A phylogenetic tree is a diagram used to visually represent the evolutionary history and relationships among organisms, species, or genes. These branching diagrams map out inferred ancestry, showing how different lineages descended from common predecessors. The structure is composed of lines, called branches, and junction points, which are the nodes. Nodes serve as fundamental reference points for interpreting the connections and divergences shown in the tree.
Defining the Evolutionary Event
The most important function of a node is to represent a speciation or divergence event in the history of life. This point on the tree marks the moment when an ancestral lineage split into two or more distinct descendant lineages. The node itself symbolizes the last common ancestor that all the descendant groups stemming from that point shared before they separated.
To understand this concept, consider a family tree where a node is the grandparent from whom two lines of descendants emerge. In biology, this common ancestor is often a hypothetical species or population that no longer exists. Relationships are inferred by analyzing similarities in physical traits or genetic sequences of the organisms being studied.
The node is not the ancestor itself, but the graphical representation of the point in time when that ancestral group ceased to be a single lineage. Following the split, each new branch begins to accumulate unique evolutionary changes. The position of a node is a precise hypothesis about when a shared evolutionary history ended and separate paths began.
Internal and Terminal Node Distinctions
Phylogenetic trees contain two primary types of nodes, each representing a different kind of evolutionary unit: internal nodes and terminal nodes. The distinction between them is based on their location within the diagram and what they represent in terms of existence.
Internal Nodes
Internal nodes are the branching points located within the body of the tree, where two or more branches connect. These nodes represent the inferred most recent common ancestors of the taxa that descend from them. Since these ancestors are not directly observable today, internal nodes are considered hypothetical taxonomic units.
If a tree is rooted, the oldest internal node, situated at the base, is the root node. This node represents the common ancestor of all organisms shown in the tree. Internal nodes mark every divergence that led to the present-day organisms. The age of an internal node establishes a temporal progression, as it is always older than any node or tip that descends from it.
Terminal Nodes
Terminal nodes, often simply called tips or leaves, are found at the very ends of the branches. These points represent the organisms, species, populations, or genes that are currently being analyzed. They are the extant (living) or sometimes extinct taxa that were sampled to construct the tree.
Unlike internal nodes, terminal nodes represent actual, observable data points, such as modern species or sequenced genes. They signify the end of the evolutionary pathway shown for that particular lineage. The placement of these tips reflects the current status of the groups whose relationships are being mapped.
Nodes and the Interpretation of Relationships
The arrangement of nodes provides a visual tool for determining how closely different organisms are related in an evolutionary sense. Evolutionary relatedness is not determined by how close two tips are on the page, but by tracing back to the node they share. Two species are considered more closely related if they share a more recent common ancestor, which corresponds to a node closer to the tips of the tree.
The concept of a clade is directly tied to a specific internal node. A clade is a complete group that includes a single ancestral node and all of its descendant branches and terminal tips. Identifying a clade involves isolating all the lineages that branch off from that single node.
In a rooted tree, the placement of nodes relative to the root provides directionality for interpreting the flow of time. Time proceeds from the root node toward the terminal tips, meaning deeper nodes are older than shallower nodes. The relative depth of a shared node is the definitive measure of how far back in time two lineages last shared a common ancestor.