Chapter Summary
21.1 All of Life Is Connected through Its Evolutionary History
Phylogeny is the history of evolutionary relationships among organisms or their genes. Groups of evolutionarily related species are represented as branches in a phylogenetic tree. Review Figures 21.1, Focus: Key Figure 21.2
Named species and groups of species are called taxa. A taxon that consists of an ancestor and all of its evolutionary descendants is called a clade. Review Figure 21.3
Homologies are similar traits that have been inherited from a common ancestor. Review Figure 21.4
A derived trait that is shared by two or more taxa and is inherited from their common ancestor is called a synapomorphy.
Distantly related species may show similar traits that do not result from common ancestry. Convergent evolution and evolutionary reversals can give rise to such traits, which are called homoplasies.
21.2 Phylogeny Can Be Reconstructed from Traits of Organisms
Phylogenetic trees can be constructed from synapomorphies using the logic of parsimony. Review Figure 21.5, Activities 21.1, 21.2
Sources of phylogenetic information include morphology, patterns of development, the fossil record, behavioral traits, and molecular traits such as DNA and protein sequences.
Phylogenetic trees can also be constructed with maximum likelihood methods, which find the tree most likely to have generated the observed data under a specific model of evolution.
Phylogenetic methods have been tested in both experimental and simulation studies, and have been shown to be accurate under a wide variety of conditions. See Animation 21.1
21.3 Phylogeny Makes Biology Comparative and Predictive
Phylogenetic trees are used to make comparisons among living organisms. Review Figure 21.9
Phylogenetic trees are used to reconstruct the past and to understand the origin of traits. Review Figure 21.10
Biologists can use phylogenetic trees to reconstruct ancestral states.
Phylogenetic trees may include estimates of divergence times of lineages determined by molecular clock analysis. Review Figure 21.12
21.4 Phylogeny Is the Basis of Biological Classification
Biologists use phylogenetic relationships to organize life into a coherent classification system.
Taxa in modern classifications are expected to be monophyletic groups. Paraphyletic and polyphyletic groups are not considered appropriate taxonomic units. Review Figure 21.13, Activity 21.3
Several sets of rules govern the use of scientific names, with the goal of providing unique and universal names for taxa.
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