I start out this lesson by projecting an image in which individual species show noticeable variability. For example, I might show them this image of Coquina clams and ask them to find specific similarities and differences. As third graders often seem to want a number, I ask them to find at least five similarities and five differences and to write these down in note or word form in their journal. (I do reiterate that I would much prefer three quality observations to a ten item list that is obvious and not well-considered. It is a constant but important undertaking to remind them that quality is far more important than quantity). Here is another example of a lesson opener (butterfly similarities).
Young learners (though I would say all learners) benefit from examining a topic through the lens of categorization/sorting and this kind of visual task makes certain that the content is available to all students. While I stress specificity of language I want to be clear that I do this with an awareness of each individual student's starting point. My sentence expectations for ELL students or any students who doesn't yet have well-developed language skills are different than they are for students with a richer vocabulary. The end goal remains the same, but the degree of scaffolding varies.
For example, The response, "Most of the moths have a circle on their upper wing," might be appropriate for some students, while I might push another child towards, "Most of the moths have a circular patch/ eye spot in the upper corners of their wings and the scales in these eye spots are (distinctly) different in color from the surrounding scales/pattern.
So, after the students have had a chance to list their details, I ask them to turn to a partner and share, using the following sentence stems:
A similarity I notice is that all the clams...
I circulate to make certain that the students are engaged and the number of examples I use is highly dependent on the level of engagement.
If you are operating within rigid time constraints for science, then this part of the lesson can be done well even if you use only one visual example.
We share out the similarities and differences as a whole group but again, if you have a large class or simply prefer small groups, this could easily be done within small groups as well.
Now that we've listed the observable similarities and differences of the moths, or the clams, the students are ready for the next step. I present them with the following scenario, or something similar:
Coquina Clam Scenario: These colorful little clams, usually ½ to 1 inch in length, live in many locations, including the Atlantic coast of the United States. As some of you may agree, clams are delicious! Humans eat coquina clams but they have many additional predators, including ghost crabs. Imagine this: A Ghost Crab is scuttling along the tidal zone of a beach in Florida. It is hunting Coquina clams. The beach is covered with round, colorful pebbles in many shades of yellow, light orange, gray and white. Which Coquina clams are most likely to be seen by the Ghost Crab and which are more likely to escape her notice because they blend in with the pebbles?
I ask students to think without raising their hands. I usually time them for at least 1 minute, even though some of them immediately want to share. I ask them to be prepared to explain their choice when they point out a particular clam. For example: "I think that this clam would quickly be eaten by the Ghost Crab because its darkish red color would be very easy to see on top of the light-colored rocks."
Depending on the level of your class, you can easily carry this discussion one step further. You can cross out all the clams that your students think would be eaten because they are too visible. You can then point out that perhaps the light colored clams will have light-colored offspring (the trait is probably not inherited in such a straightforward, one generation way, but the gist of the idea is on track). Over several generations (or, okay, millennia) the lighter colored clams will survive to reproduce in this particular location and the darker colored clams will not. Eventually, it would be possible that all the clams in this region of light colored substrate would be light colored themselves. It's a nice way to segue into simple explanations of changes over time that allow an animal (or plant) to survive that happen as a long-term response to the environment. (Evolution!)
You never know what is going to interest students, and as I had several children who were very interested in learning more about clams, it's possible that you may as well! If so, here's an interesting website that's not too far about 3rd grade level that provides more details about the habitat, movement, predators, prey, and unique coloration of coquina crabs.