AP Syllabus focus:
‘Natural selection acts on phenotypic variations that exist within populations of organisms.’
Natural selection can only “choose among” differences that already exist. Understanding what phenotypic variation is, where it comes from, and when it is heritable explains why populations (not individuals) evolve.
Core idea: selection acts on variation within a population
Natural selection changes trait distributions across generations because individuals differ in traits that affect survival and reproduction. If every individual had the same phenotype for a trait, there would be no consistent advantage for one form over another, and no evolutionary change for that trait.
Phenotype: The observable traits of an organism, produced by the interaction of its genotype and environment (e.g., morphology, physiology, behaviour).
Phenotypes are what the environment “filters.” Predators, climate, pathogens, and resource availability interact with organismal traits, making some phenotypes more likely to leave offspring than others.
Phenotypic variation is the “raw material”
Natural selection requires:
Variation in a trait among individuals in the same population
Differential success linked to that variation (some phenotypes leave more offspring)
Heritability of at least part of the variation so trait differences can persist across generations
Phenotypic variation: Differences in phenotypes among individuals in a population for a given trait.
Not all phenotypic differences contribute to evolution; natural selection produces evolutionary change only when phenotypic differences reflect underlying genetic differences that can be passed on.
Where phenotypic variation comes from (AP-level focus)
Phenotypic variation commonly arises from two overlapping sources:
Genetic differences among individuals (different alleles and allele combinations)
Environmental differences (nutrition, temperature, light, social conditions), which can alter trait expression
Many traits reflect genotype × environment effects, meaning the same genotype can produce different phenotypes in different environments, and different genotypes can respond differently to the same environment.
A reaction-norm plot illustrating genotype-by-environment interaction: each line represents the phenotype expressed by a genotype across an environmental gradient. Non-parallel (and potentially crossing) lines indicate that genotypes differ in how strongly they respond to environmental change. This is a core graphical way to represent phenotypic plasticity and G×E effects. Source
Types of phenotypic variation
Traits can vary in different ways, which affects how variation is described and measured:
Discrete variation: distinct categories (often influenced by one or a few genes), such as blood type
Continuous variation: a spectrum of values (often polygenic and environment-influenced), such as height or enzyme activity
Because selection acts on phenotypes, continuous traits are often considered as a distribution in a population (e.g., many intermediate individuals and fewer extremes), rather than as simple categories.
Three classic modes of natural selection shown as changes in a population’s phenotype-frequency distribution over time. Stabilizing selection narrows variation around the mean, directional selection shifts the mean toward one extreme, and diversifying selection produces a bimodal distribution by favoring extremes. This makes clear why selection is described in terms of population-level distributions rather than individual change. Source
Why heritable variation matters for evolution
Natural selection can favour a phenotype in one generation, but long-term evolutionary change depends on whether individuals with that phenotype tend to produce offspring with similar phenotypes due to shared genes. Therefore:
Environment-only differences (e.g., a temporary increase in body mass due to abundant food) may affect individual success but may not shift allele frequencies
Genetically influenced differences (e.g., a heritable coat colour) can change trait frequencies over generations if associated with reproductive success
Key implication for AP Biology
Selection acts on individual phenotypes, but evolution is observed as a change in the population’s trait distribution across generations. Phenotypic variation is essential because it creates the differences on which natural selection can act.
FAQ
They use designs such as common-garden or reciprocal-transplant studies.
By holding environment constant (or swapping environments), differences that persist are more likely genetic.
A reaction norm describes how one genotype’s phenotype changes across environments.
Different genotypes can have different reaction norms, increasing variation within the population.
Different allele combinations can produce similar trait values, especially for polygenic traits.
This can mask genetic differences unless genetic data or controlled breeding are used.
Imprecise measurements inflate apparent variation.
Reducing error involves standardised protocols, repeated measures, and careful sampling across ages/sexes.
Yes, chemical modifications affecting gene expression can alter phenotypes.
If epigenetic states are stable through cell divisions (and sometimes inherited), they may influence variation available to selection.
Practice Questions
Define phenotypic variation and state why it is required for natural selection. (2 marks)
Correct definition of phenotypic variation as differences in phenotypes among individuals in a population (1)
States that without variation there is no differential survival/reproduction to favour one phenotype, so no selection-driven change (1)
Explain how phenotypic variation within a population can lead to evolutionary change over generations. Your answer should refer to selection acting on phenotypes and the importance of heritability. (6 marks)
States that individuals within a population show phenotypic differences for a trait (1)
Explains that the environment causes differential survival and/or reproductive success among phenotypes (1)
States that natural selection acts on phenotypes (not directly on genotypes) (1)
Links higher reproductive success of a phenotype to more offspring contributing to the next generation (1)
Explains that if phenotypic differences are heritable (genetically influenced), offspring tend to resemble parents for the trait (1)
Concludes that the frequency/mean of the trait (and associated alleles) in the population shifts over generations (1)
