Biology · Organisms, Environments & Ecological Niches
Acclimation, phenotypic plasticity and evolutionary adaptation
An imagined genetic background has a trait value of two in one environment and five in another. That change can occur without replacing the background. A population can also change its mix of genetic backgrounds. Both alter an average trait value, but through different accounts.
Life, Evidence & Biological Scale introduces observations, models and biological organisation. Energy, Enzymes & Metabolism introduces energy use and material requirements. Evolutionary Processes & Adaptation supplies the distinction between individual change and inherited population change. The numerical examples here need only counting, fractions and ordinary graph reading; the next chapter develops population accounts in more detail.
Back to the Biology mapBy the end, you can
Distinguish environment-dependent phenotype expression from genetic frequency change, and ask separately whether a response improves fitness.
The same genetic background can express different traits
Phenotypic plasticity means that a genotype can produce different phenotypes in different environments. The response can concern development, physiology or behaviour. Calling a capacity innate does not remove its environmental context, and calling a response plastic does not remove its genetic basis.
A reaction norm describes how a specified phenotype changes across environments for a genotype or genetic background. In the original model, G1 has values two and five; G2 has four and four. Each line compares the same assigned background across two environments.
These are phenotype values, not allele frequencies or fitness scores. A change along a line describes environment-dependent expression. A difference between lines at one environment describes a difference between supplied backgrounds. The crossed ranking makes it impossible to call one background universally larger without naming the environment.
The same background can have different expressions
Read the diagram in words
G1 has values two and five, an environmental contrast of +3. G2 has four and four, contrast zero. G2 is larger at the lower category, while G1 is larger at the higher. The background identities remain fixed across each line.
| Assigned environment category | Genetic background G1: Assigned phenotype units | Genetic background G2: Assigned phenotype units |
|---|---|---|
| Lower environment category | 2 | 4 |
| Higher environment category | 5 | 4 |
| Genetic background G1 | 3 |
|---|---|
| Genetic background G2 | 0 |
The larger phenotype depends on the environment
Connections link two categories rather than a time course
Two original stipulated stable genetic backgrounds and phenotype values at two environment categories. The lines are background comparisons, not necessarily repeated observations of one individual, continuous responses, fitness effects, real genotypes or acclimation procedures.
Individual adjustment has its own time and scope
Here acclimation refers to a physiological adjustment within an individual following changed conditions. Such adjustment is one form of plasticity. Developmental responses can also be plastic without being quickly reversible. Terminology concerning controlled and natural settings varies, so the stated individual response matters more than a label alone.
A two-environment reaction norm need not describe one individual moved back and forth. It could describe comparable individuals with the same background in different environments. The teaching lines stipulate that background comparison; they supply no time course, reversibility, exposure procedure or memory effect.
To claim a particular individual acclimated, a record must distinguish within-individual change from replacement by different individuals or changes in what was measured. To claim the adjustment is beneficial, it additionally needs a relevant performance or fitness comparison. A changed phenotype alone settles neither question.
Evolution concerns inherited population composition
Genetic evolution concerns changes in inherited variation across generations in a population. An individual's adjustment to its surroundings does not by itself establish such change. Plasticity and evolution can nevertheless interact because response capacities can differ genetically.
The frequency model supplies two inherited background categories in one stipulated population. In the first generation there are sixty G1 and forty G2 individuals; in the second, thirty G1 and seventy G2. The total is one hundred in each, while the category frequencies change.
The supplied categories are assumed genetically identified and inherited. Their changing frequencies therefore represent genetic population change in the model. The counts do not identify natural selection as the cause: reproduction, migration, random change and other history would need their own evidence.
The population mix changes while expression stays fixed
Read the diagram in words
The first generation contains sixty G1 and forty G2, fractions 0.6 and 0.4, mean phenotype 4.6. The second has thirty G1 and seventy G2, fractions 0.3 and 0.7, mean 4.3. G1's frequency changes by minus 0.3, thirty percentage points, or half its starting frequency. The cause and fitness consequences remain unspecified.
| Inherited background | Complete count | Fraction of this generation | Phenotype at the fixed higher environment |
|---|---|---|---|
| Inherited background G1 | 60 | 0.6 | 5 |
| Inherited background G2 | 40 | 0.4 | 4 |
| First supplied generation | (60 × 5 + 40 × 4)/100 = 4.6 |
|---|
| Inherited background | Complete count | Fraction of this generation | Phenotype at the fixed higher environment |
|---|---|---|---|
| Inherited background G1 | 30 | 0.3 | 5 |
| Inherited background G2 | 70 | 0.7 | 4 |
| Second supplied generation | (30 × 5 + 70 × 4)/100 = 4.3 |
|---|
Inherited background G1: 0.3 − 0.6 = -0.3; -0.3/0.6 = -0.5
Count-weighted phenotype mean: 4.3 − 4.6 = -0.3
Environment and background-specific expression remain fixed
One original stipulated population with two genetically identified inherited background categories, complete counts in two generations and fixed higher-environment expression values. No cause of frequency change, sampling estimate, allele-count conversion or fitness consequence is supplied.
A change is not automatically an improvement
Plastic responses can improve fitness in some settings and fail to do so in others. Evolutionary adaptation is a claim connecting inherited traits with selection and fitness in a context. It cannot be inferred from a trait becoming larger or from a population average changing.
At the higher environment category, G1 expresses five and G2 four. Changing the genetic mix from sixty percent G1 to thirty percent lowers the population's supplied average from 4.6 to 4.3. Within G1, changing environments instead raises the expressed value from two to five.
Those directions coexist without conflict because their comparisons differ. None of the values is a fitness measurement. The reaction norm and frequency account also omit mechanisms linking environmental information, inheritance and outcomes; they help identify those missing links rather than manufacture an adaptive story.
Worked example
Two routes to a different average
For G1, the higher-environment value minus the lower-environment value is 5 − 2 = 3. For G2 it is 4 − 4 = 0. The supplied environmental contrast depends on background, and their ranking reverses between the two endpoints.
At the fixed higher environment, the first population mean is (60 × 5 + 40 × 4)/100 = 4.6. The second is (30 × 5 + 70 × 4)/100 = 4.3. The decline of 0.3 comes from the changed background frequencies while the background-specific phenotype values stay fixed.
G1's frequency falls from 0.6 to 0.3, a decline of thirty percentage points or half its starting frequency. These are distinct descriptions. The model supplies no fitness effects, adaptive benefit, selection coefficient or prediction of rescue under changing conditions.
Check your understanding
Does the G1 trait change from two to five prove that its inherited background changed?
Show reasoning
No. The model holds G1's background fixed and changes its environment. This is an assigned plastic phenotype comparison, not a change in genetic composition.
Why can the fixed-environment population mean fall while G1's response to the higher environment is positive?
Show reasoning
Different quantities change. G1's environmental contrast is +3, but the population contains a smaller fraction of the background with value five. Weighting five and four by the new frequencies gives 4.3.
Do the background-frequency changes prove evolutionary adaptation by natural selection?
Show reasoning
They represent genetic population change under the model's inheritance assumptions. They do not identify its cause or measure fitness, so they cannot establish adaptation or a benefit.
What to carry forward
Track phenotype expression, inherited population composition and fitness consequences in separate accounts before connecting them.
References
Resources supporting this lesson’s explanations and examples.
- Phenotypic plasticity for plant development, function and life history (Trends in Plant Science 5(12):537–542, 2000; doi: 10.1016/S1360-1385(00)01797-0)S. E. SultanScientific review
- Beyond buying time: the role of plasticity in phenotypic adaptation to rapid environmental change (Philosophical Transactions B 374(1768):20180174, 2019; doi: 10.1098/rstb.2018.0174)Rebecca J. Fox, Jennifer M. Donelson, Celia Schunter, Timothy Ravasi and Juan D. Gaitán-EspitiaScientific review