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Reclassification (HL) (HL IB Biology)

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Naomi H

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Naomi H

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Biology

Cladistics & Reclassification

  • The development of DNA sequencing technology means that classification can now be carried out on the basis of evolutionary relationship
    • Organisms classified in this way are grouped into clades
  • Historically, organisms would have been classified on the basis of observed traits, which often led to organisms being classified into groups that were not true clades
  • The use of DNA sequencing to classify organisms has led to discoveries of classification errors and the need to reclassify organisms
    • Some species have been reclassified into different groups of organisms
    • Some groups of organisms have been split 
    • Some groups have been merged
  • Classifying organisms correctly according to their clade is important to ensure that groups of organisms are close evolutionary relatives

Reclassification of the figwort family

  • The figwort (pronounced 'figwurt') family, also known as the Scrophulariaceae, was at one point the 8th largest family in the angiosperm phylum
  • When the group was originally classified in the late 1700s it contained 16 genera, which later expanded to 275 genera
    • The classification of the figworts was based on observable traits such as a tube-shaped flower structure
    • Examples of members of the original figwort family include foxgloves and yellow rattle
  • When DNA sequence analysis began, plant scientists discovered that the shared features of the figwort family were in fact not evidence of shared ancestry
    • Three chloroplast genes were analysed
    • It was discovered that the original figwort family was not a true clade
      • The old figwort family was shown to be paraphyletic, meaning that the group did not contain all descendants of a common ancestor
        • The group contained plant species that should have been placed on separate branches of a cladogram
    • The figworts were found to contain several separate plant families
      • New families were created
      • Several genera were moved into other existing families
      • The remaining genera were grouped together with two previously missed genera to form the new figwort family, still known as the Scrophulariaceae
  • The new figwort family is less than half its original size

Figwort family diagram

Examples of plant species; foxgloves and yellow rattle have been reclassified while figwort remains in the Scrophulariacaeaa

DNA sequencing data led to the reclassification of many species within the original figwort family

NOS: Theories and other scientific knowledge claims may eventually be falsified

  • A theory is an explanation of observed phenomena that is supported by evidence
  • This means that when new evidence is found that no longer supports the theory, it needs to be changed to take the new evidence into account
  • Reclassification of organisms on the basis of DNA sequencing data is a good example of this
    • Scientists theorise that a group of plants should be classified together on the basis of existing evidence; historically this would have been the evidence of observable traits such as flower shape
    • New evidence from DNA sequence analysis shows that these plant groups are not true clades, and so are not descendants of a common ancestor
      • The observable similarities between these plants are instead due to convergent evolution
    • The historical classification theory has therefore been falsified and needs to be changed to take the new evidence into account
    • New evidence from DNA and computer analysis is used to calculate the most likely plant evolutionary relationships, and this evidence is used to form a new theory regarding the classification of plants
  • Analysis of new DNA data continues all the time, and if evidence is found that doesn't fit with current classification theories then more falsification and reclassification will take place

Exam Tip

Note that you are not expected to memorise the specific details of the figwort case study shown here; the case study is provided to help develop understanding of the reclassification process.

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Naomi H

Author: Naomi H

Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.