A3.2 - Classification and Cladistics

Need for classification of organisms

Classification involves placing organisms in groups according to shared traits or evolutionary relationships.

It is needed because of the immense diversity of organism. After classification is completed, a broad range of further study is facilitated.

Difficulties classifying organisms into the traditional hierarchy of taxa

The traditional hierarchy of kingdom, phylum, class, order, family, genus and species does not always correspond to the patterns of divergence generated by evolution.

Advantages of classification corresponding evolutionary relationships

The ideal classification follows evolutionary relationships, so all the taxonomic groups have evolved from a common ancestor.

Characteristics of organisms within such a group can be predicted because they are shared within a clade.

Clades

Groups of organisms with common ancestry and shared characteristics

(evidence comes from base sequences of genes or amino acid sequence of proteins) - morphological traits can also be used.

Gradual accumulation of sequence differences

Change in sequence of DNA is caused by mutations.

This change gradually accumulates and becomes the basis for estimates of when clades diverged from a common ancestor.

Molecular clock

If change of DNA base sequence occurs at a constant rate, the number of differences can be used to estimate the time of divergence.

Molecular clock only used for estimates.

Factors that affect mutation rates are:

Length of generation time, size of population, intensity of selective pressure

Cladogram

A branching diagram that represents ancestor-descendant relationships

Terminal branch

Ends that represent individual clades.

Nodes

branching points on a cladogram - two clades that are linked at a node are relatively close, clades that are connected via series of nodes are less related.

Root

The base of a cladogram - hypothetical common ancestor of all the clades.

Cladogram analysis

Some cladograms are drawn to scale.

The pattern of branching in a cladogram is assumed to match phylogeny of the organisms - the evolutionary origins of each species.

Reclassification

Cladistics can be used to investigate whether the classification of groups corresponds to evolutionary relationships.

Base sequence or genome data allowed researchers to check traditional classifications of plants and animals.

Three domains

All organisms can be classified into three domains using evidence from rRNA base sequence.

Domain

The revolutionary reclassification with an extra taxonomic level above kingdoms that was proposed in 1977.

Eubacteria, Archaea, Eukaryotes