“”” AP Syllabus focus: ‘The same genotype can produce multiple phenotypes when environmental conditions alter patterns of gene expression.’ ”””
Phenotypic plasticity explains why organisms with identical DNA can look and function differently across environments. In AP Biology, the emphasis is on how external conditions regulate gene expression to generate alternative phenotypes without changing genotype.
Core Concept: Environment-Responsive Phenotypes
Phenotype can shift when environmental cues modify which genes are turned on/off, when they are expressed, and how much product is made. This produces variation within a genotype and can occur during development or throughout life.
DEFINITION
Phenotypic plasticity: The capacity of a single genotype to produce different phenotypes in different environments due to environmentally influenced changes in gene expression.
Plasticity does not require mutation; instead, the environment changes cellular conditions that influence transcription, translation, and protein activity, leading to different traits.
How the Environment Alters Gene Expression
Signal detection and response
Cells detect environmental signals (e.g., temperature, nutrient availability, light, crowding, toxins) through receptors and sensors, then activate intracellular pathways that change gene expression.
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page_url: https://openstax.org/books/biology-2e/pages/9-1-signaling-molecules-and-cellular-receptors
image_identifier: Figure 9.4 (Hydrophobic signaling molecule binds intracellular receptor; complex binds DNA)
Hydrophobic signaling molecules (such as steroid hormones) can diffuse through the plasma membrane and bind intracellular receptors. The ligand–receptor complex then enters the nucleus and acts as a transcription factor by binding regulatory DNA to change gene expression. This is a direct route from environmental cue to altered transcription.
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Key steps often include:
Signal transduction that activates transcription factors
Changes to promoter/enhancer activity that increase or decrease transcription
Shifts in mRNA stability and translation rates
Altered protein modification (e.g., phosphorylation) affecting protein activity and trait expression
Epigenetic regulation (environment-sensitive)
Environmental conditions can also influence epigenetic marks that regulate access to DNA.
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page_url: https://openstax.org/books/biology-ap-courses/pages/16-3-eukaryotic-epigenetic-gene-regulation
image_identifier: Figure 16.7 (Nucleosomes pack tightly vs. loosely; methylation vs. acetylation)
Epigenetic “tags” on DNA and histones can shift chromatin between a closed, transcriptionally silent state and an open, transcriptionally active state. The diagram emphasizes that methylation tends to compact chromatin (blocking transcription-factor access), while histone acetylation loosens packing (increasing access to regulatory DNA).
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DNA methylation often reduces transcription by limiting transcription-factor binding
Histone modification can open or compact chromatin, changing transcription rates These changes can be stable through many cell divisions within an individual, supporting long-lasting plastic responses.
Patterns of Plasticity Students Should Recognise
Continuous vs. discrete plasticity
Continuous plasticity: Gradual phenotype shifts across an environmental gradient (often seen in growth rate or enzyme activity).
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page_url: https://pmc.ncbi.nlm.nih.gov/articles/PMC3011025/
image_identifier: Figure 1 (Reaction norms of three hypothetical genotypes: a, b, and c)
A reaction norm graph plots phenotype across an environmental gradient for different genotypes. Differences in slope represent differences in plasticity (how strongly phenotype changes with environment), while crossings illustrate genotype-by-environment interactions. This kind of plot is the standard visualization for continuous phenotypic plasticity.
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Discrete plasticity (polyphenism): Distinct “either/or” phenotypes produced under different conditions, driven by switching gene-expression programs.
Developmental vs. reversible plasticity
Developmental plasticity: Early-life environments set long-term phenotypes by establishing persistent expression patterns.
Reversible plasticity (acclimation): Phenotypes adjust repeatedly as conditions change, typically through regulatory changes in gene expression and protein function.
Why Phenotypic Plasticity Matters Biologically
Plasticity can increase fitness in variable environments by producing traits better matched to current conditions, especially when:
Environmental change is frequent within a lifetime
Reliable cues predict future conditions
The costs of maintaining responsiveness are outweighed by benefits
However, plasticity can be constrained by:
Energetic costs of sensing/responding and maintaining regulatory machinery
Limits on how much gene expression can shift without disrupting other functions
Trade-offs when a phenotype advantageous in one environment is disadvantageous in another
Common AP Biology Pitfalls
Plasticity is not “genes changing”; it is gene expression changing in response to environment.
A plastic trait can still be heritable in its capacity for plasticity (genotypes can differ in responsiveness), even though the environment influences the observed phenotype.
Not all phenotype differences are plastic; some reflect different genotypes. Evidence often requires controlling genotype while varying environment.
