Aim for FiveBiology

Analyze a model or visual representation

A cladogram of land plants

  • Unit 7
  • 4 points
  • About 10 minutes

You can use a calculator on this question, just like on exam day.

A short question built around a model or diagram of a biological process. You describe what the model shows, explain how its parts relate, add to or use the model, and connect it to a bigger biological idea. On the exam: Question 5 of 6 in Section II; about 10 minutes suggested.

The question and its sources

Scientists use shared characteristics and DNA sequences to build models of how groups of organisms are related. Figure 1 is a cladogram of five groups of plants.

Figure 1 (described)

The cladogram is rooted with a green alga as the outgroup, which branches off first. Moving up the main line, mosses branch off next.

After the moss branch, a hash mark labeled 'vascular tissue' appears on the main line, and then ferns branch off.

After the fern branch, a hash mark labeled 'seeds' appears, and then pine trees branch off.

After the pine branch, a hash mark labeled 'flowers' appears. The main line then ends at a final node that splits into two branches: lilies and roses.

Source: Simplified model

Table 1. Number of differences in a 400-base-pair DNA sequence between a newly described plant (Plant N) and four groups

GroupNumber of differences from Plant N
Mosses71
Ferns58
Pine trees12
Roses41

Source: Hypothetical data

Suggested time: 10 minutes

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Part (a)

1 point

Describe what the node where lilies and roses split represents.

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Part (b)

1 point

Using Figure 1, explain why the presence of vascular tissue in ferns, pine trees, lilies and roses supports the hypothesis that these four groups share a common ancestor that is not shared with mosses.

0 / 2,500 characters

Part (c)

1 point

Plant N has vascular tissue and seeds but no flowers. Using Table 1 and Figure 1, describe where you would add Plant N to the cladogram.

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Part (d)

1 point

Explain why a cladogram such as Figure 1 is considered a hypothesis rather than a fixed description of evolutionary history.

0 / 2,500 characters

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