Beneficial Acclimation Hypothesis
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The beneficial acclimation hypothesis (BAH) is the
physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...
hypothesis A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educated guess o ...
that acclimating to a particular environment (usually thermal) provides an
organism An organism is any life, living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have be ...
with advantages in that environment. First formally tested by Armand Marie Leroi, Albert Bennett, and Richard Lenski in 1994, it has however been a central assumption in historical physiological work that acclimation is adaptive. Further refined by Raymond B. Huey and David Berrigan under the strong inference approach, the hypothesis has been falsified as a general rule by a series of multiple hypotheses experiments.


History and definition

Acclimation is a set of
physiological Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...
responses that occurs during an individual's lifetime to chronic laboratory-induced environmental conditions (in contrast to
acclimatization Acclimatization or acclimatisation ( also called acclimation or acclimatation) is the process in which an individual organism adjusts to a change in its environment (such as a change in altitude, temperature, humidity, photoperiod, or pH), ...
). It is one component of
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...
. While physiologists have traditionally assumed that acclimation is beneficial (or explicitly defined it as such), criticism of the adaptationist program by
Stephen Jay Gould Stephen Jay Gould ( ; September 10, 1941 – May 20, 2002) was an American Paleontology, paleontologist, Evolutionary biology, evolutionary biologist, and History of science, historian of science. He was one of the most influential and widely re ...
and Richard Lewontin led to a call for increased robustness in testing
adaptationist Adaptationism is a scientific perspective on evolution that focuses on accounting for the products of evolution as collections of adaptive traits, each a product of natural selection with some adaptive rationale. A formal alternative would be to ...
hypotheses.Gould, S.J and Lewontin, R.C. (1979). ''The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme.'' Proc. Roy. Soc. London B 205. pp. 581-598. The initial definition of the BAH, as published in 1994 in the Proceedings of the National Academy of Sciences by Leroi et al., is that
"acclimation to a particular environment gives an organism a performance advantage in that environment over another organism that has not had the opportunity to acclimate to that particular environment."
This definition was further reworked in an article in American Zoologist 1999 by Raymond B. Huey, David Berrigan, George W. Gilchrist, and Jon C. Herron.Huey, R. B., Berrigan, D., Gilchrist, G.W. and Herron, J.C. (1999). ''Testing the Adaptive Significance of Acclimation: A Strong Inference Approach.'' Am. Zool. 39. pp. 323-336 They determined that, following Platt's strong inference approach, multiple competing hypotheses were needed to properly assess beneficial acclimation. These included:
1. Beneficial Acclimation. Acclimating to a particular environment confers fitness advantages in that environment.
2. Optimal Developmental Temperature. There is an ideal temperature to develop at so individuals reared at an optimal temperature compete better in all environments.
3. Colder (bigger) Is Better. In ectotherms, individuals reared in colder environments tend to develop to a larger body size. These individuals therefore have a fitness advantage in all environments.
4. Warmer (smaller) Is Better. The inverse of Colder Is Better. Smaller individuals have a fitness advantage.
5. Developmental Buffering. Development temperature does not affect adult fitness.


Experimental tests

The majority of tests of the beneficial adaptation hypothesis have, following Krogh's principle, centered on the model organisms ''
Drosophila melanogaster ''Drosophila melanogaster'' is a species of fly (an insect of the Order (biology), order Diptera) in the family Drosophilidae. The species is often referred to as the fruit fly or lesser fruit fly, or less commonly the "vinegar fly", "pomace fly" ...
'' and ''
Escherichia coli ''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
''. More specifically, experimental tests have centered on easily measured temperature adaptation (although other systems have been studied; see Kristensen, T.N., Hoffmann, A. A., Overgaard, J., Sorensen, J.G., Hallas, R., and Loeschke, V. (2008). ''Costs and Benefits of Cold Acclimation in Field-Released Drosophila.'' PNAS. 105 (1). pp. 216-221.3.). Of the several experimental tests of the beneficial acclimation hypothesis, most have rejected it as a universal rule (see reviews Woods, H.A. and Harrison, J.F. (2002). ''Interpreting Rejections of the Beneficial Acclimation Hypothesis: When is Physiological Plasticity Adaptive?'' Evolution. 56(9). pp. 1863-1866.Wilson, R. S. and Franklin, C. E. (2002).'' Testing the Beneficial Acclimation Hypothesis.'' TREE. 17(2). pp. 66-70.). Initial experiments by Leroi ''et al.'', the first scientists to address this problem, tested only the beneficial acclimation hypothesis and not the subsequent hypotheses developed by Huey ''et al.'' Colonies of ''E. coli'' were acclimated for seven generations in two different temperature conditions: 32 °C and 41.5 °C. Colonies were then competed against each other at those temperatures. He found that, agreeing with the beneficial acclimation hypothesis, colonies acclimated at 32 °C competed better at 32 °C. However, at 41.5 °C, colonies acclimated at 32 °C competed better as well. This led to the authors' rejection of the generality of beneficial acclimation. Huey ''et al.'' examined four previously conducted studies, applying the five competing hypotheses, and found that none of the results of the studies could be entirely explained by beneficial acclimation. Instead, a combination of hypotheses were required to explain the observed patterns of acclimation.


Why acclimation is not beneficial

While it seems intuitive that acclimation would provide benefits to individuals, the majority of empirical tests of the hypothesis have rejected its general application. Why then is acclimation not beneficial? H. Arthur Woods and Jon F. Harrison examined the possible reasons in a 2002 ''
Evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
'' paper. They suggested that:
1. The timescale of
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...
is much longer than that of environmental variation.
2. Environmental cues for timing of adaptive acclimation are unreliable.
3. It may cost more to adapt than not to.
4.
Migration Migration, migratory, or migrate may refer to: Human migration * Human migration, physical movement by humans from one region to another ** International migration, when peoples cross state boundaries and stay in the host state for some minimum le ...
by adjacent populations may swamp out
genes In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
for adaptive acclimation.


Criticism

In response to continuing rejections of the beneficial acclimation hypothesis, a number of common criticisms of experimental tests have been developed:
1. The majority of studies have actually been examining developmental acclimation. That is, rather than acclimating an adult individual and testing, they suggest that developmental switches triggered by particular temperatures result in a different mechanism of acclimation. More recently, it has been found that adult acclimation and developmental acclimation lead to support for different hypotheses.
2. Most studies have included stressful temperatures. Acclimation to those temperatures may decrease fitness in an individual.
3. Finally, a variety of traits are examined in these studies that may only be indirectly linked to fitness. For example, examining
longevity Longevity may refer to especially long-lived members of a population, whereas ''life expectancy'' is defined Statistics, statistically as the average number of years remaining at a given age. For example, a population's life expectancy at birth ...
as a fitness measure in ''D. melanogaster'' may be irrelevant since
fertility Fertility in colloquial terms refers the ability to have offspring. In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to reproduce, which is termed fecundity. The fertility rate ...
declines rapidly with age in this species.


Current state

The majority of studies have concluded the beneficial acclimation hypothesis is not true in all cases, and that alternate hypotheses should be tested. In addition to this, recent studies of the hypothesis have provided additional complications, such as trade-offs evident only in field environments and interactions with behavior and life history traits.Deere, J.A. and Chown, S.L. (2006). ''Testing the Beneficial Acclimation Hypothesis and Its Alternatives for Locomotor Performance.'' Am. Nat. 168(5)Marais, E. and Chown, S.L. (2008). ''Beneficial Acclimation and the Bogert Effect.'' Ecol. Lett. 11. pp. 1027:1036. The study of developmental and phenotypic plasticity continues.


See also

* Acclimatisation *
Adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...
*
Ecophysiology Ecophysiology (from Greek , ''oikos'', "house(hold)"; , ''physis'', "nature, origin"; and , '' -logia''), environmental physiology or physiological ecology is a biological discipline that studies the response of an organism's physiology to envir ...
* Evolutionary physiology * Fitness *
Physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...


References

{{reflist Physiology Ecology