Module Overview

How does natural selection lead to the formation of new species?

What makes a species?

What is a Species?

This short video introduces the many ways to define a species and the murkiness inherent in trying to do so. Do not focus your students on the various types of species definitions presented in the video. Rather, use the short worksheet to focus their attention on the “big idea”: that there is no one perfect definition of a species.

Project the video to the whole class and have students fill out the associated worksheet. You may discuss the worksheet questions as a class instead of printing out individual worksheets.

  • “Species” is a human construct for which there is no one perfect definition.

10 minutes

Internet access, projector, speakers

What is a Species? (video)

Make one copy per student or pair:
Student Worksheet (fillable pdf)

Same or Different Species?

Students read cards describing pairs of organisms, then place them along a speciation continuum, ranging from “Definitely the same species” to “Definitely different species.” The cards include information about the organisms’ habitat, heritable traits, DNA differences, and ability to interbreed. There isn’t one particular way to build the continuum, or a “right” answer. Rather, this activity is an exercise in weighing evidence and recognizing that speciation is a process.

Have students work individually or in pairs to build the continuum. Debrief as a class. The Teacher Guide includes an “answer key,” notes, and discussion questions.

  • One way to define species is a group that includes individuals capable of reproducing with one another.
  • Speciation is a process.
  • Speciation can result from natural selection acting on multiple heritable traits over time.

20 minutes


Teacher Guide (pdf)

Student Pages (pdf)
Make one copy per student or pair. Cut out the cards on pages 2-3 (cards may be re-used)

Materials for digital whiteboards (ZIP file)

Reproductive Barriers

This presentation outlines five different barriers that can prevent populations from interbreeding. Project the slide show to the class and discuss. Rather than focus on the types of reproductive barriers, draw students' attention to how each barrier decreases allele mixing, and how natural selection may shape each population's traits differently.

  • Speciation begins when barriers to reproduction within a population lead to two reproductively isolated populations whose alleles are no longer mixing.
  • Reproductively isolated populations may independently gain or lose alleles through mutation and natural selection.
  • Over time, reproductively isolated populations become increasingly different in their DNA and their traits.

15 minutes

Internet access, projector

Are Flies From Apple and Hawthorn Fruits Becoming Two Different Species?

Hawthorns to Apples

This short video introduces the story of hawthorn and apple flies, setting up the following New Host, New Species? activity. Through this case study, students will examine whether or not the population of flies living on apples is becoming a new species. Project the video to the whole class immediately prior to the New Host, New Species? activity. Review the main points of the video and proceed.

  • Opportunities exist to observe speciation happen in real time.
  • Species differ from one another across multiple heritable traits.

5 minutes

Internet access, projector, speakers

Hawthorns to Apples (video)

New Host, New Species?

Students examine several lines of evidence to determine whether or not a population of flies that moved to apples is differentiating into a new species, then they write an argument with supporting evidence.

This activity involves multiple steps, which are explained in greater detail in the Teacher Guide:

  1. Students evaluate one of three lines of evidence in small groups, filling out a worksheet and a Speciation Organizer for their line of evidence.
  2. Small groups report out to the class, and students add information from groups onto the Speciation organizer they have begun.
  3. Students consider all evidence presented, and write an argument that includes claim, evidence and reasoning, using the completed Speciation organizer for support.

Answer keys and extension ideas are also included in the Teacher Guide

  • A population is a group of individuals that live in the same area and whose alleles are mixed through reproduction.
  • Speciation begins when barriers to reproduction within a population lead to two reproductively isolated populations whose alleles are no longer mixing.
  • Speciation is the process through which new species form. A speciation event represents a branch point, where one genetic lineage splits into two.
  • Barriers to reproduction, selection for different heritable traits, reduced ability to make hybrid offspring, and reduced allele mixing contribute to speciation.
Cause & Effect

Students use data to decide whether or not the introduction of a new environmental factor (cause) is leading a population to diversify into separate species (effect).

Analyzing and Interpreting Data; Argumentation

Students analyze data from actual studies and generate a claim based on those data.

90-120 minutes

Copies, document camera (optional)

Teacher Guide (pdf)

Make one copy per student:
Speciation Organizer (fillable pdf)
Note: If students use the pdf electronically, they can use the highlight tool to mark their answers for the “consider the evidence” questions.

Divide the class into three "Expert Groups." Make enough copies to ensure one copy per group member:
Fruit Preference (fillable pdf)
Life Cycle Timing (fillable pdf)
Alleles (fillable pdf)

Unity and Diversity of Life

This video summarizes key concepts from speciation, describing how the same processes carried out over very long periods of time have led to the diversification of life from common ancestors.

Project to the class and discuss.

  • The continual branching and independent evolution of new genetic lineages has led to the diversity of life.
  • After diverging from a common ancestor, independently evolving lineages may accumulate many trait changes through natural selection acting over the course of many generations.
  • Differences accumulate over time.
  • Evolutionary time is immense.
  • Given enough time, many changes can happen.

15 minutes

Internet access, projector, speakers

Before moving on...

Before moving on, make sure your students understand the following:

  • Species differ from one another across multiple heritable traits.
  • Mutation, allele shuffling, and natural selection acting on multiple traits over many generations in reproductively isolated populations cause the divergence in characteristics of living things.

Summative Assessment

This final assessment for the unit asks students to explain how evolution causes species to gain or lose traits over time.

The summative assessment uses ACORNS items that are designed to be analyzed with EvoGrader. For more information about EvoGrader, visit

For more information about naive and science-based ideas that EvoGrader analyzes, see tables 1 and 2 of the following article: href="

20 minutes