Phylogeny and Evolutionary History

Chapter Summary

1. Darwin’s idea of branching descent with modification provided a theoretical foundation for the hierarchical patterns of classification that Linnaeus suggested. The study of phylogeny is the study of these branching relationships of populations as they give rise to descendant populations over evolutionary time. Phylogenetic systematics casts that classification scheme in terms of evolutionary history.
2. The study of phylogeny rests on our observations of traits displayed by organisms. A homologous trait is a trait that is found in two or more species because those species share a common ancestor. Analogous traits are shared by two or more species, not because of a history of common descent, but instead because they have arisen independently in each species.
3. Both the process of reconstructing phylogenetic trees, and the process of mapping evolutionary events onto trees, generate hypotheses. For example, by looking at where a given trait appears on a tree, we can generate a hypothesis about when and how this trait has evolved.
4. Evolutionary biologists use synapomorphies— shared, derived traits—to infer the structure of phylogenetic trees.
5. There are many equivalent ways to draw the same phylogenetic tree.
6. The points where a phylogentic tree branches—the nodes—represent common ancestors to the species that come after the branch point. All branch tips arising from a given branching point are descendants of the common ancestor at that branching point.
7. A monophyletic group or clade is defined as a taxonomic group that consists of a unique common ancestor and each and every one of its descendant species, but no other species. A clade always consists of a group of species that share a single common ancestor.
8. A paraphyletic group is one that does include the common ancestor of all its members, but it does not contain each and every species that descended from that ancestor.
9. Rooted trees indicate the direction of time; unrooted trees do not. The base of a rooted tree is called the root; this is the common lineage from which all species indicated on the tree are derived. We can "root" an unrooted tree at different points on the tree, generating different rooted trees in each case. Each of these different rooted trees represents a different hypothesis about which nodes are most ancestral.
10. Many trees are shown with all of the branch tips aligned. Such trees, called cladograms, convey only the pattern of relationships among the various species displayed. Phylograms are drawn with branches of different lengths; in a phylogram, branch lengths represent the amount of evolutionary change—measured as the actual or estimated number of changes in DNA sequence or other characters—that has occurred along a given branch.
11. Vestigial traits are those that have no current function but appear to have been important in the evolutionary past. Such traits allow us to test evolutionary hypotheses about common origin.