A3.2 Classification and cladisticsIB Biology HL: Revision notes

Classification is needed because of the immense diversity of species. Once organisms are classified, a broad range of further study is facilitated: features can be predicted, organisms identified and information retrieved.

The traditional hierarchy is kingdom, phylum, class, order, family, genus, species. It does not always correspond to patterns of divergence generated by evolution. NOS: a fixed ranking is arbitrary because variation is gradual. Cladistics uses unranked clades instead, a paradigm shift in classification.

A clade is a group of organisms with common ancestry and shared characteristics: a common ancestor and all its descendants. The ideal classification follows evolutionary relationships, so all members of a group evolved from a common ancestor, and characteristics can be predicted because they are shared within the clade.

The most objective evidence comes from base sequences of genes or amino acid sequences of proteins. Morphological traits can also be used, but similarity may result from convergent evolution.

Sequence differences accumulate gradually, so the number of differences between two groups can estimate when they diverged: the molecular clock. It gives only estimates because mutation rates are affected by generation time, population size, the intensity of selection and other factors.

Cladograms are built from base or amino acid sequences. Parsimony analysis selects the most probable cladogram: the one that accounts for the observed variation with the smallest number of sequence changes. NOS: different criteria can lead to different hypotheses.

Terms: the root is the common ancestor of all the organisms shown; a node is a branching point representing a hypothetical common ancestor; terminal branches end at the organisms (or groups) being classified. Any node plus everything branching from it is a clade.

Cladistics can test whether traditional groups match evolutionary relationships. The figwort family (Scrophulariaceae) was grouped by flower structure; chloroplast gene sequences showed that similar morphology arose by convergent evolution, and over half the genera (e.g. foxgloves, snapdragons) were moved to other families such as the plantain family. NOS: scientific claims can be falsified. You do not need to memorise the details.

In 1977 Carl Woese used ribosomal RNA (rRNA) base sequences to classify all organisms into three domains: Archaea, Bacteria and Eukarya. This revolutionary reclassification added a taxonomic level above kingdoms. rRNA is suitable because it occurs in all organisms and changes slowly.