Unit 4 Evolution

A phylogenetic tree is a branching diagram that shows possible evolutionary relationships among groups of organisms.

New evidence has changed how scientists think about taxonomic groupings. Historical groupings, such as reptiles and fish, are no longer considered valid because they are not monophyletic.

A cladogram is constructed by grouping taxa into nested hierarchies using information from synapomorphies, characters that some members of the clade share because of common ancestry.

Character states in different species may be similar because they were derived from a common ancestor of the species (synapomorphy) or because each species converged independently on the same character state (homoplasy).
Scientists reconstruct evolutionary trees to develop and test hypotheses about how major evolutionary transformations took place.

Tetrapods belong to a clade called lobe-fins, which includes coelacanths and lungfishes.

Some tetrapod traits, such as toes and legs, evolved while the ancestors of tetrapods still lived in water.

The bones of the mammalian ear evolved from bones of the lower jaw of ancestors to mammals.

Sometimes traits that arise for one function are later co-opted for a different function (exaptations), and phylogenies can help reveal this.

Feathers evolved before flight. Dinosaurs used these early feathers for other functions, perhaps for insulation, courtship display, and nest brooding.