Natural Selection

AP Biology· AP Biology CED — Natural Selection· 14 min read

1. Core Principles of Natural Selection★☆☆☆☆⏱ 3 min

Natural selection is the primary mechanism of adaptive evolution, defined as differential survival and reproduction of individuals within a population due to differences in their heritable phenotypes. Co-developed independently by Charles Darwin and Alfred Russel Wallace, the theory rests on three core observations:

Populations have variation in heritable traits
More offspring are produced than can survive, leading to competition for limited resources
Traits that improve survival and reproduction will be passed on to more offspring, increasing their frequency in the population over generations

In the AP Biology CED, natural selection contributes 13–20% of the total Unit 7 exam score, with natural selection-specific questions making up 4–8% of the full exam. It appears in both multiple-choice (MCQ) and free-response (FRQ) questions.

2. Biological Fitness and Selective Pressures★★☆☆☆⏱ 3 min

📘 Definition

Biological Fitness

$w$ (relative fitness)

The relative reproductive success of a genotype compared to other genotypes in the same population. Fitness is measured by the number of viable surviving offspring produced, not physical strength or longevity.

Example:

A genotype that produces 8 surviving offspring has higher fitness than one that produces 2.

📘 Definition

Selection Coefficient

$s$

Measures the strength of selection against a genotype, calculated as $s = 1 - w$ . Higher values indicate stronger selection against the genotype.

Selective pressures are environmental factors that cause differential reproductive success. They can be abiotic (temperature, drought, toxin exposure) or biotic (predation, competition, parasitism), and act on existing phenotypic variation, not directly on genotypes.

📐 Worked Example

In a population of wild sunflowers, three genotypes for seed size have the following average number of surviving offspring per individual: AA = 8, Aa = 6, aa = 2. Calculate the relative fitness of each genotype and the selection coefficient against aa.

1
Identify the maximum absolute fitness in the population: 8 from genotype AA, which has a relative fitness of 1 by definition.
2
Apply the relative fitness formula:
3
$w = \frac{\text{Absolute fitness of genotype}}{\text{Maximum absolute fitness in the population}}$
4
Calculate relative fitness for all genotypes:
5
$w_{AA} = 8/8 = 1, \quad w_{Aa} = 6/8 = 0.75, \quad w_{aa} = 2/8 = 0.25$
6
Calculate the selection coefficient for aa:
7
$s_{aa} = 1 - w_{aa} = 1 - 0.25 = 0.75$

Exam tip:

If asked to explain what a fitness of 0 means, never say 'the individual dies young' — it means the individual produces zero surviving offspring, so the genotype cannot be passed to the next generation.

3. Modes of Natural Selection★★☆☆☆⏱ 3 min

Natural selection is categorized by how it changes the distribution of a continuous (polygenic) phenotype in a population. There are three core modes tested on the AP exam:

Directional selection: A single extreme phenotype is favored over all others, shifting the average phenotype of the population over time toward that extreme. Example: selection for smaller body size in marmots responding to climate warming.
Stabilizing selection: The intermediate phenotype is favored, and both extreme phenotypes are selected against. This reduces phenotypic variation and keeps the average phenotype near the original optimum. Example: human birth weight, where intermediate-weight babies have higher survival.
Disruptive selection: Both extreme phenotypes are favored, and the intermediate phenotype is selected against. This increases phenotypic variation and can lead to sympatric speciation. Example: beak size in finches when only large and small seeds are available.

📐 Worked Example

A researcher studies body length in a population of garter snakes. After 10 years of a new invasive predator that preys most heavily on medium-sized snakes, the population changes: original average length = 60 cm, range = 40–80 cm. After selection, 12% of the population is 40–50 cm, 15% is 70–80 cm, and only 3% is 50–70 cm. Identify the mode of selection and predict its long-term effect if the predator remains.

1
Identify which phenotypes have higher frequency after selection: both the short (40–50 cm) and long (70–80 cm) extremes are much more common than the intermediate 50–70 cm range.
2
Match to the correct mode: when both extremes are favored over intermediates, this is disruptive selection.
3
Predict long-term effect: Disruptive selection will maintain or increase phenotypic and genetic variation in the population. If gene flow between the two size morphs is reduced, this can lead to sympatric speciation as the two populations diverge genetically over time.

Exam tip:

When identifying a selection mode from a graph, remember: x-axis = phenotype value, y-axis = frequency. One shifted peak = directional; one narrow peak at the original center = stabilizing; two separate peaks at extremes = disruptive.

4. Modeling Selection with Hardy-Weinberg Equilibrium★★★☆☆⏱ 4 min

The Hardy-Weinberg principle states that allele and genotype frequencies remain constant from generation to generation only in the absence of evolutionary forces, including natural selection. When selection is acting, we use the Hardy-Weinberg framework to calculate how allele frequencies change between generations.

First, adjust each initial genotype frequency by its relative fitness, then normalize the adjusted frequencies by the mean fitness of the population ( $\overset{w}{ˉ}$ ), which is the sum of all adjusted frequencies. The general formula for post-selection genotype frequencies is:

f(AA)_{\text{post-selection}} = \frac{p^2 w_{AA}}{\bar{w}}

Where mean fitness is calculated as:

\bar{w} = p^2 w_{AA} + 2pq w_{Aa} + q^2 w_{aa}

$p$ = initial frequency of the A allele, $q = 1 - p$ = initial frequency of the a allele. After calculating post-selection genotype frequencies, the new allele frequency is:

p' = f(AA) + \frac{1}{2}f(Aa)

📐 Worked Example

In a population, the initial frequency of the recessive a allele is $q = 0.5$ , and relative fitnesses are $w_{AA} = 1$ , $w_{A a} = 1$ , $w_{aa} = 0.25$ (selection against aa). Calculate the new frequency of the a allele ( $q^{'}$ ) after one generation of selection.

1
Calculate initial genotype frequencies from Hardy-Weinberg: $p = 1 - q = 0.5$ , so:
2
$p^2 = 0.25, \quad 2pq = 0.5, \quad q^2 = 0.25$
3
Calculate mean fitness $\overset{w}{ˉ}$ :
4
$\bar{w} = (0.25)(1) + (0.5)(1) + (0.25)(0.25) = 0.8125$
5
Calculate post-selection genotype frequencies for aa and Aa:
6
$f(aa) = \frac{0.25 \times 0.25}{0.8125} ≈ 0.0769, \quad f(Aa) = \frac{0.5 \times 1}{0.8125} ≈ 0.6154$
7
Calculate new q:
8
$q' = f(aa) + \frac{1}{2}f(Aa) = 0.0769 + 0.5(0.6154) ≈ 0.385$

Exam tip:

Always remember to normalize by mean fitness after multiplying by relative fitness. Skipping this step leaves you with total genotype frequencies that do not add up to 1, which will cost points on FRQs.

5. AP-Style Practice Questions★★★☆☆⏱ 4 min

📐 Worked Example

A population of limpets (aquatic snails) has shell color ranging from white to dark brown, with tan being intermediate. Limpets live on intertidal rocks that are either light-colored (covered in white barnacles) or dark-colored (bare rock). Birds (predators) can easily see and eat tan limpets on either background, but rarely see white limpets on barnacles or dark brown limpets on dark rock. Which of the following correctly predicts the mode of natural selection and its effect on shell color variation?
A) Directional selection, variation in shell color will decrease
B) Stabilizing selection, variation in shell color will increase
C) Disruptive selection, variation in shell color will increase
D) Disruptive selection, variation in shell color will decrease

1
First, identify which phenotypes are favored: the two extreme phenotypes (white and dark brown) are favored, while the intermediate (tan) is selected against. This eliminates options A (directional) and B (stabilizing).
2
Disruptive selection removes intermediate phenotypes and preserves the two extreme phenotypes already present in the population, so it increases overall phenotypic variation. This eliminates option D, which claims variation decreases. The correct answer is C.

📐 Worked Example

Bacteria develop resistance to antibiotics through natural selection. A researcher studies a population of E. coli growing in medium with a low concentration of the antibiotic rifampicin. She genotypes the rpoB gene (which confers resistance when mutated) and counts the number of viable colonies formed by each genotype after 24 hours. She finds: RR (homozygous resistant) = 120 colonies, Rr (heterozygous) = 90 colonies, rr (homozygous susceptible) = 20 colonies.
(a) Calculate the relative fitness of each genotype and the selection coefficient against rr. Show your work.
(b) The initial allele frequency of R is p = 0.2. Predict the change in frequency of R after one generation of selection. Justify your prediction.
(c) Explain why the frequency of the resistance allele would decrease if the antibiotic is removed from the growth medium. Connect your answer to fitness.

1
(a) Absolute fitness is proportional to the number of colonies, so the maximum absolute fitness is 120. Using $w = w_{ab s} / w_{ma x}$ :
2
$w_{RR} = 120/120 = 1, \quad w_{Rr} = 90/120 = 0.75, \quad w_{rr} = 20/120 ≈ 0.17$
3
Selection coefficient against rr: $s_{r r} = 1 - w_{r r} = 1 - 0.17 = 0.83$ .
4
(b) The frequency of R will increase from 0.2 to a higher value. Genotypes carrying the R allele have much higher fitness than rr in the presence of rifampicin, so R alleles are passed on to more offspring. Calculations confirm this: $\overset{w}{ˉ} = (0. 2^{2}) (1) + 2 (0.2) (0.8) (0.75) + (0. 8^{2}) (0.17) = 0.3888$ , and $p^{'} \approx 0.41$ , so p increases.
5
(c) Resistance mutations almost always have a fitness cost: the mutated rpoB gene produces a less functional RNA polymerase even in the absence of rifampicin. This means RR and Rr genotypes have lower relative fitness than wild-type rr when there is no antibiotic. Because rr has higher fitness, natural selection will increase the frequency of r and decrease the frequency of R over generations.

📐 Worked Example

Climate change is increasing average annual temperatures in mountain habitats. Researchers measured body mass in a population of marmots at 2000m elevation in 1980 and 2020. In 1980, average adult body mass was 3.5 kg, with a range from 2 kg to 5 kg. By 2020, average adult body mass was 3.1 kg, the proportion of the population with mass between 2 kg and 3 kg increased from 25% to 45%, and the proportion between 4 kg and 5 kg decreased from 25% to 8%. Smaller body size increases heat tolerance because it raises the surface area to volume ratio, allowing more efficient heat loss. Identify the mode of selection acting on marmot body mass, calculate the relative fitness of 4–5 kg marmots if 2–3 kg marmots have an absolute fitness of 4.2 surviving offspring per individual and 4–5 kg marmots have an absolute fitness of 1.8, and explain what this fitness value means in context.

1
The smaller body mass (one extreme of the original phenotypic range) is favored, while larger body mass (the opposite extreme) is selected against, and the population average has shifted toward smaller size. This is directional selection.
2
The maximum absolute fitness is 4.2 (for 2–3 kg marmots), so relative fitness of 4–5 kg marmots is:
3
$w = 1.8 / 4.2 ≈ 0.43$
4
A relative fitness of 0.43 means that 4–5 kg marmots produce only 43% as many surviving offspring as the 2–3 kg marmots that are most favored under current warmer temperatures. This matches the observed shift in average body mass over 40 years: directional selection favoring smaller body size in response to the selective pressure of anthropogenic climate change is leading to adaptive change in the marmot population.

6. Common Pitfalls

Wrong move:

Defining fitness as 'how strong/healthy an individual is' instead of reproductive success.

Why:

Common language uses 'fitness' to mean physical condition, so students default to this incorrect definition.

Correct move:

Always define fitness as 'the relative reproductive success of a genotype, measured by the number of surviving offspring produced' in any FRQ answer.

Wrong move:

Claiming natural selection creates new traits that a population 'needs' to survive.

Why:

Students often anthropomorphize selection, thinking it acts toward a predetermined goal.

Correct move:

Always state that selection acts on existing heritable variation; new variation comes from random mutation, and selection only increases the frequency of variants that already improve reproductive success.

Wrong move:

Saying selection acts on genotypes instead of phenotypes.

Why:

Students focus on genetic variation when discussing evolution, so they misstate the level at which selection acts.

Correct move:

Remember the mantra: 'Selection acts on phenotypes, which are coded by genotypes, leading to changes in genotype frequencies over time.'

Wrong move:

Confusing disruptive selection with directional selection when a graph shows two peaks.

Why:

Students remember directional selection favors an extreme, so they mix up the two when both extremes are favored.

Correct move:

Count the number of peaks after selection: one shifted peak = directional, two peaks at opposite ends = disruptive, one narrow peak at the original center = stabilizing.

Wrong move:

Claiming stabilizing selection changes the average phenotype of a population.

Why:

Students confuse the effect of stabilizing selection on variation vs. average phenotype.

Correct move:

Stabilizing selection reduces variation around the existing average, it does not shift the average phenotype of the population.

Wrong move:

Forgetting to divide by mean fitness when calculating post-selection allele frequencies.

Why:

Students correctly multiply initial frequencies by relative fitness, but stop there, forgetting selection removes genotypes so the total must be renormalized.

Correct move:

After calculating the sum of (initial frequency × fitness), always divide each adjusted frequency by that sum (mean fitness) to get post-selection frequencies.

7. Quick Reference Cheatsheet

Category	Formula/Definition	Notes
Relative Fitness	$w = \frac{Absolute fitness of genotype}{Maximum absolute fitness in population}$	Ranges 0–1; 1 = highest fitness, 0 = no surviving offspring
Selection Coefficient	$s = 1 - w$	Higher $s$ = stronger selection against a genotype
Mean Population Fitness	$\overset{w}{ˉ} = p^{2} w_{AA} + 2 pq w_{A a} + q^{2} w_{aa}$	Used to normalize post-selection genotype frequencies
Post-selection AA frequency	$f (AA) = \frac{p ^{2} w _{AA}}{w ˉ}$	All post-selection frequencies sum to 1 after normalization
Post-selection A allele frequency	$p^{'} = f (AA) + \frac{1}{2} f (A a)$	$q^{'} = 1 - p^{'}$ always holds
Directional Selection	One extreme phenotype favored	Shifts average phenotype in the population
Stabilizing Selection	Intermediate phenotype favored	Reduces variation, does not shift average
Disruptive Selection	Both extreme phenotypes favored	Increases variation, can lead to speciation

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · MCQ
Identify mode of natural selection
2022 · FRQ
Calculate relative fitness and selection
2021 · MCQ
Core principles of natural selection

What's Next

Natural selection is the foundation of all evolutionary biology, and understanding it prepares you to explore more advanced topics in AP Biology Unit 7, including speciation, evidence for evolution, and population genetics. Beyond the exam, natural selection explains real-world phenomena like the evolution of antibiotic resistance in bacteria, pesticide resistance in agricultural pests, and adaptive responses to climate change in wild populations. Solid understanding of fitness and selection modes is also required for interpreting phylogenetic trees and understanding how macroevolutionary patterns emerge from microevolutionary processes. Use the links below to continue your study of Unit 7 topics.