The Origin of Species: Lizards in an Evolutionary Tree
Film Guide
Educator Materials
biologists consider sympatric speciation very difficult to occur because ongoing gene flow prevents species
from evolving different features that would then preclude genetic exchange.
• Stress to students that Losos has been able to do what few evolutionary biologists have done. He has been
able to conduct experiments to test how natural selection drives adaptations. Losos uses islands that have
recently been cleared of anoles by hurricanes as natural laboratories to test natural selection. In one
experiment, Losos and his team transplanted breeding pairs of male and female long-legged anoles from a
larger island to several small hurricane-scrubbed islands which had only bushes left. In the film, we see how
after only a few generations, the populations on islands where the anoles were introduced evolve shorter and
shorter legs—adaptations driven by natural selection that help the anoles be successful in their new
environment.
• Students may ask whether there are examples of convergent evolution in groups other than the anoles. Ask
students to come up with explanations for the torpedo body shape among groups of unrelated species like
dolphins, sharks, tunas, penguins, and extinct ichthyosaurs. Students may also find it fascinating that the
branching pattern found in trees today evolved independently in several groups of plants and evolved several
different times throughout the evolutionary history of plants.
• The most famous case of repeated adaptive radiation is the cichlids of the East African Great Lakes, which
comprise approximately 2,000 species. In Lake Tanganyika, 250 cichlid species have evolved in the past 9
million to 12 million years, and in Lake Malawi 1000 species have evolved in the past 2 million to 5 million
years. Remarkably, Lake Victoria, which is less than 200,000 years old, has 500 to 1000 species. These lakes
have experienced independent evolutionary radiations and have each produced a dazzling array of similar
ecomorph classes, including plankton grazers, algae scrapers, sand filterers, egg predators, and so on, and
many of these ecomorphs have evolved convergently in two or all three lakes. However, our understanding of
this convergent evolution of cichlids lags behind that of anoles.
• The film’s narrator, Sean Carroll, explains that two groups of organisms are defined as different species when
individuals from one group don’t mate with those from the other, or they do not produce fertile offspring if
they do. This is the biological species concept first introduced by evolutionary biologist Ernst Mayr and the
definition most often taught to students. This concept emphasizes reproductive isolation. However, in
practice, evolutionary biologists focus more on size, shape, and color to group individuals into distinct species.
In general, they assume that species that look distinct are unlikely to interbreed. Differences in leg length,
body size, body color, and dewlap size and color distinguish different species of anoles. When studying the
fossil record, for example, morphological species criteria are the only ones available. For the most part, data
from various approaches for defining species, such as observing behavior, studying physical characteristics,
and even testing the ability to mate and produce fertile offspring, agree with one another. Species definitions
are trickiest for phyla other than animals, such as protozoans, plants, fungi, and of course bacteria.
• Near the end of the film, the narrator, Sean Carroll, says, “The simple reason why there are so many species
in the world is that there are so many habitats. And each habitat provides myriad ways to survive.” Carroll
gives the example that in the Serengeti, zebras eat the tallest, coarsest grass, wildebeests the medium-height
grass, and Thomson's gazelles the shortest. Ask students to list as many different habitats as they can think of
that exist on the land that surrounds your city or town. Ask students to then list all the habitats they can think
of that were on that same land 200 years ago. What, if anything, has changed? Ask students how some of the
evolutionary processes they learned about in the film, like adaptation and natural selection, may have been
affected by changes in the local and regional habitats. You may also consider having students perform the
same exercise using your entire state as the landscape.
• Jonathan Losos has shown that contrary to what Charles Darwin thought, species can evolve rapidly. When he
conducts his experiments on the islands, leg length changes within one or two generations. Even so, it is
important to help students understand that most speciation events are infrequent, and it often requires long