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40°C, and no effect of lineages’ historical thermal environment on either their improvement at 20°C or the extent of their trade-off at high temperature. We do not yet know the underlying mechanisms responsible for the trade-off, but they are sufficiently prevalent to drive a general effect. However, approximately one-third of the experimental lineages achieved low-temperature adaptation without detectable high-temperature trade-offs; therefore, it cannot be necessary that every change conferring benefit in cold environments has a negative effect on function in warmer environments.

Evolutionary adaptation to a new environment necessarily involves the enhancement of certain traits, leading to improved function and an increase in fitness. However, adaptation may be accompanied by deterioration in other traits, which are presumably of less or no importance in the new environment. This decline in some characters during adaptation is termed a trade-off and is often viewed as a cost or constraint associated with adaptation (e.g., Futuyma and Moreno, 1988; Stearns, 1992; Futuyma, 1998; Sibly, 2002; Novak et al., 2006).

The assumption of cost associated with gain has been a fundamental premise of biological and evolutionary thought for centuries. For example, Darwin (1859b, pp. 147–148) states that “… natural selection is continually trying to economise in every part of the organisation. If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless structure.” The assumption of trade-offs continues to be an important component of thinking about adaptive evolution: “… improvements cannot occur indefinitely, because eventually organisms come up against limitations…. At that point, improvements in one trait may be achievable only at the expense of others—there is a trade-off between the traits” (Sibly, 2002). This way of thinking has embedded itself into the models and mindsets we use to study life history and morphological and physiological evolution. For instance, in regard to environmental adaptation, Levins’ (1968) principle of allocation explicitly incorporates fitness trade-offs and consequent niche shifts. Adaptation to cold environments, for instance, is predicted to entail the loss of performance in warm environments. Subsequent models in evolutionary physiology about thermal niche structure and biological responses to climate change have usually assumed trade-offs (e.g., Lynch and Gabriel, 1987; Pease et al., 1989; but see Gilchrist, 1995).

Although evolutionary trade-offs are widely assumed, demonstrating their existence can be difficult. Several approaches have been used,

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