= Solution
\b[<behavioural plasticity> can slow inherited change by buffering selection, or accelerate it by permitting persistence and exposing organisms to new selection. There is no general ranking without specifying the trait, environment and timescale.] The immediate adjustment of an individual's <animal behaviour> is a change in expression. <evolution> of that response requires a change in inherited properties of a <biological population>. A learned innovation spreading through <cultural inheritance> is another form of transmitted change, which should be distinguished from a change in <allele frequencies>.
A <reaction norm> makes this distinction explicit. Suppose a behavioural measurement in environment $E$ is $z=a+bE$, with inherited variation in baseline $a$ and responsiveness $b$. For illustration, let the favoured value be $E$, and let relative <genotype fitness> be:
$$
w(a,b,E)=\exp\{-k[a+bE-E]^2-cb^2\},\qquad k>0,\quad c\geq0.
$$
The first term penalizes mismatch; the second represents a possible cost of responsiveness. This is a deliberately simple model, not a claim that all <animal behaviour> has one optimum. Its <phenotypic selection gradient> on baseline, holding $b$ fixed, is:
$$
\frac{\partial\log w}{\partial a}=2k[(1-b)E-a].
$$
Immediately after a change to $E=1$, individuals with $a=0,b=1$ match the optimum and experience no mismatch selection on $a$; otherwise identical individuals with $b=0$ experience a gradient $2k$. Thus <phenotypic buffering> can reduce selection for inherited baseline change even when the response is highly beneficial. If $c>0$, selection on responsiveness can nevertheless persist. Buffering one dimension of the <phenotype> does not stop all <evolution>.
This impeding effect is biologically plausible when animals can avoid adverse conditions. Moving to a cooler microhabitat can reduce exposure to heat and therefore weaken selection on heat tolerance. Switching food can similarly reduce selection on digestive performance for an avoided resource. These are conditional mechanisms: the relevant evidence would be reduced differences in survival or <reproduction> among inherited physiological variants, rather than mere observation that animals move or switch foods. <learning> can also allow several inherited variants to reach similar behavioural performance, weakening selection among them. Such buffering may preserve <genetic variation>, which can become consequential after later environmental change; slow immediate change can therefore coexist with greater long-term evolutionary potential.
Acceleration begins with the demographic problem. An inherited advantage cannot spread in a lineage that disappears before it is expressed or transmitted. A flexible response can allow individuals to survive and reproduce after an environmental shift, providing time for subsequent inherited <adaptation> and possibly <evolutionary rescue>. This need not increase the selection coefficient of a particular variant. Increasing the number of surviving individuals or generations available for selection can matter even while the per-generation gradient becomes smaller. A comparison of change per generation and total <adaptation> over a fixed number of years can consequently give different answers.
<behavioural plasticity> can also change which environments impose selection. A newly learned <foraging> technique may expose animals to a new resource, making inherited differences in handling, digestion or sensory discrimination important. <habitat preference> can alter encounters with competitors and mates; repeated use of a new host can influence <gene flow> when mating occurs there. The <host fidelity> of <Rhagoletis pomonella> provides evidence that host-associated actions can constitute a premating barrier. It demonstrates a route from <animal behaviour> to evolutionary consequences, but does not by itself establish that the initiating host shift was a plastic response. <Host fidelity is an effective premating barrier between sympatric races of the apple maggot fly by Jeffrey L. Feder and others>[PNAS 91, pp. 7990–7994; three mark-and-recapture studies and host-associated gene flow]{ref} Establishing that historical sequence is necessary before counting the case as <plasticity-led evolution>.
In the <Baldwin effect>, acquired adjustments change survival and the selection subsequently experienced by inherited variants. <genetic accommodation> can then refine the threshold, specificity or efficiency of an environmentally induced response. Its outcome may be greater <phenotypic plasticity>, a differently tuned response, or <genetic assimilation>, in which expression becomes less dependent on the original inducing condition. No step requires a learned action to be copied directly into <DNA>.
Experiments on <spadefoot toad> tadpoles provide relevant, though not exclusively behavioural, evidence. Populations that expressed the carnivore ecomorph more frequently showed refinement of associated resource-use performance and morphology under laboratory comparison. Such correspondence is predicted if recurrent induction exposes the expressed <phenotype> to selection. The comparison supports inherited refinement following environmental responsiveness, but contemporary populations are not a direct film of the ancestral transition, and their prior ecological histories could differ. <Plasticity-led evolution: evaluating the key prediction of frequency-dependent adaptation by Nicholas A. Levis and David W. Pfennig>[laboratory competition and expression of the carnivore ecomorph in <spadefoot toad> tadpoles]{ref} Classic selection on heat-induced crossveinless wings in <Drosophila melanogaster> also showed that a previously induced <phenotype> could eventually appear without the inducing treatment. That is evidence for <genetic assimilation>, not direct evidence that behavioural innovations generally accelerate <evolution>. <Genetic assimilation of an acquired character by Conrad Hal Waddington>[Evolution 7, pp. 118–126; heat-induced crossveinless wings and selection across generations]{ref}
The balance depends on cue reliability, environmental persistence and <costs of phenotypic plasticity>. A response based on a formerly reliable cue can become maladaptive after environmental change. A persistent new environment may favour a cheaper constitutive solution, whereas recurring variation can favour continued flexibility. Response delays, limited sensory discrimination and trade-offs between alternative activities can leave substantial residual selection. Conversely, sufficiently effective and inexpensive flexibility can remove much of the benefit of further inherited change. The availability of relevant <genetic variation> and its <genetic covariance> with other traits also constrain the response.
A decisive study would separate immediate adjustment from inherited divergence. Replicated populations could experience stable and fluctuating environments, with variation in access to a learned response or environmental cue. Descendants should then be assayed in a <common garden experiment>, both with and without that cue. Repeated measures estimate within-individual change; <pedigrees> or crosses estimate inherited <reaction norm> differences. Survival, <reproduction>, population persistence and changes in other traits must all be measured. Manipulating responsiveness requires controls for sensory damage, <physiological stress>[stress] and general incapacity: disabling a neural process and then observing extinction would not isolate an evolutionary benefit of flexibility. Comparisons across <species> additionally require control for <phylogenetic nonindependence> and ecological opportunity.
\b[The strongest conclusion is conditional: effective adjustment often weakens immediate selection on the buffered trait, while persistence, new ecological exposure and inherited refinement can increase cumulative evolutionary change.] Those mechanisms can operate together within the same lineage; their relative importance is an empirical question rather than an intrinsic property of flexible <animal behaviour>.
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