Reclassification (HL) (HL IB Biology)

• 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

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