Preview
IB DP Biology — Unity and Diversity | SL | A3.2
📋 What You Will Learn
- Identify the features used to construct cladograms and explain what a node, clade, and outgroup represent
- Explain how shared derived characteristics (synapomorphies) are used to infer evolutionary relationships
- Analyse cladograms to deduce the most recent common ancestor of two or more taxa
- Compare traditional Linnaean classification with modern cladistic classification and evaluate the evidence for reclassifying organisms
- Apply molecular sequence data (DNA, amino acid) as evidence for phylogenetic relationships
🔍 Guiding Question How can the evolutionary history of life be determined, and what evidence is used to classify organisms into groups that reflect their shared ancestry?
🌍 Why This Matters When COVID-19 emerged, scientists sequenced its genome within days and placed it on a phylogenetic tree of coronaviruses — identifying its closest relatives, tracing its likely origin in bats, and predicting which proteins might be targeted by vaccines. Classification is not just naming things. It is reading the history of life itself.
📋 Real-Life Context
For most of human history, we grouped organisms by what they looked like. Whales were once classified with fish. Bats were grouped with birds. These arrangements felt logical — until we started asking a deeper question: not "what does this organism resemble?" but "who is this organism related to?" That shift in thinking gave birth to cladistics, and it changed everything.
The Problem with Appearance: Why Traditional Classification Fails
Imagine two organisms that both have wings. A bat and a butterfly both fly, both have wings, and both navigate through air. But their wings evolved completely independently — from entirely different ancestral structures, through entirely different evolutionary pathways. Grouping them together because of wings would be like grouping a submarine and a dolphin because they both move through water.
This is the central problem that cladistics was designed to solve. Shared features that arise from independent evolution — called analogous structures — tell us nothing about common ancestry. What matters is shared features inherited from a common ancestor. Before we can understand how cladistics solves this, we need the right vocabulary.
📖 Key Definition — Homologous Structure A structure in two or more species that is similar because it was inherited from a common ancestor, regardless of its current function. Example: the pentadactyl limb found in humans, whales, bats, and frogs — all derived from the same ancestral forelimb structure.
📖 Key Definition — Analogous Structure A structure in two or more species that performs a similar function but evolved independently, not from a common ancestor. Example: the wings of birds and insects — similar function, completely different evolutionary origin.
📖 Key Definition — Convergent Evolution The independent evolution of similar features in unrelated lineages, driven by similar selective pressures. Convergent evolution produces analogous structures and can mislead classification based on appearance alone.
Traditional Linnaean classification grouped organisms into a hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. This system was revolutionary in the 18th century and remains useful for naming. But it was built on morphology — physical appearance — and morphology alone cannot reliably reveal evolutionary history. Cladistics offers a solution grounded in evidence of shared ancestry.
Cladistics: Classification by Ancestry
Here is the core idea: if two organisms share a feature that their common ancestor also had, that shared feature is evidence of their relationship.