Phenomenon
The pattern of unity and biodiversity observed on Earth today has existed in the long history of life on Earth as evident by the fossil record. Over time, the number of species on Earth has fluctuated, going up and down over time.
Sub-Phenomenon: Modern birds are believed to be descendants of ancient dinosaurs.
Question
What happens when the number of species on Earth changes and specifically how do new species form?
–Sub-Question: Where did all these different species of birds come from?
Model Ideas
- New species come from existing species.
- New species come about when existing species branch off and each branch changes over time. Those branches may persist in a similar form, change, branch again, or go extinct.
- When one group of organisms is separated into different populations, the separated populations may be under different natural selection pressures, leading to a change in traits over time.
- When the changes between populations are such that they can no longer mate and produce fertile offspring, they are considered distinct species.
Advanced Planning
There are two activities in this unit. The first is a slight variation on the Wormeaters game from Natural Selection. All the same equipment is needed plus 4 new items to replace the rubber bands as prey. See notes for details. The second activity only requires some color copies of data cards for students.
1: How has life on Earth changed over time?
Overview: We begin by considering what the large animal life was like on Earth just before the last mass extinction and then compare that to the life on Earth right now.
What we figured out... The kinds of living things have changed a lot on Earth over time and the number of species on Earth has decreased at times and increased at others. We briefly explored what it means when species numbers go down and talked through extinction. Then we thought about what happens after big extinction events and realized we need to figure out how new species form.
2: What is the definition of a species?
Overview: We realize we need some clarity about this thing we call species. We show examples of organisms that look alike but are separate species, and some that look very different but are members of the same species. The morphological species concept is thus problematic. We show “non-examples” that may be familiar to students, mules and ligers.
What we figured out... We produced a definition for species. A species is a group of organisms with similar characteristics that can interbreed and produce fertile offspring. We acknowledge that this definition is not perfect for all cases – for example, asexual organisms.
3: How did all the birds get here?
Overview: We now look at the specific example of the evolution of birds from dinosaurs and come up with our first model statement.
What we figured out... We learned that modern birds are descendants of dinosaurs. We came up with our first model idea about how traits can change over time and groups of organisms branch off from each other (basically, “descent with modification”). Moving forward we ask the more specific question of how new species form from existing species.
4: Is selective advantage still part of the story?
Overview: We return to the Galapagos and wonder why there are so many kinds of ground finches. We play Wormeaters (the sequel) and figure out what happens to adaptive advantages when the environment changes for different groups? We then apply these ideas to the finches.
What we figured out... We see that under different conditions different variations may have an advantage. We also have our second model idea which is something like: when natural selection acts on different populations in different ways, they may diverge in their characteristics.
5: But really, how do new species form?
Overview: Here we wonder what conditions lead to new species not just trait divergence. We look at vignettes covering scenarios around reproductive isolation and add a model idea.
What we figured out... We looked at reproductive isolation vignettes and came up with another model idea which was that when a population of a single species is reproductively isolated into two populations, each population will evolve independently until the two populations cannot produce viable offspring anymore.
6: Putting it all together
Overview: Revisit dino to bird in the context of our driving question. We see that once divergence occurs over a long enough period; we get a radiation of species. We use our speciation model to explain something about the dino to bird phenomenon.
What we figured out...We answered our driving question and considered how our ideas relate to our Unity and Diversity question.