HL Only: Why classification is needed

• The immense diversity of species makes biological classification necessary.

• Classification organizes organisms so their similarities, differences and relationships can be studied systematically.

• Once organisms have been classified, a broad range of further biological study is facilitated.

HL Only: Evolutionary classification and clades

• An ideal classification follows evolutionary relationships rather than relying only on traditional taxonomic ranks.

• Members of a taxonomic group should have evolved from a common ancestor.

• A clade is a group of organisms united by common ancestry and shared characteristics.

• Characteristics can be predicted for members of a group because those characteristics are shared within the clade.

HL Only: Molecular clocks

• Sequence differences gradually accumulate after clades diverge from a common ancestor.

• A molecular clock uses this accumulation to estimate when two clades diverged.

• Molecular clocks give estimates, not exact divergence times, because mutation rates are not constant.

• Mutation rates are influenced by generation time, population size, selective pressure and other factors.

HL Only: Reading a cladogram

• A cladogram can be analysed to deduce evolutionary relationships, common ancestors and clades.

• The root represents the ancestral lineage from which all organisms shown on the cladogram ultimately descend.

• A node is a branching point and represents a hypothetical common ancestor.

• A terminal branch ends at one of the organisms or groups represented in the cladogram.

• Exam questions may require relationships or common ancestors to be deduced directly from a diagram.

Follow branches back toward their shared nodes to identify common ancestry. The diagram provides a clear practice model for recognizing branches and branching points. Source

HL Only: The three-domain classification

• Evidence from rRNA base sequences was used to classify all organisms into three domains.

• The domain system introduced an additional taxonomic level above kingdoms.

• This revolutionary reclassification was proposed in 1977.

The diagram connects rRNA sequence evidence with the division of organisms into three domains. Focus on the major branches rather than memorizing every terminal group. Source

HL Only: Traditional hierarchy versus cladistics

HL Only: Evidence for placing organisms in clades

• The most objective evidence for placing organisms in the same clade comes from molecular sequence data.

• Biologists compare base sequences of genes when investigating evolutionary relationships.

• They can also compare amino acid sequences of proteins.

• Morphological traits may also be used to assign organisms to clades, but sequence evidence provides the most objective basis.

HL Only: Constructing cladograms and parsimony

• Gene base sequences or protein amino acid sequences can provide the data used to construct cladograms.

• Simple sample sequence data can be compared to generate hypotheses about evolutionary relationships.

• Different criteria for judging the same evidence can produce different evolutionary hypotheses.

• Parsimony analysis selects the most probable cladogram by explaining observed sequence variation with the smallest number of sequence changes.

Parsimony favours the evolutionary hypothesis that accounts for observed variation using the fewest sequence changes. Source

HL Only: Testing and revising classifications

• Cladistics can test whether an existing classification actually corresponds to evolutionary relationships.

• A possible case study is the transfer of plant species between families, such as reclassification of the figwort family; case-study details need not be memorized.

• Similar morphology can arise through convergent evolution rather than common ancestry.

• Cladistic evidence can therefore falsify an earlier classification that was based on misleading morphological similarities.