---
title: "Artificial Selection - AP Biology Required Lab Guide"
description: "Review Artificial Selection for AP Biology with CED-aligned concepts, lab skills, data analysis, and AP exam connections."
canonical: "https://fiveable.me/ap-bio/required-labs/artificial-selection/study-guide/cyqdArpgong5HkwDVOKG"
type: "study-guide"
subject: "AP Biology"
unit: "Required Labs"
lastUpdated: "2026-06-17"
---

# Artificial Selection - AP Biology Required Lab Guide

## Summary

Review Artificial Selection for AP Biology with CED-aligned concepts, lab skills, data analysis, and AP exam connections.

## Guide

# AP Biology Lab Guide: Artificial Selection Investigation

This lab is really about one core question: what happens to a population when humans decide which individuals get to reproduce? You will use a model organism (often Wisconsin Fast Plants or a similar system) to simulate how selecting for specific traits changes the distribution of those traits across generations. The bigger picture is understanding how selection, whether by humans or nature, shifts **[allele frequencies](/ap-bio/key-terms/allele-frequencies "fv-autolink")** and reshapes **[phenotypic variation](/ap-bio/key-terms/phenotypic-variation "fv-autolink")** in a population over time.

## Why This Lab Matters for the AP Exam

[Artificial selection](/ap-bio/unit-7/artificial-selection/study-guide/YdhzRk9EPvFMpXZ8Cthc "fv-autolink") shows up on the AP exam as a direct model for natural selection. If you understand how human-driven selection changes a population, you can apply that same logic to any scenario involving **[selective pressure](/ap-bio/key-terms/selective-pressure "fv-autolink")**, fitness, and population change. The exam loves to give you data about trait distributions across generations and ask you to explain what happened and why. This lab gives you the hands-on experience to actually reason through those questions instead of just memorizing definitions.

You should also expect free-response questions that ask you to connect artificial selection to broader evolutionary concepts like **[genetic diversity](/ap-bio/key-terms/genetic-diversity "fv-autolink")**, **[population dynamics](/ap-bio/unit-7/extinction/study-guide/CpKuTxKrClQmBnYbY5tb "fv-autolink")**, and the risks that come with reducing variation in a population.

## CED Connections

This lab directly supports three topics in [Unit 7](/ap-bio/unit-7 "fv-autolink").

**Topic 7.2 (Natural Selection)** is the foundation. Learning Objective LO 7.2.A asks you to describe the importance of **phenotypic variation** in a population. The essential knowledge here is that natural selection acts on phenotypic variation (EK 7.2.A.1), that environments apply **selective pressures** (EK 7.2.A.2), and that some variations increase or decrease fitness in specific environments (EK 7.2.A.3). In this lab, you are the environment. You decide which [phenotypes](/ap-bio/key-terms/phenotypes "fv-autolink") survive to reproduce, which makes the connection to natural selection very concrete.

LO 7.2.B and EK 7.2.B.1 connect to the molecular side: variation in the types and amounts of molecules within cells (like [enzymes](/ap-bio/unit-3/enzyme-structure/study-guide/jsjBfuk2jmYAZVrmVwtF "fv-autolink") or pigments) is what produces the [phenotypic differences](/ap-bio/unit-6/gene-expression-specialization/study-guide/g4BjZXbPu9cSt2JcU9pU "fv-autolink") you are selecting on.

**Topic 7.3 (Artificial Selection)** is the direct CED match. LO 7.3.A asks you to explain how humans affect diversity within a population, and EK 7.3.A.1 states that through artificial selection, humans affect variation in other species. This is the core claim your lab data should support.

**Topic 7.11 (Variations in Populations)** is where the consequences come in. LO 7.11.A asks you to explain how **genetic diversity** affects a population's ability to withstand **[environmental pressures](/ap-bio/key-terms/environmental-pressure "fv-autolink")**. EK 7.11.A.1 breaks this down into three key ideas: low diversity puts populations at risk of decline or **[extinction](/ap-bio/key-terms/extinction "fv-autolink")** (part i), genetically diverse populations are more **resilient** (part ii), and **adaptive alleles** in one environment can become **deleterious alleles** in another (part iii).

## What You Need to Be Able to Do

This lab builds several skills that show up directly on the AP exam.

- **Identify variables**: Know what you are selecting for (the trait), what you are measuring ([phenotype](/ap-bio/unit-4/signal-transduction/study-guide/OSq09o306uHFrgypolNe "fv-autolink") distribution), and what you are holding constant (growing conditions, generation time, etc.).
- **Collect and organize data across generations**: You need to track how a trait's distribution shifts from one generation to the next, not just record a single snapshot.
- **Graph trait distributions**: Histograms are your main tool here. You should be able to draw and interpret a histogram showing how a trait like trichome density or leaf width changes across generations of selection.
- **Calculate and compare means**: Comparing the average trait value before and after selection is how you quantify whether your selection worked.
- **Make a claim with evidence**: The AP exam expects you to use your data to support a specific claim about how selection changed the population. This is the claim-evidence-reasoning (CER) format.
- **Connect population-level change to [allele frequency](/ap-bio/key-terms/allele-frequency "fv-autolink") change**: You need to explain why the phenotype distribution shifted in terms of which **[alleles](/ap-bio/key-terms/allele "fv-autolink")** became more or less common.
- **Evaluate the trade-offs of selection**: Specifically, you should be able to explain why reducing genetic diversity through selection can make a population more vulnerable to new environmental pressures.

## Core Concepts

### Phenotypic Variation and Why It Matters

**Phenotypic variation** is the range of observable differences in a trait among individuals in a **population**. In this lab, you might measure things like trichome (tiny hair) density on leaves, plant height, or seed size. The key point is that not all individuals look the same, and that variation is what selection acts on.

That variation comes from **[genetic variability](/ap-bio/key-terms/genetic-variability "fv-autolink")**, meaning differences in the [DNA sequences](/ap-bio/key-terms/dna-sequences "fv-autolink") individuals carry. Specifically, different versions of a gene are called **alleles**. If everyone in a population had identical alleles for every gene, there would be no variation for selection to act on, and evolution could not happen.

### Selective Pressure and Selective Advantage

A **selective pressure** is any environmental factor (or in this case, human choice) that causes some individuals to survive and reproduce more than others. When you choose which plants to breed from, you are applying a selective pressure.

A **[selective advantage](/ap-bio/key-terms/selective-advantage "fv-autolink")** is what an individual has when its phenotype makes it more likely to survive and reproduce under a given selective pressure. If you are selecting for plants with the most trichomes, then high trichome density is a selective advantage in your experiment.

### Directional Selection

**Directional selection** is what happens when selection consistently favors one extreme of a trait distribution. If you always pick the plants with the most trichomes to breed, you are pushing the population in one direction. Over generations, the average trichome count goes up, and the distribution shifts. This is the type of selection most artificial selection experiments demonstrate.

### Allele Frequency and Population Change

**Allele frequency** is how common a particular allele is in a population, expressed as a proportion. If 60 out of 100 alleles at a given gene are the "high trichome" version, that allele's frequency is 0.60.

When you apply selection, individuals with the favored phenotype reproduce more. Their alleles get passed on at a higher rate. Over generations, the frequency of **adaptive alleles** (alleles that increase fitness under the current selective pressure) goes up, while the frequency of **deleterious alleles** (alleles that reduce fitness) goes down. This shift in allele frequency across generations is evolution.

### Genetic Diversity and Resilience

**Genetic diversity** refers to the total variety of alleles present in a population. **Population diversity** is the broader idea that a population contains many different individuals with different traits.

Here is the trade-off that the AP exam loves to test: artificial selection increases the frequency of favored alleles, but it also reduces genetic diversity. When you repeatedly select for one trait, you are essentially removing alleles from the gene pool. A population with low genetic diversity has less **resilience**, meaning it is less able to adapt if the environment changes. This is why monoculture crops (like the Irish potato famine example) are so vulnerable. If a new disease or **environmental pressure** arrives, a genetically uniform population may have no individuals with the right alleles to survive.

The illustrative examples from the CED make this concrete: California condors, black-footed ferrets, and prairie chickens all went through population bottlenecks that reduced their genetic diversity, making them more vulnerable. The potato blight and corn rust examples show what happens when a pathogen hits a low-diversity crop population.

### Mutation as a Source of Variation

**Mutation** is the original source of new alleles. Without mutation, there would be no new variation entering the gene pool. Mutations are random, but selection is not. Selection determines which mutations stick around and spread. This is an important distinction: mutation provides the raw material, and selection shapes what happens to it.

### Evolutionary Fitness

**Evolutionary fitness** is not about being strong or fast in a general sense. It specifically means the ability to survive and reproduce in a particular environment. An allele that increases fitness in one environment might decrease fitness in another. This is why **adaptive alleles** and **deleterious alleles** are always defined relative to a specific environment and its selective pressures.

## How the Lab Works

The investigation puts you in the role of the selective agent. You start with a population of organisms (often Wisconsin Fast Plants, *Brassica rapa*) that shows natural variation in some measurable trait. Trichome density is a common choice because it is easy to count and shows clear variation between individuals.

Your job is to choose which individuals reproduce based on that trait. If you are selecting for high trichome density, you only allow the plants with the most trichomes to produce seeds for the next generation. If you are selecting for low trichome density, you only breed from the plants with the fewest trichomes. You then grow the next generation and measure the same trait again.

The logic is simple: if the trait is heritable (meaning it has a genetic basis), then selecting which individuals reproduce should shift the trait distribution in the direction you selected. If the average trichome count goes up after one round of selection for high trichomes, that is evidence that the trait is heritable and that selection changed the population.

Some versions of this lab also include a control group, a population where you select randomly (or do not select at all). Comparing your selected population to the control is how you isolate the effect of selection from random variation.

The lab might run for one or two generations depending on your class timeline. Even one generation of strong selection can produce a measurable shift, which is part of what makes this investigation so effective.

## Data and Analysis Moves

### Setting Up Your Measurements

For each generation, you measure the trait (like trichome density) for every individual in your population. Record the value for each plant. You need individual-level data, not just averages, because you are going to look at the whole distribution.

### Graphing: Histograms Are Your Best Friend

Plot a histogram of trait values for each generation. Put trait value (like trichome count) on the x-axis and number of individuals on the y-axis. When you compare the histogram from generation 1 to generation 2, you should see the distribution shift in the direction you selected.

A shift in the mean is the key signal. If you selected for high trichomes and the mean went from 4.2 to 6.8, that is your evidence that selection worked.

### Calculating the Selection Response

The basic comparison is:

$$\text{Mean trait value (generation 2)} - \text{Mean trait value (generation 1)}$$

A positive number when selecting for high values (or a negative number when selecting for low values) supports your claim that selection changed the population.

You can also calculate the **selection differential**, which is the difference between the mean of the whole population and the mean of only the individuals you selected to breed. A larger selection differential usually produces a larger response, especially if the trait has high heritability.

### Controls and Variables

- **Independent variable**: Whether selection was applied (and in which direction)
- **Dependent variable**: The trait distribution (mean, range, shape of histogram) in the next generation
- **Controlled variables**: Growing conditions, generation time, [population size](/ap-bio/unit-8/population-ecology/study-guide/JiYkhCa7zQ0XPgs6OpbK "fv-autolink"), measurement method

Your control group (random selection or no selection) is critical. Without it, you cannot rule out that the trait distribution changed for reasons other than your selection.

### Connecting Data to Allele Frequency

Your data shows phenotype change, but the AP exam will ask you to explain this in terms of alleles. The reasoning goes like this: the phenotype shifted because individuals with certain alleles reproduced more, so those alleles became more common in the next generation. You are not directly measuring allele frequencies in this lab, but you are expected to explain your phenotypic data using allele frequency logic.

### Evaluating Genetic Diversity

After several rounds of selection, think about what happened to the variation in your population. The histogram probably got narrower (less spread). That narrowing represents a loss of genetic diversity. This is where you connect to Topic 7.11: a population that has been heavily selected is less likely to contain individuals that can survive a new, unexpected environmental pressure.

## Common Mistakes

**Confusing phenotype with genotype.** You are selecting on phenotype (what you can see and measure), not directly on genotype. The connection to allele frequency is an inference you make, not something you directly observe in this lab.

**Saying selection "creates" new traits.** Selection does not create anything. It changes the frequency of traits that already exist in the population. New alleles come from mutation, not selection.

**Forgetting that fitness is environment-specific.** A high trichome density might be adaptive in one environment (where it deters [herbivores](/ap-bio/unit-8/energy-flow-through-ecosystems/study-guide/A1PeQD1Zy3BIMu1zSMzd "fv-autolink")) and neutral or costly in another. Do not say a trait is "better" in an absolute sense. Always tie fitness to a specific environment and selective pressure.

**Mixing up artificial and natural selection.** They work by the same mechanism (differential reproduction based on phenotype), but the selective agent is different. In artificial selection, humans choose. In natural selection, the environment determines who survives and reproduces. The AP exam may ask you to compare them, so keep that distinction clear.

**Ignoring the control group.** If you only report data from your selected population, you cannot make a strong claim that selection caused the change. Always compare to your control.

**Overstating one generation of results.** One generation of selection might show a small or statistically uncertain shift. Be careful about making absolute claims from limited data. Acknowledge that more generations would strengthen your conclusion.

**Thinking reduced genetic diversity is always bad in the short term.** In the current environment, a selected population might perform very well. The risk shows up when the environment changes. Make sure you frame the diversity trade-off correctly: it is about long-term resilience, not immediate performance.

## Quick Review Checklist

- Artificial selection works by humans choosing which individuals reproduce based on a specific phenotype, shifting allele frequencies over generations.
- Phenotypic variation in a population is the raw material that selection acts on; without variation, selection cannot change a population.
- Directional selection consistently favors one extreme of a trait distribution, moving the population mean in that direction over time.
- A histogram of trait values across generations is the key graph for this lab; a shift in the mean and a narrowing of the distribution are the signals you are looking for.
- Adaptive alleles increase in frequency under a given selective pressure; deleterious alleles decrease in frequency.
- Artificial selection reduces genetic diversity, which decreases a population's resilience to new or changing environmental pressures.
- The same allele can be adaptive in one environment and deleterious in another, depending on the selective pressures present.
- Always connect your phenotypic data back to allele frequency logic when writing explanations on the AP exam.