How does natural selection act on heritable variation to change a population over time?
Use mathematical and conceptual models to explain how natural selection acts on heritable variation to change the traits of a population over generations (GSE SB6.d).
A Georgia Milestones Biology EOC answer on natural selection: the four conditions (variation, overproduction, differential survival and reproduction, inheritance), what fitness really means, how selection produces adaptation, and the key idea that populations evolve while individuals do not.
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What this topic is asking
Standard SB6.d asks you to use models (conceptual and mathematical) to explain how natural selection acts on heritable variation to change a population over generations. For the Georgia Milestones Biology EOC that means knowing the four conditions for natural selection, what fitness really means, how selection produces adaptation, and the crucial idea that populations evolve, individuals do not. Items almost always give a scenario (camouflage, food supply, a shift in allele frequency) and ask you to predict or explain the change.
The four conditions for natural selection
If a scenario has all four, it is natural selection. The trait must be heritable (genetic), and the change happens across generations in the population, not within one individual's lifetime.
Fitness and adaptation
In biology, fitness does not mean strength or health; it means reproductive success, how many offspring an individual leaves that themselves survive to reproduce. A trait that raises fitness in a given environment is favored by selection.
Over generations this produces adaptation: a heritable trait that improves an organism's chances of surviving and reproducing in its environment, such as camouflage, a thick coat in a cold climate, or a beak shape suited to the available food. Adaptation is the visible result of natural selection acting on variation over time.
Tracking selection as a change in allele frequency
SB6.d expects you to model selection quantitatively as a shift in allele frequencies. If is the frequency of a favorable allele, selection that favors it makes increase generation by generation. For example, a favorable allele might rise as:
The allele is not "trying" to spread; rather, the individuals carrying it leave more surviving offspring, so the proportion of that allele in the population's gene pool grows. A harmful allele behaves the opposite way, with its frequency falling over generations. Describing evolution as a change in allele frequency over generations is the population-genetics definition the EOC rewards.
Where the variation comes from
Natural selection can only act on variation that already exists. That variation arises from mutation (the source of new alleles) and is shuffled by meiosis and sexual reproduction into new combinations. Selection does not create the variation; it filters it, favoring the variants that work best in the current environment. This is why mutations matter for evolution even though most are neutral or harmful: the rare beneficial ones supply raw material.
Populations evolve, not individuals
This is one of the most common EOC misconceptions to avoid. A single beetle does not turn dark; rather, dark beetles become a larger fraction of the population over generations because they survive and reproduce more. Always describe the change at the population level.
Try this
Q1. State the four conditions required for natural selection to occur. [2 points]
- Cue. Variation in the population; overproduction and competition for resources; differential survival and reproduction (fitness); and inheritance of the advantageous heritable traits.
Q2. Explain what biologists mean by fitness. [2 points]
- Cue. Fitness is reproductive success, how many surviving offspring an individual produces, not its strength or health; higher fitness means a trait is favored by selection.
Exam-style practice questions
Practice questions written in the style of GaDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
GA Biology EOC (style)1 marksA population of beetles lives on dark tree bark. Most beetles are dark, but a few are light, and birds eat the beetles they can see most easily. Over many generations, the population is most likely to: (A) become mostly light. (B) become mostly dark. (C) stay exactly the same. (D) have every beetle change color at once.Show worked answer →
A 1-point selected-response item on natural selection.
The correct answer is B. On dark bark, dark beetles are better camouflaged, so birds eat fewer of them; dark beetles survive and reproduce more, passing on the dark-color alleles. Over generations the dark-color allele increases in frequency and the population becomes mostly dark. C ignores the selection pressure, and D wrongly suggests every individual changes at once (individuals do not evolve; populations do).
GA Biology EOC (style)2 marksA trait controlled by a single allele has a starting frequency of 0.30 in a population. The trait improves survival, and after several generations its frequency is 0.65. (a) Explain why the frequency increased. (b) State why this is natural selection and not a change in any one individual.Show worked answer →
A 2-point item linking allele frequency to natural selection.
(a) 1 point: individuals carrying the advantageous allele survive and reproduce more, so they pass that allele to more offspring; over generations the allele's frequency rises (here from 0.30 to 0.65).
(b) 1 point: this is natural selection because heritable variation is being filtered by a selection pressure, changing the population's allele frequencies; no single individual changed its own alleles, the proportions in the population shifted across generations.
Markers reward linking higher survival and reproduction of the favorable variant to a rising allele frequency, and stating that the population, not the individual, changes.
Related dot points
- Develop and use mathematical models to support explanations of how undirected genetic changes, including genetic drift and gene flow, alongside natural selection, lead to changes in populations of organisms (GSE SB6.d).
A Georgia Milestones Biology EOC answer on the mechanisms that change allele frequencies: mutation as the source of new alleles, genetic drift (including bottleneck and founder effects), gene flow, and natural selection, plus the Hardy-Weinberg idea of a non-evolving population for comparison.
- Construct an argument using valid and reliable sources to support the claim that evidence from comparative morphology (analogous vs. homologous structures), embryology, biochemistry, and genetics supports common descent (GSE SB6.c).
A Georgia Milestones Biology EOC answer on the evidence for evolution: the fossil record, homologous, analogous, and vestigial structures, embryological similarities, and molecular evidence from DNA and proteins, and what each line shows about common descent.
- Analyze and interpret data to explain patterns in biodiversity that result from speciation, and develop a model to explain how natural selection causes biological resistance such as pesticide and antibiotic resistance (GSE SB6.b, SB6.e).
A Georgia Milestones Biology EOC answer on speciation and resistance: how reproductive isolation (often a geographic barrier) splits one species into two, how speciation builds biodiversity, and how natural selection produces antibiotic and pesticide resistance, a fast, real-world example of evolution.
- Construct an argument that mutations (changes in DNA sequence and chromosomal alterations) may result in phenotypic variation, and classify gene mutations as beneficial, harmful, or neutral (GSE SB2.b).
A Georgia Milestones Biology EOC answer on mutations: point mutations (substitution, insertion, deletion), frameshift effects, chromosomal mutations, causes (mutagens and replication errors), and how mutations can be beneficial, harmful, or neutral sources of variation.
- Explain the role of meiosis in producing gametes and in generating genetic variation through crossing over and independent assortment (GSE SB3.a).
A Georgia Milestones Biology EOC answer on meiosis: how it halves the chromosome number to make gametes, the difference from mitosis, and how crossing over, independent assortment, and random fertilization create genetic variation.
Sources & how we know this
- Biology Georgia Standards of Excellence (GSE) — Georgia Department of Education (2024)
- Georgia Milestones Biology EOC Assessment Guide — Georgia Department of Education (2024)