5.4 CLADISTICS

Clade: Group of organisms that have evolved from a  common ancestor

These groups of species can be identified by looking for shared characteristics

Clades include all the species alive today and species that have evolved and became extinct

• Species that have a recent common ancestor are expected to have few amino acid sequence differences

• Species that have some similarities but diverged from a common ancestor a long time ago are expected to have many differences

Mutations occur at a relatively constant rate so they can be used as molecular clocks

Molecular clocks: The number of genetic differences can predict how long ago two species diverged

The larger the number of differences between two species the longer the time since they diverged from a common ancestor

If the base sequences are similar then few mutations have occurred which means the species only diverged recently

LIMITATIONS

• Different genes or proteins may change at different rates
• The rate of change for a certain gene may differ between two groups of organisms
• May question the accuracy of early predictions based on more recent findings

Homologous structures: Structures that are similar because of similar ancestry

Might have different functions

Analogous structures: Structures that are similar because of convergent evolution [different organisms independently evolve similar traits] but evolved independently

Same structure and functions such as human and octapus eyes

Cladograms: Tree diagrams that show most probable sequence of divergence in clades

Features:

• Root: The initial ancestor common to all organisms
• Node: Hypothetical common ancestor that speciated to give rise to two or more daughter taxa
• Outgroup: Most distantly related species which functions as a point of comparison
• Clades: A common ancestor and all of its decendents (node and its branches)

Classification allows us to see evolutionary relationships

Organisms that are grouped together share many homologous features, however only classifying based on structure is problamatic 

Nowadays DNA evidence is far more accurate and trustworthy

• Figworts had 275 Genera
• Taxonomists discovered that the Figwort species should be classified into 5 different clades when examining chloroplast genes
• Reclassification helped to simplify and better understand the Figwort species