Evolution - Homologous Structures & Embryology
Lesson 11 of 17
Objective: Students will be able to identify similarities is morphology and early embryo development as evidence for evolution
In order to engage the students in today's lesson, I display the following slideshow, starting at slide 5.
Slide 5 is set to be animated, so that I can wait for students to look at their notes and/or talk to one another about what they remember. After a volunteer or two offer up the evidence, I click and also click so that the check mark appears to indicate what we have already covered in depth. This is a simple exercise aimed just at setting the stage for learning.
I display slide 6.
I ask students to help me remember what the Greek root word "homo" means. This is usually met with giggles, but I remind the students that this is science class, so we need to understand root words, regardless of the connotation they may have in other contexts. Once the giggles die down and we agree that "homo" means same, I ask the students to Think-Pair-Share what they think Homologous Structures might mean. After about a minute of think time, students turn to their elbow partner, and talk about their ideas. Two minutes after that, I use popsicle sticks to elicit a response from a couple of pairs. This TPS allows students to make connections between what they know about words and their potential meaning.
I continue to present, giving the formal definition of homologous structures in the slide. I proceed to the next slide, when I distribute the "Color dem bones" sheet. I tell the students that their job is to try to identify the different homologous structures they see. Before they start coloring, they must discuss their observations with their tables, "This is not about choosing five colors and coloring things in, it is about identifying the patterns you see in the different bone structures." (CCC Patterns Graphs, charts and images can be used to identify patterns in data.) I also tell the students that as I am walking around, I should hear conversations like, "I think this is probably the ulna, although it is shorter and thicker than the human one, it looks to be in the same place. What do you think?" (SP7).
Note to teachers: Giving students sentence frames for the conversation encourages students that might not be ready to discuss a scaffold to actively participate. I use sentence frames like this one to clarify the expectations and help guide the discussion between the students. In this video, you see how the students that might struggle are guided and able to use the sentence frames provided.
The student work shows that they were able to see the similarities between homologous structures.
Once the students have finished their coloring, I regain the attention of the class. I ask for a reporter from each table to share their completed drawing and their findings. As the students listen to each other's findings, I act as moderator, moving from one reporter to the next without offering my opinion, but rather letting the students ask questions from each other. (SP8: Engage in discussion with scientific peers).
At the end of the discussion, I present slide 8, and ask the students, "Based on what we have discussed, and the evidence you see here, who do you think we (humans) are more closely related to, bats or chickens?" I ask students to discuss their answers before they commit them to the Bats or Chickens handout. This half-sheet ACE exercise requires that students evaluate information to form hypotheses (SP6), and lets me know whether they have understood the concepts being discussed. I also lets me understand whether students are able to evaluate details in the evidence for evolution (SW1, SW2, SW3). Watch as students discuss their evidence and realize that this is only part of the story.
Evidence from Embryology
I continue with slide 9, and briefly explain that when looking at embryos, it is hard to tell different organisms apart.
I present a segment from NOVA "Common Past, Different Paths", before navigating to Guess the Embryo. I play this brief the interactive with the whole class, asking students to give evidence for their guesses. Finally, I present the last few slides in the presentation. These slides are intentionally left without captions until the end, to drive the point across: "In the early stages of development, embryos with a shared ancestry look very similar."
At the end of the presentation, I have a slide that again asks the students what are the four lines of evidence for evolution we studied, and confirm that we have covered them all.
At the end of the lesson, I post the following question on Edmodo:
"In your opinion, what is the most compelling evidence for evolution. Why?" Use the ACE strategy to answer."
Note to teachers: This question usually ends up being homework. In their answers I am looking more for the evidence they cite to support their opinion than anything else. This question is about students being able to examine their own understanding in light of the evidence presented (SP7), as there is really no right or wrong answer. A few may even mention that on its own, no single line of evidence is more compelling than another, but that when taken together, the four lines of evidence provide strong support for evolution.
You may notice in their answers that the students came to the conclusion that "embryology provides the most compelling evidence for evolution". This was probably influenced by the presentation (slide 9 - mentions Darwin's opinion). Although I would have liked to see some other opinions, the students referenced the evidence they were provided with so, in terms of examining their understanding in light of the evidence presented, their responses are well supported.