Transcript

Phylogeny describes the ancestral relationships between organisms. The connections often take the form of a tree with tips, branches, nodes and roots called the phylogenetic tree.

Specifically, the tips of the tree represent extant or living taxa and the branches denote evolutionary changes between ancestors and descendants such as a change in the DNA sequence or the evolution of a characteristic, like feathers.

Species that share an immediate common ancestor, sister taxa, are their closest relatives and share nodes, points where branches meet, like lizards and birds and rodents and humans. A basal node corresponds to the most recent common ancestor of all organisms in the tree. The Metaspriggina, in this case.

Phylogeny is concerned with the evolutionary diversification of organisms or groups of organisms. A group of organisms with a name is called a taxon. Taxa can span different levels of the evolutionary hierarchy. For instance, the group containing all birds is a taxon (comprising the class Aves), and the group of all species of daisies (the genus Bellis) is a taxon. Phylogenies can likewise include just one genus (i.e., depict species relationships) or span an entire kingdom.

Typically, phylogeny is represented by a tree. Each taxon of interest is depicted at the tip of the branch, and the branches represent the relationship between these taxa. Closely related taxa share a larger portion of their genome, resulting in morphological similarities. Sister taxa share an immediate common ancestor, represented by a shared node.

Nodes in the tree depict a morphological characteristic or molecular change that differentiates taxa. All taxa that directly connect to a specific node share a recent common ancestor. Hence, the root of the tree is the ancestor shared by all taxa in the tree. A tree without a root represents the relationship of taxa but does not provide information on the shared common ancestor.

The length of the branches of a tree may or may not have a meaning. In a scaled tree, the length of the branch may either represent time or the number of changes that occurred since the taxa evolved from a common ancestor.

Suggested Reading

Gaucher, Eric A., James T. Kratzer, and Ryan N. Randall. “Deep Phylogeny—How a Tree Can Help Characterize Early Life on Earth.” Cold Spring Harbor Perspectives in Biology 2, no. 1 (January 2010). [Source]

Lyubetsky, Vassily, William H. Piel, and Dietmar Quandt. “Current Advances in Molecular Phylogenetics.” BioMed Research International 2014 (2014). [Source]