Reclassification (DP IB 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

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