IB Biology HLClassification & CladisticsPaper 1 & 2~11 min read
Reclassification
The figwort family was once the eighth largest family of flowering plants, held together by a shared flower shape. Then someone sequenced three chloroplast genes, and the family turned out to be less than half the size anyone thought — because the flower shape had evolved more than once.
📚 What you need to know
DNA sequencing technology allows classification by evolutionary relationship, grouping organisms into clades.
Historically organisms were classified on observed traits, which often produced groups that were not true clades.
DNA evidence has revealed classification errors and the need to reclassify: some species moved to different groups, some groups split, some groups merged.
The figwort family (Scrophulariaceae) was found to be paraphyletic — it did not contain all the descendants of a common ancestor.
Its shared features turned out to be the result of convergent evolution, not shared ancestry.
The family was split; new families were created, genera were moved into existing families, and the new figwort family is less than half its original size.
A theory is an explanation of observed phenomena supported by evidence. When new evidence no longer supports it, the theory is falsified and must change.
Why groups need revising
Before DNA sequencing, groups were built from what could be seen. That produced plenty of groups whose members were not each other’s closest relatives, because similar-looking organisms are not always related.
Sequencing has since exposed those errors, and fixing them takes three forms:
Some species have been reclassified into different groups of organisms.
Some groups have been split into several.
Some groups have been merged into one.
The goal is that every group is a true clade, so that the organisms inside it really are close evolutionary relatives.
Paraphyletic groups are not random mistakes. They happen whenever a group is defined by a visible feature that some descendants happen to have lost, or that others gained independently.
The figwort family
The figwort family, or Scrophulariaceae, was at one point the eighth largest family in the angiosperm phylum. When first classified in the late 1700s it held 16 genera, which later grew to 275.
That classification was based on observable traits — principally a tube-shaped flower structure. Foxgloves and yellow rattle were both counted as members.
Then plant scientists analysed three chloroplast genes, and the shared features turned out not to be evidence of shared ancestry at all.
The original figwort family was not a true clade.
It was paraphyletic: the group did not contain all the descendants of a common ancestor, and included plants that should have sat on separate branches of a cladogram.
The figworts in fact contained several separate plant families.
Nothing about the plants changed. The evidence changed, and the groups had to follow it.
The outcome was a rearrangement rather than a deletion:
New families were created from groups inside the old one.
Several genera were moved into other existing families.
The remaining genera, together with two previously missed genera, form the new figwort family, still called the Scrophulariaceae — and less than half its original size.
You are not expected to memorise the figwort details. The case study exists to show you the shape of a reclassification: a group built on appearance, new molecular evidence, the group found not to be a clade, and the group rebuilt. If an unfamiliar example appears in an exam, that shape is what you write about.
Theories can be falsified
This is the nature of science point for this topic, and reclassification is the perfect example of it.
Theory
An explanation of observed phenomena that is supported by evidence
When new evidence appears that no longer supports a theory, the theory has to change to take that evidence into account. Follow it through with the figworts:
Scientists theorised that a group of plants belonged together, based on the existing evidence — historically, observable traits such as flower shape.
New evidence from DNA sequence analysis showed these plant groups were not true clades, and so not all descendants of a common ancestor.
The observable similarities were instead the result of convergent evolution.
The historical classification theory was therefore falsified, and had to be changed.
New evidence from DNA and computer analysis is used to calculate the most likely evolutionary relationships, forming a new theory of plant classification.
Analysis of new DNA data continues all the time. Whenever evidence turns up that does not fit current classification theories, more falsification and reclassification will follow.
🧠
Split, move, merge
Reclassification does one of three things to a group: split it, move species out of it, or merge it with another. Whatever the organism, the answer is one of those three.
Worked examples
WE 1
Explain why a group was reclassified
Explain why DNA evidence led to the original figwort family being reclassified. (3 marks)
Point 1: how it was grouped
The family was originally classified on observable traits such as a tube-shaped flower structure.
Point 2: what the DNA showed
Analysis of three chloroplast genes showed the shared features were not evidence of shared ancestry, so the family was not a true clade.
Point 3: the nature of the error
The group was paraphyletic — it did not contain all the descendants of a common ancestor — so it was split and genera were moved into other families.
Grouped on looks, found not to be a clade, so rebuilt“paraphyletic” and “not a true clade” are the two phrases being tested
WE 2
Apply falsification
Use the reclassification of plant families to explain what is meant by the falsification of a theory. (4 marks)
Point 1: what a theory is
A theory is an explanation of observed phenomena that is supported by evidence.
Point 2: the original theory
Plants were theorised to belong together based on observable traits such as flower shape.
Point 3: the new evidence
DNA sequence analysis showed the groups were not true clades, and the similarities were caused by convergent evolution.
Point 4: the consequence
The theory no longer fitted the evidence, so it was falsified and replaced with a new classification based on DNA and computer analysis.
New evidence contradicted the theory, so the theory changedfalsification means the theory is revised in light of evidence, not that scientists were careless
WE 3
Predict an outcome
DNA analysis shows that a group of beetles currently classified as one family does not include all the descendants of its common ancestor. Suggest what taxonomists should do. (3 marks)
Point 1: identify the problem
The group is paraphyletic and therefore not a true clade.
Point 2: the action
The family should be reclassified — either split into separate families, or expanded to include the missing descendants.
Point 3: the reason
Groups should be true clades so that their members are genuinely close evolutionary relatives, and classification should follow the evidence.
Reclassify so the group becomes monophyleticgive the reason, not just the action — the third mark is for why it matters
💡 Exam tips
Use paraphyletic to describe a group missing some descendants.
Give all three outcomes of reclassification: species moved, groups split, groups merged.
Attribute misleading similarities to convergent evolution.
Define a theory as an explanation supported by evidence before discussing falsification.
Say classification should follow the evidence.
You do not need the figwort details memorised — you need the process.
⚠ Common mistakes
Saying falsification means the old scientists were wrong to try. They followed the evidence available at the time.
Confusing paraphyletic with monophyletic. Monophyletic includes every descendant.
Claiming the plants changed. The evidence changed, not the organisms.
Saying reclassification only means renaming. Groups are split, merged or moved.
Treating current classification as final. More DNA data means more revision.
Using a theory to mean a guess. It is an evidence-supported explanation.
Up next: Classification System — how the top of the hierarchy went from two kingdoms to three domains, and why the archaea got a domain of their own.
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