AP Syllabus focus:
‘Flower color, seasonal fur color, and similar traits can shift when environmental conditions such as soil pH or temperature change.’
Coloration is a visible phenotype shaped by pigments and tissue structure. In many organisms, environmental conditions alter pigment production, pigment chemistry, or hair/feather replacement patterns, changing appearance without changing DNA sequence.
Core idea: environment can shift colour phenotypes
Environmental variation can change colour by affecting:
Pigment synthesis (enzyme activity, gene expression, precursor availability)
Pigment chemistry (molecular form, binding partners, pH-dependent structure)
Pigment deposition (where and how much pigment accumulates)
Replacement of coloured tissues (seasonal moults, new hairs/feathers)
Pigments and where they come from
Common biological colour sources include:
Melanins (common in animals; browns/blacks; also affect UV protection)
Carotenoids (reds/oranges/yellows; often obtained from diet)
Flavonoids/anthocyanins (common plant pigments; reds/purples/blues; sensitive to cellular chemistry)
Pigment: A molecule that absorbs specific wavelengths of light and reflects others, producing colour.
Different pigment types respond differently to the environment because they rely on distinct pathways and materials.
Plant coloration: soil chemistry and pigment behaviour
Soil pH and flower colour
This graph summarizes how hydrangea flower color varies with soil pH: acidic soils (lower pH) are associated with blue flowers, while more alkaline soils (higher pH) are associated with pink flowers, with intermediate pH producing purple hues. It provides an at-a-glance example of an environmentally driven color phenotype that reflects changes in pigment chemistry and/or metal-ion interactions within petal tissues. Source
In some plants, soil pH changes the chemical environment around pigments and can shift flower colour. Mechanisms include:
pH-dependent pigment structure: certain pigments change form as acidity changes, altering the wavelengths they absorb.
Ion availability: soil pH can change the availability of ions (for example, metal ions) that bind pigments and modify colour.
Transport into tissues: environmental conditions can influence uptake and movement of ions or precursors into petals.
These effects are phenotypic because the genes encoding pigment pathways can remain the same while the chemical context in petal cells changes.
Environment-dependent gene expression in petals
Environmental conditions can also alter transcription of pigment-pathway genes in developing petals, changing:
which pigments are produced
relative pigment concentrations
patterning intensity (darker veins, lighter edges) when local cellular conditions differ across the petal
Animal coloration: temperature, seasons, and pigment deposition
Temperature effects on pigment production
Temperature can influence colour by changing protein function in pigment pathways:
Enzymes involved in pigment synthesis may show temperature-sensitive activity, changing pigment amounts.
Temperature can affect cellular transport and pigment deposition in skin or hair follicles.
Melanin: A class of pigments produced by animal cells that contributes to dark coloration and can protect tissues by absorbing UV radiation.
Temperature-linked colour changes are especially noticeable when pigment production differs across body regions that experience different temperatures.
Seasonal fur colour change
This photograph shows a snowshoe hare in its winter-white coat, illustrating a seasonal phenotype that improves camouflage in snowy environments. In AP Biology terms, the color change reflects environmentally cued timing of moulting and pigment deposition in newly grown hairs, rather than a change in the hare’s DNA sequence. Source
Seasonal shifts (often winter white to summer brown/grey) typically reflect environmental control of moulting schedules and pigment deposition in newly grown hairs:
Photoperiod (day length) is a key cue that synchronises moulting with seasons.
Temperature and food availability can modify timing or completeness of colour change.
The new coat’s colour depends on melanin levels deposited during hair growth; environmental cues influence the hormonal signals that regulate this process.
These changes can improve camouflage in snow versus vegetation, but the immediate mechanism is environmental control of hair replacement and pigmentation.
Diet-dependent coloration
Some animal colours depend on dietary pigments:
If carotenoids must be obtained from food, limited diet can reduce red/orange/yellow intensity.
Resource availability can therefore shift visible colour without any genetic change, especially in feathers, scales, or skin.
Interpreting environmental colour changes in AP Biology
When evaluating a colour shift, distinguish:
Genotype unchanged: environment alters expression, chemistry, or deposition.
Timing matters: many changes occur during development of petals, hairs, or feathers.
Trait specificity: the same environmental factor (like temperature) may affect one pigment pathway but not another.
FAQ
Hydrangea colour often depends on complexes between petal pigments and metal ions.
More available aluminium in certain soil conditions can shift pigment complexes toward blue hues.
Soil chemistry affects aluminium solubility and transport into petals.
Yes. Structural colours depend on tissue microstructure.
Environmental stress during growth (nutrition, temperature) can alter the spacing/organisation of nanostructures in feathers or scales, shifting reflected wavelengths.
Some microbes alter pigment chemistry or host resources.
They can reduce available dietary pigments (e.g., carotenoids) by affecting digestion.
Inflammation can disrupt pigment deposition during feather or hair growth.
Not as DNA sequence changes, but some effects can persist via non-genetic mechanisms.
For example, epigenetic marks or maternal provisioning might influence offspring pigment production under similar conditions, though stability varies across taxa.
If snow cover duration changes faster than day length, photoperiod-driven moults may become mismatched.
This can produce periods where animals are white on snowless ground (or vice versa), increasing predation risk and potentially shifting selection on cue sensitivity.
Practice Questions
Explain how soil pH can change flower colour without changing the plant’s genotype. (2 marks)
Soil pH changes the chemical environment of pigments (e.g., alters pigment form/structure or binding), leading to different light absorption/reflection (1).
pH can alter availability/uptake of ions or pigment precursors that modify petal pigment chemistry or concentration, changing colour phenotype (1).
A mammal species has brown fur in summer and white fur in winter. Describe how environmental cues can produce this seasonal colour change and outline one way you could test which cue is most important. (5 marks)
Seasonal colour change occurs through a moult producing a new coat (1).
Photoperiod can act as a cue, via hormonal signalling that initiates moulting/pigment changes (1).
Temperature can influence pigment synthesis or deposition (e.g., melanin levels) in new hairs (1).
Describe a test: manipulate one cue while controlling the other (e.g., constant temperature with changing day length, or constant day length with changing temperature) (1).
Measure outcome objectively (e.g., quantify fur colour/pigment levels in regrown fur) and compare between treatments (1).
