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1.7.3Inquiry Process

Reclassification — IB Biology HL Revision Notes & Topical Questions

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Section 1 of 5Why Groups Change

1.7.3 Reclassification

Classification is a hypothesis about evolutionary relationships, so it must change when evidence changes.

New molecular data
  • DNA and protein sequences give much larger amounts of evidence than appearance alone, and can reveal hidden relationships.
Convergent evolution
  • Organisms may look alike because of similar environments rather than close ancestry, and so be misplaced.
Paraphyletic groups
  • A group that excludes some descendants of its common ancestor is not natural and must be revised.
Cladistics
  • Cladograms built from sequence data show branching order and test whether a group is a true clade.

Section 2 of 5Examples

Examples
Archaea
  • In the 1970s rRNA comparisons showed that archaea are as different from bacteria as from eukaryotes, leading to three domains.
Plant families
  • The Scrophulariaceae (figwort family) were split and many genera moved to other families after DNA analysis.
Birds and reptiles
  • Birds descend from within the reptile lineage, so the traditional class Reptilia excluding birds is not a natural group.
Whales
  • DNA studies place whales close to hippopotamuses within the even-toed ungulates.

Section 3 of 5Process of Reclassification

1
Researchers sequence a gene or whole genomes from many species.
2
Computer programs align the sequences and construct a cladogram based on shared derived changes.
3
The new tree is compared with the existing classification.
4
If a group is not a clade, species are moved or the group is split.
5
Scientists publish the revision and the community debates and tests it with more data.
Old basisNew basis
AppearanceDNA/protein sequences
Two kingdomsThree domains
Prokaryote groupBacteria and Archaea

Section 4 of 5Worked Example

Question

  • Two groups of plants were placed together in one family because they have similar flowers.
  • DNA sequencing shows that one group shares a more recent ancestor with a different family.
  • State what taxonomists should do and why.

  1. Similar flowers may be the result of convergent evolution, so they are weaker evidence than DNA sequences.
  2. A natural classification must include all descendants of a common ancestor and no unrelated groups.
  3. The group whose DNA shows a closer link to another family should be moved into that family.
  4. This is reclassification based on new evidence.

Answer Move the group to the family with which it shares a recent common ancestor, because classification should reflect evolutionary relationships.

Section 5 of 5Exam Checklist & Tips

1.7.3 – Key Points for the Exam

  • Reclassification Moving organisms to new groups after new evidence.
  • Clade A group containing an ancestor and all its descendants.
  • Paraphyletic A group that leaves out some descendants of its ancestor.
  • rRNA Ribosomal RNA, used to compare species.

Common Mistakes

  • Saying scientists changed the groups because the organisms changed.
  • Thinking classification is fixed and final.
  • Forgetting that similar appearance may result from convergent evolution.
  • Mixing up clade and paraphyletic group.

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