Teaching about natural selection gives a mechanism for how species change but it doesn't directly address how one species can diverge into two. Speciation, then, needs its own lesson. This post shows you how I've taught it my IB biology (16–18) students using diagrams and dialogue, culminating in a stock and flow model.
Speciation—what will happen?
I didn't just tell students about speciation because meaning can't be transmitted. Instead, I provoked them with a difference; some variation. Below is the drawing I produced to begin the lesson. The students had learnt about natural selection and mutations previously, but they hadn't learnt about speciation. This was something new for them to discern.
I told the students that there was a population on the left and a sample of the population managed to disperse around a geographical barrier. They survived and established a new population on the other side.
In scenario 1 there are different selective pressures on either side of the geographical barrier.
In scenario 2 the selective pressures are the same. What happens next, in terms of evolution?
Take note. This is a key to a pedagogy of meaning making using variation. You should present two situations that differ in only one aspect (or as few as possible), and then ask questions along the lines of: What's the difference? and, What difference does it make?
I gave the students time to converse in pairs, then we discussed the scenarios as a class. I asked a student to present their idea of the implications of scenario 1. As is typical, they said that the two populations would begin changing in different ways as they both evolve to differing pressures. When I asked the class to vote if they agreed or not, they all did.
I asked the class if they thought they would evolve into different species, given enough time. In other words, if we reunited the populations physically after enough generations, would they continue to reproduce & engender fertile offspring? The students thought this was quite obvious but when I asked why, they weren't sure of a mechanism.
This led us to scenario 2. I asked what would happen and a student explained that they populations would remain as the same species. Afterwards, the class again voted that they agreed. Now it was time to provoke the students with more variation; I drew wrote this: Scenario 1: Different mutations + different selective pressures = speciation
Scenario 2: Different mutations + same selective pressures = ...
The original provocation had shifted my students awareness to the difference in selective pressures between the scenarios. It was time to shift that focal awareness to the aspect of mutation. In discussion, we agreed that mutations are random and therefore in each population the mutations would be different.
To help students see this however, I provoked them with variation in the idea:
I gave them a third scenario: same mutations + same pressures,
and finally a forth: no mutations + same pressures.
Random mutations, then, are the difference maker. Given enough time, the gene pools are sent along different paths in pursuit of maintaining congruent adaptation with an environment. Through conversing and varying the aspect of mutations, the students discerned this for themselves.
A concrete example
We watched this video showing how speciation occurs through reproductive isolation in the context of the birds of paradise. This helped concretise the concept and lead us into the context of islands.
Enacting a model
From here, it was time to formalise the ideas into causal model. Specifically, I wanted to build a stock and flow diagram (see Difference Maker for how to embed these in your teaching). I began with a single stock: Species Richness with its inflow and outflow. I told students it represented the number of species on an island.
I asked the students to name the flows. Here I'm asking students to draw a distinction between what directly and indirectly affects the number of species. For example, students may offer answers like reproductive isolation or geographical barriers. I reminded the students that a stock and flow model is like a bathtub, if water is in the bath, water must flow through the tubes.
In our model, species are in the stock, so species must flow through the tubes. So what could we call the flows? Through conversing, they agreed that the inflow is speciation and the outflow is extinction. Flows occur over time, so these are: speciation rate and extinction rate. This model suggests then that if the speciation rate is higher than the extinction rate, the number of species would rise.
I could now ask the students for the cause of speciation and we agreed on "reproductive isolation". I added the variable to the model but asked the students if the connection was "same" or "opposite": Same: More X, more Y; or less X, less Y.
Opposite: More X, less Y; or less X, more Y.
The students voted for "same", all good so far. The larger the extent of reproductive isolation, the higher the speciation rate. However, I wanted to define this more specifically for students, so I added a note to the model; reproductive isolation is a reduction in gene flow between populations.
The question was obvious, what would cause an increase in reproductive isolation? I gave students time to converse and they came up with a number of ideas. For example, the number of geographical barriers on the island. Eventually, by making reference to island biogeography, I suggested "the number of available habitats". I added it to the model and asked students to vote if the connection was same or opposite.
What if, I asked, a single species arrived to an island and there were many available (differing) habitats? Following the model with our fingers, we saw that at very large number of available habitats would lead to many reproductive isolation events. This, in turn, would increase the speciation rate. I told the students this was called adaptive radiation and referred to Darwin's finches as an example.
I wanted to complete the model with reference to island biogeography because it allows students to make inferences about their world. Therefore, I asked them what would allow an island to have more habitat types. Many reasons can be given here but I told them that a likely indicator is the size of the island. So, we added "area size" to the model.
Yet, larger islands will likely have a high species richness for two reasons. The larger an island, the more likely it can carry larger population sizes. And the larger the population, the less likely it will go extinct. We added this to the model also, again, by asking students to vote on whether connections were same or opposite.
Finally, I asked students what variable would increase the rate of extinction. We concluded that that rate of environmental change would do so. The faster the rate of change, the more likely species would go extinct. We related this to climate change and the current rates of extinction.
To finish the lesson, I gave my students time to self-explain the diagrams we'd enacted together (as if they were explaining it to someone who had missed the lesson). This is teaching with diagrams and dialogue; do you want to co-construct meaning without lecturing, slide decks, or leaving students to discover for themselves? Learn how and why in my books. Download the first chapters of each book here.