IB Biology HLClassification & CladisticsPaper 1 & 2~14 min read
Cladistics
Should a bat be grouped with birds because of its wings, or with mammals because of its fur? Looking at organisms will not settle that. Reading their DNA will — and that is the whole idea behind cladistics.
📚 What you need to know
DNA sequencing technology means classification can be based on evolutionary relationship rather than appearance.
A clade is a group of organisms that have all descended from a common ancestor. Clades are monophyletic: they contain all the descendants of that ancestor.
Cladistics is the branch of science that places organisms into clades. Taxonomy names groups; cladistics identifies evolutionary relationships.
Clades can include living and extinct species, and can be large or small depending on which common ancestor you start from.
The most objective way to build clades is sequence data: DNA bases, mRNA bases, or amino acids in polypeptides.
Primitive traits evolve early and are found in all clade members; derived traits evolve later and differ between members. Closely related species share more derived traits.
The constant rate of mutation acts as a molecular clock, giving estimates of divergence times. DNA hybridisation measures the number of differences.
A cladogram is an evolutionary tree. Branch points are nodes, representing common ancestor species. Cladograms are built by computer using the principle of parsimony.
Grouping by ancestry
Historically, organisms were classified on morphology, which often produced groups whose members were not close relatives at all. DNA sequencing changed that. Organisms grouped by evolutionary relationship form clades, and every member of a clade shares a recent common ancestor.
Clade
A group of organisms that have all descended from a common ancestor
A common ancestor is simply a shared ancestor. The most recent common ancestor of two siblings is their parents; for two cousins it is their grandparents. Clades are monophyletic, which means they contain all the descendants of that ancestor — not most of them.
Taxon
Clade
Grouped on
Shared features
Shared descent from a common ancestor
Concerned with
Classifying and naming organisms
Identifying evolutionary relationships
Given a rank
Yes — kingdom, phylum, class and so on
No — clades are unranked
If taxonomy is done correctly, all the members of a taxon should form a clade. Because of historical errors, that is not always the case.
Careful with lookalikes. Characteristics within a clade are often inherited from a common ancestor, so members tend to share them. But the reverse does not follow — not all species with shared characteristics are closely related. Grouping organisms on analogous characteristics, which evolved separately, is exactly the mistake cladistics is designed to avoid.
Building clades from evidence
Sequence data
The most objective method uses sequence data from DNA bases, mRNA bases or amino acids in polypeptides. For all three, the rule is the same:
The rule
The more similar the sequences, the more closely related the species
Groups with very similar sequences separated into separate species more recently. Groups separated for longer have had more time to accumulate mutations in their DNA, mRNA and amino acid sequences.
Morphology
Species sharing a recent common ancestor are more likely to share similar morphologies, so appearance can sometimes identify clade members. It is more subjective, though, and has caused classification errors: similar morphology can be a sign of convergent evolution rather than recent shared ancestry.
It is also hard to decide which characteristic should decide the grouping. Bats have wings like birds and fur like mammals — which one counts? The answer comes from separating two kinds of trait.
The same feature can be either kind. Fur is a primitive trait within the mammal clade, because all mammals have it — but a derived trait within the wider vertebrate clade.
Primitive trait
Derived trait
When it evolved
Early in the lineage of a clade
Later
Who has it
All members of the clade
Can differ between clade members
Example
All vertebrates have spinal cords; all insects have six legs
Within the vertebrate clade, birds have feathers while mammals have fur
More closely related species share a larger number of derived traits. That is the rule that decides the bat question: fur is a derived trait shared with mammals, so bats belong with mammals.
The molecular clock
The number of differences between two sets of sequence data tells you how closely related two species are. More differences means the species diverged longer ago.
Those differences arise from mutations, and evidence suggests mutations occur at a constant rate. So the number of mutations indicates how much time has passed since two species diverged. Scientists call that constant rate the molecular clock.
DNA hybridisation
Sections of single-stranded DNA from corresponding parts of the genome are taken from two species.
The two complementary strands are allowed to bind together, or hybridise.
Points where the strands do not bind show where the bases differ.
The number of differences is recorded.
Always call molecular clock results estimates. The assumed rate of mutation does not always match the real rate, and the rate at which mutations accumulate is affected by generation time, population size and selection pressures. That caveat is nearly always worth a mark.
Cladograms
A cladogram is an evolutionary tree showing the probable order of divergence from ancestral species, and therefore the probable relationships between species.
Count the nodes you have to trace back through. One node between two groups means a close relationship; several means a distant one.
How they are constructed
The information most often comes from base or amino acid sequence data, because observable characteristics can mislead.
Sequence data shows how different species are and how much time has passed since divergence, using the molecular clock.
Reliability varies with the amount of sequence data used. A cladogram based on one gene is less reliable than one based on several genes.
Computers build the most likely cladogram using the principle of parsimony: the simplest explanation is preferred. The computer produces the shortest possible cladogram, with the smallest number of divergence events that fit the data.
Cladograms therefore show the most probable divergence times and relationships, not definite conclusions, and may change if new evidence appears.
Parsimony in one example. It is more likely that fur evolved once than that it evolved separately several times. So a cladogram places all mammals as descendants of one common ancestor rather than scattering them across the tree. Parsimony provides the criteria for construction — without it, cladograms built from the same data could look completely different.
Reading one
Feature
What it tells you
Node
The point where two branches separate; it represents a common ancestor species
A node immediately adjacent to two clades
Those two clades share a recent common ancestor and are more closely related to each other than to any other clade shown
Several nodes traced back before two clades join
A more distant relationship
Root
At the base; the common ancestor of every organism in the cladogram, from a long way back in evolutionary history
Terminal branch
The most recent species in an evolutionary lineage
Numbers along branches
The number of base or amino acid changes between one node and the next, usable as a molecular clock
Time scale
Shows how many millions of years have passed
🧠
Count nodes, not gaps
Two species printed next to each other may not be close relatives. What matters is how many nodes you trace back before their lines meet. Fewer nodes means a closer relationship.
Worked examples
WE 1
Define and apply the term clade
Define the term clade and explain why a group containing only some descendants of a common ancestor is not one. (3 marks)
Definition
A clade is a group of organisms that have all descended from a common ancestor.
The requirement
Clades are monophyletic, meaning they contain all of the descendants of that ancestor.
Why the group fails
Leaving some descendants out means the group is not monophyletic, so it is not a true clade and does not reflect the real evolutionary relationships.
All descendants, or it is not a cladethe word “all” is what the mark is for — do not soften it
WE 2
Interpret a cladogram
Using the cladogram above, deduce which species is most distantly related to species A, and justify your answer. (3 marks)
Step 1: the deductionSpecies E is the most distantly related to species A.
Step 2: the justification
Tracing back from A to E, you have to pass through several nodes before the two lines join, and they only meet at the root.
Step 3: the comparison
A and B join at a single node, so they share the most recent common ancestor and are the closest pair.
E, because the two lineages only meet at the rootjustify using nodes, not by how far apart the labels are printed
WE 3
Evaluate a cladogram’s reliability
A cladogram is constructed from the sequence of a single gene. Evaluate how reliable its conclusions are likely to be. (3 marks)
Point 1: what is good about it
Sequence data is the most objective basis for grouping species, and is more reliable than morphology.
Point 2: the limitation
Reliability depends on the amount of sequence data; a cladogram from one gene is less reliable than one built from several genes.
Point 3: the judgement
Cladograms show the most probable relationships rather than definite conclusions, and may change if new evidence emerges.
Objective, but built on too little data to be treated as settledthe phrase “most probable rather than definite” fits almost any cladogram evaluation
💡 Exam tips
Use monophyletic and say a clade contains all the descendants.
Name all three types of sequence data: DNA bases, mRNA bases, amino acids.
Distinguish primitive from derived, and say closely related species share more derived traits.
Call molecular clock results estimates, and give a reason why.
Use parsimony by name and explain it as “the simplest explanation is preferred”.
Read cladograms by counting nodes.
⚠ Common mistakes
Judging relatedness by label position. Only the branch points matter.
Saying a clade is any group of similar organisms. It is defined by shared descent.
Treating a cladogram as definite. It shows the most probable relationships.
Mixing up primitive and derived. Primitive traits appear in every member of the clade.
Grouping on analogous features. Convergent evolution produces similarity without ancestry.
Forgetting that clades can include extinct species, including the common ancestor itself.
Up next: Reclassification — what happens when DNA evidence shows that a whole plant family was never a clade at all.
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