Reclassification of figworts using cladistics

In this lesson students first try to make a cladogram using the physical appearance of flowers from the figwort family. This illustrates how classification has been done for many years. In the second activity students investigate how new data from DNA sequences has led to new evidence which upsets the accepted classification of Figworts and other species. Finally students consider how the evidence from modern cladistics has shown that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species and try to decide what is the best way to advance our biological knowledge.

Lesson Description

Guiding Questions

  • Can you give any examples of how classification of organisms has changed in the light of evidence from cladistics?
  • Why are features of flowers thought to be homologous structures, but the wings of a pterodactyl and bat wings are considered analogous - similar structures which developed independently?

Activity 1 - Introduction to the Figwort Family

Look at this gallery of photos and the diagram of figwort family flower structure below.
Which of the plants looks the most different? Which look most like they would be part of the Figwort family?

  • Biologists have been classifying plants using the external features for many years. If these features are similar the assumption is that the plants are closely related, and that they share many genes. The genotype actually does influence the phenotype, so this makes logical sense.
  • The diagram below shows the generalised features of flowers which have been used to identify members of the figwort family for a long time.

Activity 2 - Using DNA similarities to build a cladogram

A new method is beginning to provide a lot of new evidence about how groups of living things are related. This method takes a few genes and compares the base sequence of these genes in several related species.

Carry out the short activity using DNA sequences to build cladograms​.

Activity 3 - What shall we do? Dilemma

Read the following short quotation from a botanist then answer the questions which follow.

Figwort Family Breaks Up: Read All About It

http://www.tulipsinthewoods.com/science/figwort-scrophulariaceae/

Lumpers and Splitters have come into the garden, wreaking as much havoc as deer. DNA sequencing shows that the Figworts must be in five separate groups, some of them entirely new groups made for the occasion, like children of a second marriage assigned a name they’ve never known. Even these five groups leave the mimulus on the outskirts : mimulus is still unclassified. And to make the splitup even more fraught, Lumpers and Splitters have been tussling over the genus Mimulus for years. Some say that the shrubby ones should be called Diplacus, and the herby ones Mimulus. (For more about this epic struggle, check out this post.)

When botanists create new orders (so to speak), gardeners (conservatives of the botanical world) often stubbornly hold on to the old ones. The gardeners have a point: there were reasons why these plants were bound together under one roof. They have similarities that help us to identify them, know some of their properties, and have a good idea where they like to grow.

The botanists have a point, too. If these plants are different down to their DNA, it makes sense to me that there would be distinctions in their personalities and properties – how could there not be?

Questions

Should we make these changes to the classification and put figworts and plantain together in the same taxonomic group?

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What data do we need to have to be sure that the new classification is really correct?

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Why do gardeners stubbornly hold on to the old classification?

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Extension Activity - Read the original paper which resulted in the reclassification.

Read one of the following links and make some notes for your TOK teacher

ToK to think about

  • The group Archaea was recognised in 1977 by Carl Woese as a distinct domain different from bacteria. This division was not immediately accepted by scientists. What are the benefits of this 'conservatism'?
  • Consider this quote, about the struggle to get a theory accepted, "A theory is a species of thinking, and its right to exist is coextensive with its power of resisting extinction by its rivals".
    Thomas Henry Huxley,Science and Culture, and Other Essays (1890),
  • How closely does this fit with the development of theories of classification?

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Teachers notes

The two activities with worksheets are quite easy to follow but this is a complex topic which has had to be simplified to make it an introduction to cladistics and the use of DNA sequences. Common names have been used for the plants to make the idea more accessible to students

A great improvement to activity 2 would be to provide students with samples of the flowers, or a selection of them, together with a hand lens.

Model answers are available on these pages.  Please remember they are just 'model answers' there are many ways to answer these questions which will follow from the student's own interpretation of the cladograms. 

Cladgram of figwort family - model answers

Using DNA similarities -Model answers (Activity 2)

The essential points to learn are that cladograms are useful in determining evolutionary relationships and in the past these have been created using visible features. Now that we have DNA sequencing methods available at a much more affordable price we can use this data to improve our cladograms. It is also possible for two scientists using the same data to construct two different cladograms. Eventually using DNA sequences this will most likely become less common.
(This could be an interesting example to use for one of the 2017 TOK essays)
 

The readings at the end are more thorough accounts of the methods used and the changes made to classification.

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