IB Biology HL A3.2 Classification and cladistics [HL only] Notes

Classification is not just naming things. It organizes biodiversity into groups with shared characteristics so scientists can communicate, identify organisms, compare them, and study them further. A good classification also suggests functional, structural, and evolutionary relationships, so it becomes a prediction tool rather than a filing cabinet.

Classification turns huge biodiversity into groups that can be compared and studied.

Traditional taxonomy uses fixed ranks: kingdom, phylum, class, order, family, genus, and species. The problem is that evolution does not happen in neat equal steps, so rank spacing can be arbitrary and may not match real divergence. Morphological similarity can also be convergence rather than common ancestry, so organisms can look alike for functional reasons while being less closely related than they appear.

Fixed rank names do not automatically measure evolutionary distance.

Evolutionary classification aims to match phylogeny: the actual pattern of common ancestry. A natural group is monophyletic, meaning it includes one ancestor and all of that ancestor’s descendants. DNA and protein sequence evidence can correct misleading morphology, so named groups become better predictions of shared traits and evolutionary history.

Evolutionary classification is stronger when the named group matches the tree of common ancestry.

A clade is a lineage group that evolved from a common ancestor. To identify it, look for shared derived characteristics, called synapomorphies. “Derived” matters: a trait that evolved in the common ancestor and is inherited by descendants is stronger evidence than a general similarity. Synapomorphies can be anatomical, behavioural, genetic, or protein-sequence evidence.

After two clades diverge, DNA or amino acid sequence differences can accumulate. A molecular clock uses those differences to estimate divergence time. The word estimate matters: the rate must be calibrated with known dates, and rates can vary between genes, lineages, generation times, population sizes, and selection pressures. More differences may suggest earlier divergence only when the clock is appropriately calibrated.

A molecular clock is an estimate: the rate assumption must be checked before turning differences into time.

Cladograms can be built by aligning DNA, RNA, or protein sequences and comparing similarities. Computer analysis tests possible trees and infers the tree that best explains the data. Parsimony is one decision rule: prefer the tree requiring the fewest evolutionary changes, as long as it explains the observed sequence pattern.

To read a cladogram, ignore how close the tips look on the page. Nodes represent hypothetical common ancestors or speciation events. Branch points show the order of divergence, and the most recent common ancestor determines which taxa are more closely related. Roots, terminal branches, ingroups, outgroups, and sister groups help orient the tree.

Read the tree by tracing back to the most recent common ancestor, not by comparing tip positions.

Cladistics can test whether traditional taxa match evolutionary relationships. The figwort family case is the exam example: conserved chloroplast gene sequences showed that the old Scrophulariaceae grouping did not form one clade. Some similar floral traits were convergent, so species such as foxglove and yellow rattle were moved. The lesson is that molecular evidence can falsify a morphology-based group.

Reclassification happens when molecular evidence shows that an old taxon does not match ancestry.

The three-domain system is a molecular-evidence story. rRNA sequence evidence separated prokaryotes into Eubacteria and Archaea, giving three domains: Archaea, Eubacteria, and Eukarya. These domains also differ in chromosome form, histones, introns, cell walls, and membrane lipids. The point is that “prokaryote” was too broad for evolutionary classification because Archaea and Eubacteria are deeply different.

The three-domain model came from molecular evidence, then matched deeper cellular differences.

A3.2 exam answers are strongest when they sound like evidence arguments. Classification organizes diversity, but fixed ranks and morphology can mislead. Evolutionary classification should match phylogeny using monophyletic clades supported by synapomorphies. Molecular clocks estimate divergence time from calibrated sequence differences. Cladograms are built from aligned sequence data and interpreted by nodes, not tip positions. Cladistics can reclassify old taxa, and rRNA evidence supports the three-domain system.

Use this for combined HL classification and cladistics questions.