PBS Evolution A Critical Review
Viewers Guide

Activity 5: How Do You Know?

Accompanies the Viewer’s Guide, Chapter Four, “The Evolutionary Arms Race”


An important aspect of scientific literacy is being able to judge how much confidence we can place in the conclusions that we draw from empirical research or other sources. Part of this skill involves being able to spot the difference between well-supported claims and conjectural ones–those that are founded on insufficient information and reasoning.

One kind of speculation is what Harvard paleontologist Stephen Jay Gould, featured in Episode One, has called “just-so stories.”

“Evolutionary biology,” Gould said, “has been severely hampered by a speculative style of argument that records anatomy and ecology and then tries to construct historical or adaptive explanations for why this bone looked like that or why this creature lived here.”

Gould continues, “Scientists know that these tales are stories; unfortunately, they are presented in the professional literature where they are taken too seriously and literally. Then they become ‘facts’ and enter the popular literature, often in such socially dubious form as the ancestral killer ape who absolves us from responsibility for our current nastiness, or as the `innate’ male dominance that justifies cultural sexism as the mark of nature.”1

In other words, what sometimes happens is that biologists will gather information about an animal’s current traits and environment–but then go beyond the data by inventing a scenario to explain how things got the way they are.

Although Episode Four does not present us with any ancestral killer apes, it does present us with conjectural scenarios. And it does so without warning viewers. In this activity, students will view Episode Four and try to spot such scenarios.

Learning Objectives

  • Students will be aware that scientific statements may have varying levels of support.
  • Students will be aware that even scientists may present conjectural claims as factual.
  • Students will identify conjectural historical claims made in Episode Four.

Directions (will require multiple class sessions)

This activity will require you to show the first half of the episode during one class session and the second half during the next class session.

First Session

At the beginning of the first class, tell students that they will be viewing Episode Four of the Evolution series. However, instead of viewing the video the way they have in the past, students will be looking for certain kinds of things. In particular, they will be looking for examples of conjecture.

Ask students if they know what the term “conjecture” means. Have one or two volunteers define the term. Acceptable definitions should identify conjecture as a conclusion or opinion that is based upon insufficient evidence. Some students might have the idea that a conjecture is something that’s untrue. Correct this misconception by pointing out that a conjecture may be true, but that it’s unsupported by sufficient evidence.

Use the following questions to help flesh out the idea of what conjecture is:

Q. If we wanted to know how fast cheetahs can run, how would we go about finding that out?

A good answer would be something like, “We clock the speed of cheetahs when they are chasing game.”

Q. Now, let’s say someone measures the speeds of cheetahs in different parts of Africa. On the basis of his measurements, he then says that the cheetahs got their great speed due to environmental pressures that forced them to run fast to catch game and survive. Is this a well-supported claim or conjecture?

Students should identify this as conjecture. Measuring the running speed of a population of cheetahs at one point in time cannot tell us whether the speed of cheetahs has increased over several generations. That kind of information requires longitudinal research. We would have to measure the speed of cheetah populations over many years. We would also need observations to understand what environmental pressures–if any–were causing change. One way to get an estimate of cheetah speed over the years would be if paleontologists obtained fossils of cheetahs and inferred their speed from the fossilized cheetahs anatomical structure. But in that case, we’d also have to determine how well the fossils allow us to estimate the cheetah’s speed.

Instruct students to jot down any claims in the video that they think are conjecture. Tell them that the next time the class meets they should be prepared to explain why they think those claims are conjectural.

Show the first half of the video.

Second Session

At the beginning of the second class session, show the second half of the video.

After the video, have students say which parts of the video they thought were conjectural and discuss why they think so. Remind students that conjecture isn’t necessarily false, just unsupported. To keep them focused on this distinction, have students discuss how scientists could go about testing the conjectural statements.


  1. Stephen Jay Gould, “Introduction,” in Björn Kurtén, Dance of the Tiger: A Novel of the Ice Age (New York: Random House, 1980), xvii-xviii.