Reconstructing Evolutionary Trees Chapter 14

Phylogeny A phylogenetic tree is an illustrated summary of the history of a group of organisms. It describes the pattern and timing of speciation events. It describes the relationship of different taxa to one another.

The Logic of Phylogeny Inference 14.1

Challenges to Creating Phylogenic Trees Characters used in a phylogenitic analysis have to be independent of one another. Characters used in phylogeny inference have to be homologous. Traits that are shared among species because they are homologous and that are similar because they were modified in a common ancestor are called synapomorpies.

Evolutionary relationships can be inferred by analyzing synapomorphies

Synapomorphies identify evolutionary branch points Genetic separation causes homologous traits to change independently. These changed traits are synapomorphies that identify populations belonging to different descending lineages.

Synapomorphies are Nested As you move through time and trace a tree from its bottom to its top, each branching event adds one or more shared, derived traits. Cladistic method

Establishing the direction of Change

Feathers are a a Hallmark of birds Based on their structure, the feathers of modern and fossil birds are clearly homologous, and synaptomorphic. Employing outgroup analysis one can tell the direction and polarity of change through time.

Outgroup Analysis The character (feathers) in the ingroup is compared to that trait in a close relative (the outgroup) that clearly branched off earlier. Skeletal structures show that theropod dinosaurs are birds closest realtives.

Several dromaeosaur species have simple featherlike structures projecting from their skin The presence of feather in nonflying dinosaurs also confirmed that the evolution of feathers predated the evolution of flight in birds.

The problem of Homoplasy Reversals remove similarity that is caused by descent from a common ancestor

Phylogenetic hypotheses for whales