressed under conditions in
which selection acts [44, 45]. A recent study of Soay sheep
found an inverse correlation between intensity of selection
acting on a trait and heritability of that trait (body size) across
different study years, suggesting that environmental conditions which determined the strength of selection influenced
also the expression of genetic variance in the trait, thereby
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constraining the expected response to selection [46]. A similar
correlation between strength of selection and trait heritability
was also demonstrated in the breeding time of great tits,
though with the opposite effect: when selection for early
breeding was strong, heritability of breeding time also tended
to be high [47]. These two examples – showing opposing
associations between the strength of selection and levels of
expression of additive genetic variance – illustrate the difficulty in predicting selection responses: changing environmental conditions may not only influence the selection pressures,
but may also lead to changes in the genetic variability available for selection to work with. Unfortunately, we do not
currently have any general theory or framework to predict
how different environmental conditions influence expression
of genetic variance in any trait [45](but see [48]). Nevertheless,
given that environmental influences on expression of genetic
variation are widespread, discounting them as potential contributors to the paucity of positive evidence for climate change
mediated evolution would be premature.
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Genetic covariances among traits?
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Apart from issues with genetic variance, other forms of genetic
constraints are also possible. Genetic covariances among traits
have been implicated as potential constraints for adaptation
to climate change [49], but very little empirical research has
been done with them in the context of climate change
responses (but see [49, 50]). However, genetic covariances
among traits can pose severe constraints to adaptation and
even prohibit selection responses, even if the trait under
selection is heritable (e.g. [51]). Furthermore, similarly to
heritability, genetic covariances are also influenced by
environmental conditions (e.g. [52]), and hence, climate
change may also impact their sign and strength. Therefore,
while little can be said about the role of genetic correlations in
explaining the paucity of evidence for climate mediated selection, it would not be surprising if future research would
identify them as an important factors explaining unexpected
trait dynamics (e.g. [53]) or lack of adaptation to climate
change (cf. [49]).
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Other forms of genetic constraints?
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Aforementioned genetic complications aside, selection
responses may also be lacking or deviating from those
expected due to other kinds of genetic constraints. For
instance, epistatic gene interactions may lead to unexpected
evolutionary dynamics of traits under selection [54]. However,
little is known about their importance in almost in any context
due to the inherent difficulties in estimating them (e.g. [39,
55]). However, recent empirical work hints at the possibility
that effect of epistasis is often constraining evolutionary
responses to selection [56, 57]. Likewise, indirect genetic
effects – defined as a situation where the individual’s phenotype is influenced by genes of other individuals, such as its
mate or the social group to which it belongs [58] – can either
facilitate of constrain a response to selection [58, 59]. Again,
little is known about their importance in a climate change
setting, but the work by Teplitsky et al. [19] illustrates their
potential relevance in this very context.
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Phenotypic plasticity
Recent theoretical considerations highlight the potential
importance of phenotypic plasticity as a step towards adaptation to climate change: plastic responses favoured by
natural selection may become genetically assimilated in a
population and lead to genetic adaptation [33, 60]. Apart from
these theoretical considerations (see also [11, 61]), empirical
evidence regarding the importance of genetic assimilation in a
climate change context remains yet to be demonstrated.
However, there is ever increasing evidence to suggest the
importance of phenotypic plasticity as a possible means by
organisms may cope with climate change. Breeding time
changes in birds provide several good examples of this.
Considering the case studies to date, evidence strongly
suggests that observed shifts are plastic, rather than microevolutionary responses to climate change (reviewed in ref. [8]).
For instance, a detailed study based on extensive highresolution data using individually marked great tits in
Britain suggests that they track the changing phenology with
plastic responses [62]. Compelling evidence for the ‘adaptive’
nature of this plasticity is provided also by a comparative
study of European birds showing that species which have
not changed their breeding time in response to warming
climate show declining population size trends [63].
Here, one should also consider the possibility that the odds
of finding evidence for plastic responses to climate change
are probably higher than those of finding evidence for an
evolutionary response. This is probably true for two distinct
reasons. First, given that phenotypic plastic itself can be a trait
optimized by natural selection to buffer individuals and populations towards environmental heterogeneity, we would perhaps expect to see it as ‘the first line of defence’ in response to
climate change mediated fitness loss. As such, it might also
provide a partial answer to the paucity of empirical evidence
for climate-mediated microevolution: as long as individual
plasticity can alleviate the negative consequences of climate
change against fitness loss, it also acts to reduce the force
of selection on the trait under selection. Behavioural thermoregulation in ectotherms provides a good example – there
may be little need for physiological adaptation if behavioural
thermoregulation can be used as a buffer against climate
warming [64, 65].
The second reason to expect more evidence for phenotypic
plasticity than for microevolutionary changes in the context of
climate change studies relates to higher methodological and
data demands of evolutionary inference: the burden to obtain
genetic evidence (Box 1). Likewise, the interpretational complications (c.f. [23]) elaborated above are also likely to favour
detection of plastic, rather than evolutionary, responses. That
said, one should keep in mind that environmental induced
plasticity can also be demanding to demonstrate. For instance,
the effect of transgenerational plasticity – defined as the direct
effect of parental growth conditions on offspring phenotype
on subsequent generations – on offspring growth in sheepshead minnows can exceed the single-generation rate of adaptive evolution by 30% [66]. Such effects cannot be deduced
purely from observational data, but rather, require controlled
experiments to disentangle them from other (genetic and
environmental) sources of variation.
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J. Merilä
Yet, while acknowledging these considerations, numerous
bird studies have demonstrated that the observed population
mean responses to climate change can usually be accounted
for by plasticity expressed by individuals (reviewed in ref. [8]).
Likewise, most of the studies which have been able to disentangle genetic from plastic responses in the context of climatemediated selection have failed to yield evidence for a genetic
basis for these changes (e.g. [17, 62]). Admittedly, given
methodological concerns, these studies often cannot exclude
the possibility that some genetic response has actually taken
place, but with all we know and can infer, plasticity seems to
play major role. Temporal body size trends observed in longterm studies of vertebrates provide a case in point. Mean
body size in several different species and populations has
declined over time in parallel to increasing ambient temperatures [67, 68] (Table 2). These trends were initially interpreted
as adaptive responses to warming climate in the context of
Bergmann’s rule [67]: increased surface-to-volume ratio of
smaller sized individuals was postulated to be favoured by
selection. However, it is problematic to explain these trends as
adaptive without genetic evidence [8, 18] (Box 1). For instance,
the observed body size decline in red-billed gulls in Kaikoura,
New Zealand, from 1958 to 2002 was indicated to be reflection
of environmental deterioration, rather than adaptation to
climate warming [18]. In fact, there is growing evidence from
long-term studies of vertebrates to indicate that the temporal
phenotypic trends in mean body size are environmentally,
rather than genetically driven (Table 2; see also [69]).
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Perception bias and other considerations
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An interesting observation in studies finding little or no
additive genetic variance for key tolerance traits within
local populations of D. birchii and C. tigriopus is that different
populations of these species show genetic divergence with
respect to the very same tolerance traits [40, 42]. This implies
that the genetic variance within populations – which
must have been there for these populations to be able to
differentiate in the first place – must have been subsequently
lost. While such a loss of genetic variability can easily be
explained by founder events and genetic drift acting in
finite populations, one cannot dismiss the possibility – at
least not without further evidence and inference – that the
genetic divergence among these populations might have
been influenced by other processes than selection (or drift)
acting on standing genetic variability. Namely, it is possible
that that adaptation was not based on standing genetic
variability, but initially seeded by hybridization among
divergent lineages (e.g. [70]), or that cryptic genetic variability
was released during the colonization process and subsequently lost due to the process of genetic assimilation
(e.g. [71, 33]).
The point of bringing up these considerations here is
mostly rhetorical: when inferring evolutionary adaptation
from synchronic (spatial) data, we seldom have the means
to infer what processes exactly were involved. In contrast,
when dealing with allochronic (longitudinal) data, the process
of adaptation is more readily inferred. When looking at
synchronic data sets – on which most of our inference
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and understanding of evolution and adaptation is based (e.g.
[72, 73]) – we are probably looking at historical evolutionary
events in which the mechanisms and processes involved with
adaption might be more heterogeneous than those confined
to a smaller sample of allochronic studies which we now
examine, for instance, in the context of ongoing climate
change. Further, as mentioned earlier, there is likely to be a
certain ‘invisible fraction’ of populations in allochronic studies which are on their way towards extinction, but which will
never become included in synchronic studies because they
were lost to extinction. Hence, it follows that in allochronic
studies we would expect to encounter populations which will
not adapt to ongoing climate change.
The possible bias towards finding more adaptation in
synchronic than allochronic studies comes conceptually close
to the phenomenon known as the ‘space-for-time substitution
problem’ in ecology [74]. Since allochronic data (i.e. timeseries) is time-consuming and logistically hard to obtain,
ecologists have used synchronic (spatial) comparisons as substitutes to infer the influence of different factors on ecological
processes. However, covarying factors, ecological time lags as
well as unknown population histories – to name but a few –
have been identified as sources of bias in these substitutions
[74]. Hence, apart from the methodological and biological
considerations raised above, it is possible that our inference
about the evolution in response climate change – similar
to that in ecology – may to some degree be biased with the
space-for-time substation approach.
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Are we overly pessimistic or optimistic?
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Given the theoretical considerations suggesting that the current pace of climate change may be too fast for most populations to be able to track it by evolutionary adaptation [9, 11]
even without considering the constraints (e.g. lack of genetic
variance) listed above, it would be easy reach the conclusion
that the odds of a population’s ability to track the change must
be very low. While this is a reasonable conclusion on the basis
of what we currently know, there are large gaps in our knowledge, even at the relatively superficial level of consideration
presented here.
To put this into perspective, consider information from
recent geophysical studies which suggest that the speed of
current and projected climate change may not be unprecedented, as is often stated [75]. Namely, these studies indicate
that abrupt and large (4–10 8C per year) temperature changes
might have taken place at the end of the last glaciations, and
these changes do not correspond to any known major extinction events. If so, this raises the obvious question: if the
organisms were able to cope with these changes – whether
by range shifts, plasticity or adaptation – why would they not
be able do so also in the face of the current changes? While the
answer for this question must await more detailed geophysical
and paleontological studies focused on the time-period in
question, these findings might provide a glimmer of hope
with respect to the fate of biodiversity in response to ongoing
climate change. As such, they also raise questions about
our ability to fully understand the relative importance of
evolutionary adaptation – in comparison to range shifts
Problems & Paradigms
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Prospects & Overviews
....
and phenotypic plasticity – in dealing with major climatic
changes.
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Spatial heterogeneity in selection
pressures
3
It may also be interesting to note that much of the research
done on evolution in response to climate change has been
focused on terrestrial systems, and less so in aquatic systems
(cf. Table 1; but see: e.g. [76–78]). This is understandable in the
view that the long-term studies of individually marked organisms – as well as studies requiring common garden rearing –
are often (but not always) more readily conducted in terrestrial
organisms. Yet, the magnitude and pace of temperature
increases in aquatic systems are comparable to – and sometimes even exceed – those seen in terrestrial habitats [79]. For
instance, a recent study examining warming trends in large
marine ecosystems found large heterogeneity in warming
rates: land-locked or semi-enclosed seas have been warming
at rates two to four times the global mean rate [79]. This spatial
heterogeneity can also entail a great deal of heterogeneity in
selection pressures, and thereby, perhaps also in expected
rates of evolution in response to climate change.
The observed spatial heterogeneity in warming rates is
seen also in terrestrial systems, with expected rates of climate
warming being strongest at higher latitudes [80]. All else being
equal, this would also lead to the intuitive prediction that the
problems faced by organisms – and by inference, strength of
selection for adaptation – should be particularly pronounced
at higher as compared to equatorial latitudes (e.g. [81]).
However, as pointed out by Tewksbury et al. [82], this reasoning fails to account for the fact that the organisms in different
latitudes may differ in their sensitivity for temperature change
of a similar magnitude. In fact, ectothermic animals living in
the tropics have narrower temperature tolerance ranges than
those living in the temperate zone [82, 83]. Consequently, a
climate warming of similar magnitude would push a larger
fraction of tropical, rather than temperate, zone species out of
their tolerance range and perhaps to the brink of extinction
[82]. This highlights the difficulty of making generalizations
about expected selection pressures, and thereby also expected
rates of evolution in response to climate change, predictions
which are further complicated by possible latitudinal differences in genetic variability in critical tolerance traits [43].
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Conclusions
43
Taken together, the genetic evidence for climate change driven
evolution is still scarce, and there appears to be little consensus – or even compelling general explanations – among
evolutionary biologist as to why this is the case. Technical and
methodological difficulties, as well a lack of high-resolution
long-term data are likely explanations, but also biological
phenomena – such as genetic constraints and ecological
niche conservatism – provide plausible and mutually nonexclusive explanations. Good evidence and consensus about
the importance of phenotypic plasticity as a means of coping
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Prospects & Overviews
with climate change mediated selection pressures is emerging.
While phenotypic plasticity – together with dispersal to
favourable environments (cf. range shifts) – may provide
the ‘first line of defence’ to buffer organisms against fitness
losses in a warming world, they are unlikely to provide the
means for populations to continually track changing climatic
conditions unless plasticity provides a general spring-board
for evolutionary adaptation to climate change through the
process of genetic assimilation [33, 60]. Hence, although a
lot of new insight into climate change adaptation has been
gained during the past two decades, we are still far from a
situation were informed general predictions about a population’s ability to adapt to climate change can be made. In this
view, the words put forth by Holt [84] two decades ago still
provide a good a generalization about the state of affairs:
‘There is almost no species for which we know enough
relevant ecology, physiology and genetics to predict its evolutionary response to climate change’.
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Acknowledgments
I thank Andrew Moore, Scott McCairns and anonymous
reviewers for comments which improved the earlier versions
of this paper. Financial support was provided by the Academy
of Finland. I also thank the organizers – Klaus Fischer and
Wolf Blanckenhorn – of the ESEB 2011 symposium in ‘Climate
change and evolution’ for invitation to participate: this paper
was strongly inspired by the groundwork devoted to prepare
for that symposium.
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