III. Looking for Patterns in Collection Data (45-60 minutes)
Explain to students that lots of different scientists have been collecting individual specimens from each of these clades to learn more about them. Each time a specimen is caught some basic information is recorded such as their stage of development, sex, size, and depth caught. Johnson et. al. compiled this data and noticed some interesting patterns that contradicted the Three Clade Hypothesis.
The students' challenge is to analyze this same data presented as the Fish Collection Data Table on page 3 of the Student Research Notes to look for interesting patterns that might suggest an alternative to the Three Clade Hypothesis. Guide students to look for similarities and differences in the data between and among the three clades. You can use the following questions to help students in their evaluation of the data:
What is unusual about the Whalefishes that were collected? They are all females, adults, and found in deep water.
What is unusual about the Bignose fishes that were collected? They are all males, adults, and found in deep water.
What is unusual about the Tapetails that were collected? They are all immature (larval forms), their sex cannot be determined, and they are caught in the upper 200 meters of water.
Students should record the patterns they observe below the data table. Students should notice that the only Whalefishes caught are adult females, the only Bignose fishes caught are adult males, and the only Tapetails caught are immature. They should also see that almost all Tapetails were found above 200m, whereas individuals from the other two groups were all found at depths below 1000m. They may also observe that there were many more Whalefishes caught than the other two groups; that Tapetails have the smallest minimum size of all the groups, and that Whalefishes grow to be the largest.
Students should discuss as a class their ideas and record responses to the following questions. Encourage students to consult the Fish Reproduction and Life Cycle section of Fish Resource Guide as a reference:
How does this data impact the Three Clade Hypothesis? It calls into question that these fishes are in three different clades. One would expect to see both adult males and females and immature forms, among the specimens of each group, if they were indeed separate families.
What is an alternative explanation or hypothesis to explain the patterns in the data? The members of the different fish groups are really members of the same clade and represent different sexes and life stage.
Share with students that they will call this alternate hypothesis the Single Clade Hypothesis.
Three Clade Hypothesis: The three types of fishes represent three different clades. The patterns in the data could be a result of incomplete sampling (i.e. we have not yet caught representatives of each of the sexes and growth forms).
Single Clade Hypothesis: Tapetails, Whalefishes and Bignose fishes are all in the same clade. Whalefishes are adult females, Bignose fishes are adult males. The huge differences observed between Whalefishes and Bignose fishes are the result of extreme sexual dimorphism. Tapetails are the larval forms that transform into the adults.
Have students write the two alternative hypotheses on page 4 of the Student Research Notes and generate ideas for how they could test the hypotheses. Ask students to think about what type of data they would collect. They can refer to the Fish Resource Guide to have them explore different features that might be useful for testing the hypotheses. This is a great opportunity to discuss what makes a good hypothesis and the importance of alternative hypotheses (for more information, visit Understanding Science.
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