AP Biology Population Genetics

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

1. Core Concepts and Standard Notation★★☆☆☆⏱ 3 min

Population genetics is the quantitative study of the distribution and change in allele and genotype frequencies within biological populations, forming the core mathematical foundation for the study of evolution in AP Biology. It frames evolution as a measurable change in allele frequency over generations, rather than just a qualitative change in trait distribution, allowing researchers to test hypotheses about evolutionary forces. This topic accounts for ~15% of Unit 7 exam weight, appearing regularly on both multiple-choice and free-response sections.

$p$ = frequency of the dominant allele in the population
$q$ = frequency of the recessive allele in the population
$p^{2}$ = frequency of the homozygous dominant genotype
$2 pq$ = frequency of the heterozygous genotype
$q^{2}$ = frequency of the homozygous recessive genotype

2. Hardy-Weinberg Equilibrium (HWE)★★★☆☆⏱ 5 min

The Hardy-Weinberg Equilibrium is a null model that describes a population that is not evolving, meaning allele and genotype frequencies remain constant from generation to generation. For a biallelic autosomal locus, the model is derived from probability rules for independent allele sampling from a gene pool, giving two core equations:

p + q = 1

p^2 + 2pq + q^2 = 1

📘 Definition

Hardy-Weinberg Equilibrium

A null model of non-evolving populations that requires five key assumptions to hold: no new mutations, random mating, no natural selection, large population size, and no gene flow. Any violation indicates evolution is occurring.

📐 Worked Example

A population of 500 wildflowers has 120 red-flowered (RR), 280 pink-flowered (Rr), and 100 white-flowered (rr) individuals. Calculate allele frequencies for R and r, then determine if the population is in HWE for this locus.

1
Count total alleles for this diploid population:
2
$500 \text{ individuals} \times 2 = 1000 \text{ total alleles}$
3
Count R and r alleles and calculate frequencies: R alleles = $(120 \times 2) + 280 = 520$ , so $p = 520/1000 = 0.52$ . r alleles = $(100 \times 2) + 280 = 480$ , so $q = 480/1000 = 0.48$ . $p + q = 1$ , which checks out.
4
Calculate expected HWE genotype frequencies: $p^{2} = (0.52)^{2} = 0.2704$ , $2 pq = 2 (0.52) (0.48) = 0.4992$ , $q^{2} = (0.48)^{2} = 0.2304$ , which sum to 1 as expected.
5
Compare to observed frequencies: Observed $R R = 0.24$ , $R r = 0.56$ , $r r = 0.20$ . Observed and expected differ significantly, so the population is not in HWE.

Exam tip:

Always count alleles correctly for diploid organisms: multiply the number of homozygotes by 2 before adding heterozygotes to get the total allele count. Never use individual counts directly for $p$ and $q$ .

3. Mechanisms of Evolution (HWE Violations)★★★★☆⏱ 4 min

HWE acts as a null model: when we observe deviations from HWE predictions, we can infer that one or more evolutionary mechanisms are acting on the population. Each deviation corresponds to a violation of one of the five HWE assumptions, and each mechanism alters allele and genotype frequencies in predictable ways:

Mutation: Creates new alleles, changes frequency very slowly over time; it is the ultimate source of genetic variation.
Non-random mating (e.g., inbreeding): Increases homozygosity by genotype, but does not change overall allele frequency on its own.
Natural selection: Differential survival and reproduction based on phenotype causes consistent, adaptive changes in allele frequency, modeled using relative fitness.
Genetic drift: Random change in allele frequency due to sampling error in small populations; changes are non-adaptive.
Gene flow: Movement of alleles between populations reduces genetic differentiation between groups.

📐 Worked Example

A population of lizards has three genotypes for body size: $B B$ (large), $B b$ (medium), $bb$ (small). Survival rates for each genotype are: $B B$ : 60%, $B b$ : 80%, $bb$ : 40%. Current allele frequencies are $p = 0.5$ , $q = 0.5$ . Calculate relative fitness for each genotype, and find the new frequency of $b$ after selection.

1
Calculate relative fitness by dividing each survival rate by the highest survival rate (80% for $B b$ ):
2
$w_{BB} = 0.6/0.8 = 0.75, \quad w_{Bb} = 0.8/0.8 = 1.0, \quad w_{bb} = 0.4/0.8 = 0.5$
3
Pre-selection genotype frequencies are $p^{2} = 0.25$ ( $B B$ ), $2 pq = 0.5$ ( $B b$ ), $q^{2} = 0.25$ ( $bb$ ).
4
Calculate unnormalized post-selection frequencies by multiplying pre-selection frequency by relative fitness, then sum to get mean fitness $\overset{w}{ˉ}$ : $B B = 0.1875$ , $B b = 0.5$ , $bb = 0.125$ , so $\overset{w}{ˉ} = 0.8125$ .
5
Normalize to get post-selection frequencies, then calculate new $q$ : $B B \approx 0.231$ , $B b \approx 0.615$ , $bb \approx 0.154$ . New $q = f (bb) + 0.5 f (B b) \approx 0.46$ .

Exam tip:

When calculating relative fitness, always divide by the highest survival rate, not the sum of survival rates. This scales all fitness values between 0 and 1 for easy comparison.

4. Genetic Drift: Founder and Bottleneck Effects★★★☆☆⏱ 3 min

Genetic drift is random fluctuation in allele frequency caused by sampling error in small populations, and it is a frequently tested HWE violation on the AP exam. Unlike natural selection, genetic drift causes non-adaptive changes: allele frequency shifts are random and unrelated to the fitness of the allele. Two commonly tested scenarios are:

Founder effect: Occurs when a small subset of a larger population founds a new, isolated population. The founder group is not genetically representative of the original population, so allele frequencies change randomly.
Bottleneck effect: Occurs when a large population is drastically reduced in size by a random event (e.g., natural disaster, habitat destruction). The surviving population has reduced genetic diversity and altered allele frequencies due to random sampling.

📐 Worked Example

A main population of bats has an allele frequency of $p = 0.8$ for the dominant $A$ allele (fog echo-location ability) and $q = 0.2$ for the recessive $a$ allele. A small group of 10 bats is blown off course to a new isolated island, and by chance 1 bat is $aa$ , 8 are $AA$ , and 1 is $A a$ . Is this founder effect or bottleneck effect? What is the new frequency of $a$ ?

1
Classify the event: This is founder effect, not bottleneck. A new isolated population is founded by a small subset of the original population, which matches the definition of founder effect. Bottleneck refers to reduction of an existing population, not founding a new one.
2
Calculate total alleles for 10 diploid bats:
3
$10 \times 2 = 20 \text{ total alleles}$
4
Count $a$ alleles: 1 $aa$ contributes 2 $a$ alleles, 1 $A a$ contributes 1 $a$ allele, so total $a = 3$ . New frequency is $q = 3/20 = 0.15$ , a random change from the original $q = 0.2$ .

Exam tip:

The most common AP exam question distinction: founder effect = new population, bottleneck effect = existing population reduced. Both are types of genetic drift.

5. Common Pitfalls

Wrong move:

Counting individuals instead of alleles when calculating $p$ and $q$ , e.g., using 100 recessive individuals out of 500 total as $q = 0.2$

Why:

Students confuse genotype frequency (count of individuals) with allele frequency (count of alleles) for diploid organisms

Correct move:

Always multiply the number of homozygous individuals by 2, add the number of heterozygous individuals, then divide by twice the total number of individuals to get allele frequency

Wrong move:

Assuming $q^{2}$ equals the frequency of the recessive allele

Why:

Students mix up notation for allele frequency ( $q$ ) and homozygous recessive genotype frequency ( $q^{2}$ ), leading to algebra errors

Correct move:

Always label variables first; $q = q^{2}$ only when the only observed data is the number of recessive phenotypes in a HWE population

Wrong move:

Claiming non-random mating (like inbreeding) changes allele frequencies

Why:

Students confuse genotype frequency change with allele frequency change

Correct move:

Non-random mating changes how alleles are arranged into genotypes, not total allele counts. Do not list it as a mechanism that changes overall allele frequency unless selection is also acting

Wrong move:

Calling genetic drift 'natural selection that happens in small populations'

Why:

Students mix up adaptive (selection) and non-adaptive (drift) evolutionary change

Correct move:

Always note that genetic drift causes random changes in allele frequency that are not tied to allele fitness, unlike natural selection

Wrong move:

Claiming a population is in HWE because most assumptions are met

Why:

Students memorize the assumptions but forget all must hold for no evolution

Correct move:

If any one HWE assumption is violated, the population is evolving, so allele frequencies will change between generations

6. Quick Reference Cheatsheet

Category	Formula / Rule	Notes
Sum of allele frequencies	$p + q = 1$	Applies to any 2-allele locus, regardless of HWE
HWE Genotype Frequencies	$p^{2} + 2 pq + q^{2} = 1$	Only applies if all 5 HWE assumptions are met
Recessive allele from recessive phenotype	$q = q^{2}$	Only valid for HWE populations with recessive phenotypes only in homozygotes
Carrier (heterozygote) frequency	$2 pq$	Always 2 times the product of the two allele frequencies
Relative Fitness	$w = \frac{Genotype survival rate}{Highest survival rate}$	Scales fitness between 0 and 1 for selection modeling
New allele frequency after selection	$q_{new} = f (bb) + 0.5 \times f (B b)$	Same calculation as any allele frequency, after normalizing post-selection frequencies
Total allele count (diploid)	$Total alleles = 2 \times number of individuals$	Always multiply individual count by 2 for diploid organisms
HWE Assumptions	No mutation 2. Random mating 3. No selection 4. Large population 5. No gene flow	Any violation = evolution is occurring
Founder Effect	Genetic drift in a new population	New population founded by a small subset of the original population
Bottleneck Effect	Genetic drift in a reduced existing population	Random population crash causes loss of genetic diversity

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
Calculate HWE carrier frequency
2022 · FRQ
Identify type of genetic drift

What's Next

Population genetics provides the quantitative framework for all subsequent study of evolution in AP Biology Unit 7. Next, you will apply the core concepts of allele frequency change and the null model of HWE to analyze patterns of natural selection (directional, stabilizing, disruptive) and the process of speciation. Without mastering the material in this sub-topic, you will not be able to correctly interpret experimental data on evolutionary change, which is a regularly tested skill in both MCQ and FRQ sections of the AP exam. This topic also connects back to Mendelian genetics and molecular genetics, and feeds into the big idea that evolution is a measurable change in genetic variation over time.