NOVA scienceNOW: Bird Brains
Students will compare the sequence of amino acids in a gene shared between humans and six other organisms and infer evolutionary relationships among the species.
Students will be able to:
explain that different organisms often have the same genes.
understand how scientists use genetic differences to infer evolutionary relationships.
relate how shared genes may be a result of shared evolutionary history.
provide evidence suggesting that living things share common ancestors.
One class period
In the NOVA scienceNOW segment Bird Brains, students learn that organisms as diverse as mushrooms, fish, flies, and humans share a gene called FOXP2. This gene produces a type of protein called a transcription factor, which turns other genes "on" or "off." Transcription factors regulate many other genes, and because of this, they may affect multiple processes in different organisms. In animals, the FOXP2 gene is especially active during embryonic development in the brain, gut, heart, and lungs, but scientists are still unraveling which genes it regulates in each of these tissues.
As explained in the NOVA scienceNOW segment, FOXP2 also plays a role in the processes involved in human speech and birdsong: people with an altered form of the gene have difficulty with many aspects of speech, and birds whose FOXP2 activity is disrupted have trouble learning songs. Despite these and other observations, scientists still don't know which other genes FOXP2 regulates or what its function is in the numerous other species that share this gene with birds and humans. That FOXP2 is so widespread raises additional questions, not only about its role in other organisms, but also how the gene differs from one organism to the next.
All life on Earth arose from a single common ancestor, and our genes reflect this shared ancestry. As species differentiated over evolutionary time, the DNA sequences in their genes acquired slight changes. According to evolutionary theory, these changes accumulate over time: species that diverged from each other long ago have more differences in their DNA than species that diverged recently. Scientists use this degree of difference as a molecular clock to help them predict how long ago species split apart from one another. In general, scientists say the longer ago two species split, the more distantly related they are.
You may need to remind your students about the nature of DNA, genes, proteins, and amino acids and how they differ from one another. DNA is a molecule made up of four types of units called bases. The four bases—adenine (A), cytosine (C), guanine (G) and thymine (T)—collectively make up the DNA "alphabet." Genes are distinct locations along the length of a DNA molecule. The sequence of bases in a gene determines the order of amino acids in a protein, and the order of amino acids acts as the blueprint for protein assembly.
Because the DNA sequence determines a protein's amino acid sequence, a gene shared by two closely related organisms should have similar, or even identical, amino acid sequences. That's because closely related species most likely diverged from one another fairly recently in the evolutionary span. Thus, they haven't had as much time to accumulate random mutations in their genetic codes.
For years, scientists have used DNA and amino acid sequences to decipher relationships between closely related species, such as different types of reptiles, birds, and even bacteria. The approach, called "molecular phylogeny," compares sequence data and ranks organisms' degree of relatedness based on the differences in their DNA. As researchers sequence the genomes of an increasing number of organisms every year, they uncover more data to use in evolutionary studies. In the emerging field of phylogenomics, researchers simultaneously compare numerous genes—and will one day compare complete genomes—to build new evolutionary trees.
In this activity, your students will analyze a suite of amino acid sequences from a gene that makes the protein Cytochrome C. All eukaryotic organisms share this protein, which plays a central role in the energy-producing process of cellular respiration. Cytochrome C is an iron-containing molecule that carries electrons during the electron transport chain in cellular respiration. The protein is found in many lineages, including those of animals, plants, and numerous unicellular species. Its ubiquity makes it a convenient tool for studying evolution. By counting the number of amino acid differences between humans and six other species, your students will be able to make predictions about how closely related humans are to each species.
Before the Lesson
- Lead a short brainstorm session about how scientists classify organisms. What criteria might scientists use to determine how closely related two species are? They might look for similarity in physical features, behavior, mode of reproduction, or genes.
- Introduce the concept of using molecular evidence, such as DNA or amino acid sequence data, to unravel evolutionary relationships between species (see background). You might point out that for some species, physical traits alone don't offer enough clues. For example, is a horse more closely related to a dog or to a buffalo? All three have fur and walk on four legs, but these clues don't tell you much about evolution. Optional: If possible, show the short animation Biology: Molecular Differences. Ask students what additional information DNA evidence provides scientists studying evolution.
- Divide the class into pairs and distribute the Predicting Evolutionary Relationships handout.
- Work through an example as a class.
- Explain that each letter in the table Amino Acids in the Protein Cytochrome C represents an amino acid in the protein Cytochrome C. The key shows them which amino acid corresponds to each letter.
- Call students' attention to the amino acid sequences for humans and tuna. Be sure students understand that because the sequence is too long to fit on one line of text, it wraps to a second line. Explain that they will look for the number of amino acids that differ between humans and tuna. Also explain that plain-text letters represent amino acids that may vary between species, while letters in bold are amino acids that are identical in all species.
- First, count the number of differences in the sequence together. The first difference is at position 17; humans have an "I," while tuna have a "T." Be sure all students can identify the 21 differences between humans and tuna.
- Have students complete the handouts.
- To wrap up, discuss the following points as a class:
- The table lists three species of fungi: Candida, Neurospora, and baker's yeast. How similar are their Cytochrome C sequences? Their sequences are quite different, with 41 differences between neurospora and baker's yeast, 43 between neurospora and Candida, and 27 between baker's yeast and Candida. What can you say about the evolutionary relationships among the fungi compared to the relationship between the two insects on the table, the screwworm fly and the silkworm moth? The fly and the moth are more closely related in evolutionary time; there are only 14 differences between the fly and moth Cytochrome C sequences.
- Pigs, cows, and sheep have identical Cytochrome C sequences. How can they have the same sequence but be different species? The difference between species is determined by many factors; different species can still have identical sequences, especially if they diverged from a common ancestor recently in evolutionary time.
- Is it appropriate for scientists to infer evolutionary relationships based on information from only one protein? Why or why not? These animals each have thousands of genes. The fact that one gene is identical for the three animals says nothing about the other genes. It's better to look at multiple proteins or other sources of DNA evidence. Proteins evolve at different rates, and additional pieces of evidence will make a prediction about an evolutionary relationship stronger.
Divide the class into four teams. Assign each team one of the following genes: FOXP2, hemoglobin alpha, eyeless, and sonic hedgehog. Have students visit the Kyoto Encyclopedia of Genes and Genomes and look up their gene's amino acid sequence in humans. Have students research how many of the six species from their handouts share this gene with humans; for all cases in which species share the gene, have students write down the first ten amino acids listed in the database. Then have students prepare a short report about the gene, how much similarity they discovered between humans and other species, and what scientists know about the gene's function.
Human: gray whale ___9____
Human: snapping turtle: ____15___
Human-rhesus monkey: ___1__
Human: chicken/turkey: ___13____
Human: neurospora (a type of bread mold): __51_______
Student Handout Questions
- Based on the amino acid sequence data you collected, which organism are humans most closely related to? Which organisms are humans most distantly related to? Explain your reasoning.
Humans are most closely related to the monkey; there is only one amino acid difference between the two. Humans are most distantly related to Neurospora; there are 51 amino acid differences between the two.
- What additional data or information might help you confirm the statement you made above?
Information from other genes would strengthen the statement; we also could use fossil evidence or physical evidence such as similarity in physical structures and features.
- Does your answer to Question 1 above match the prediction you made in Step 2 of the Procedure? Explain your answer.
Answers will vary; look for evidence that students compare their answers and explain why they are the same, or why they are different.
- Explain how amino acid sequence data can help scientists infer patterns of evolutionary relationships between species.
An amino acid is one of the building blocks of a protein. A gene's DNA sequence determines the order of amino acids that make up a protein, so changes in the DNA sequence often result in changes in the amino acid sequence as well. By looking for amino acid sequence differences between species, scientists can infer how closely or distantly related two species are in evolutionary time.
Use the following rubric to assess each team's work.
|Completing handouts and participating in discussion
- Students clearly understand how molecular evidence relates to inferring patterns of evolution
- Students ask follow-up questions showing creativity and critical thinking
- Students miscount amino acid difference between species and do not make a connection between molecular evidence and patterns of evolution
- Students make little effort to complete handouts or participate in discussion.
The "Bird Brains" activity aligns with the following National Science Education Standards (see books.nap.edu/html/nses).
Content Standard C
- Molecular basis of heredity
Content Standard F
Science in Personal and Social Perspectives
- Personal and Community Health
Classroom Activity Author
Jennifer Cutraro and WGBH Educational Outreach Staff
Jennifer Cutraro has 12 years of experience in science writing and education. She has written text and ancillaries for Houghton Mifflin, K12, and Delta Education and has taught science and environmental education at science centers across the country. She also contributes news and feature stories about science and health to media outlets including The Los Angeles Times, The Boston Globe, Science News for Kids and Scholastic Science World.