系統図

A phylogenetic tree contains tips representing living organisms, branches showing evolutionary changes like DNA sequence shifts or trait evolution, nodes marking where branches meet and representing common ancestors, and a root representing the most recent common ancestor of all organisms in the tree.

Sister taxa are species that share an immediate common ancestor, represented by a shared node on the phylogenetic tree. They are each other's closest relatives. For example, reptiles and birds are sister taxa that diverged from a single common ancestor, making them more closely related to each other than to other organisms.

Humans and rodents share a common ancestor, but the node representing this ancestor depicts an extinct organism that was neither a rodent nor a human. The phylogenetic tree shows that both lineages evolved separately from this shared ancestor, not that one species descended directly from the other.

In a scaled phylogenetic tree, branch length can represent either the amount of time elapsed since taxa diverged from a common ancestor or the number of evolutionary changes that occurred. In unscaled trees, branch length has no specific meaning and serves only to show relationships between organisms.

A basal taxon is a lineage that evolved early from the root of the phylogenetic tree and remained unbranched. It represents an organism or group that diverged early in evolutionary history and did not subsequently split into multiple descendant lineages.

Phylogenetic trees are branching diagrams where each taxon appears at a branch tip, and branches represent relationships between organisms. Closely related taxa share a larger portion of their genome and group together on the tree. All taxa connecting to a specific node share a recent common ancestor, revealing their evolutionary history.

Phylogenetic trees represent evolutionary relationships inferred from morphological data, such as physical characteristics like feathers, or molecular data, such as DNA sequences. These sources of evidence help scientists determine how closely organisms are related and reconstruct their shared evolutionary history.