by James Morris
“Fish tails go sideways. Whale tails go up and down.” I saw this on a piece of paper on the wall of my wife’s kindergarten classroom. It says a lot about teaching and evolution.
First, teaching. It was Flex Week. This is a time that occurs once a year when the students and teachers throughout school stop what they have been doing, and instead take a week and do something different. They typically focus on a particular topic, such as giraffes or robots, go on field trips, or do a service learning project.
One year, the kindergartners studied the ocean. They made a mural covering an entire wall filled with colorful fish and other marine life; they visited the Boston Aquarium; they wrote a song about the sea to the tune of Over in the Meadow.
They started the week by sitting in a circle and making a list of things they know about the ocean, and things they don’t know, but want to know. This is a technique that my wife uses regularly with her students. It got me thinking about how it could be used more generally.
Why couldn’t a college class begin the same way? In fact, I tried it a few years ago. Instead of starting with a set syllabus, listing all of the various topics and readings for the semester, I began simply by asking students what they already know (or think they know) and what they don’t know but would like to learn.
This technique can be used in a wide range of classes, from science to history to English. It provides a good starting place, allowing the instructor to take the pulse of the classroom and meeting students where they are. And it provides a way for students to take ownership of their learning, to be involved in not just the work, but also the planning of the semester.
Second, evolution. Fish and whales are both strong swimmers with tail fins, but they took two different evolutionary paths to their current forms.
Fish evolved over 500 million years ago in the ocean. They have a backbone running from head to tail, which serves as a central axis. On either side of this axis are blocks of muscles. These muscles contract in an alternating fashion, so the animal bends one way and then the other, resulting in an undulating, side-to-side motion. As a result, fish move their tail fin in a side-to-side fashion, propelling them through the water.
When amphibians and reptiles moved onto land, they retained this basic structure and side-to-side motion, which is most obvious if you watch the way lizards and snakes move.
Whales evolved about 50 million years ago from land-based mammals that returned to the water. Their ancestors walked on four legs. The orientation of the legs, other bones, and muscles in mammals allows for easy motion of the spine forward and back (or up and down) and limited motion from side to side. Think of a swimmer doing the butterfly, with the hips and legs moving in an up-and-down motion.
As the hind legs were lost and the tail fin evolved, whales maintained the same relative orientation of the spine and muscles, so the whale tail moves up and down, as do the tail fins of other marine mammals such as dolphins, seals, and sea otters. Whales essentially gallop underwater.
Both motions (up and down and side to side) work well, allowing the animals to move through the water. They are both an effective means of propulsion, which is the primary function of the tail fin in both fish and whales.
The tail fin of fish and whales is an interesting case of convergent evolution, in which two species share a trait, but evolved it independently of each other. In other words, the two groups converged on the same “solution” (a tail fin) when confronted with a similar “problem” (how to move through water). Another example is wings in birds, bats, and insects. All three have wings, but they each evolved them on their own and did not inherit them from a common ancestor with wings.
It’s easy to be fooled by lookalikes that result from convergent evolution. For example, some people think whales are fish because of their similar appearance, perhaps best captured by Herman Melville in Moby Dick:
The uncertain, unsettled condition of this science of Cetology is in the very vestibule attested by the fact, that in some quarters it still remains a moot point whether a whale be a fish … Be it known that, waiving all argument, I take the good old fashioned ground that the whale is a fish, and call upon holy Jonah to back me.
Similarly, in the Adirondacks, I mistook American hog peanut for poison ivy because the two plants look so similar to each other, with their characteristic three leaves: “Leaves of three, let it be.” Can you guess which is which?
Although fish and whales both have tails, their different orientation and motions show us that they evolved in different ways. Similarly, although birds, bats, and insects all have wings, the structure and movement of these wings are very different because they took different evolutionary routes to get there.
Jonathan Losos, an evolutionary biologist, explores the topic of convergence in his book Improbably Destinies. There, he asks how close two structures need to be to be considered convergent: “What about the tail flukes of cetaceans and sharks, similar in design and operation, but one horizontal and moving up and down, the other vertical and swept left and right? Do these features represent slight variations on a convergent theme, or non-convergent solutions producing the same functional outcome? I suspect that most people would consider horizontal and vertical tail flukes to be fundamentally the same solution.”
Which brings us back to teaching – my wife and I both stumbled on a similar technique in an environment where we wanted students to take charge of their learning. Like the tails of fish and whales, the strategies we use are similar enough, I think, to be called convergent.
© James Morris and Science Whys, 2019