A phylogenetic tree is a scientific diagram representing the evolutionary history and relationships among a group of organisms. These diagrams are structured like family trees, illustrating how different species or groups, known as taxa, have descended from common ancestors. The branching patterns provide a map of divergence, showing which groups are more closely related to one another.
Identifying the Core Components
Every phylogenetic tree is built from a few simple, recognizable elements that form a language for evolutionary relationships. The most recognizable parts are the tips or leaves, which are the endpoints of the branches and represent the groups of organisms being studied, such as existing species or extinct groups. These tips are connected by branches or lines, which signify the evolutionary lineages leading to the taxa.
The points where branches diverge are called nodes or branch points, and each node represents the inferred most recent common ancestor shared by the lineages stemming from that point. A node represents a speciation event where one ancestral lineage split into two or more descendant lineages. In rooted trees, a single point called the root serves as the ultimate common ancestor for all organisms displayed in the entire diagram.
Interpreting Evolutionary Relationships
The relatedness of any two groups on a phylogenetic tree is determined by tracing back from the tips to find their most recent common ancestor (MRCA). Groups that share an MRCA closer to the tips are considered more closely related than those whose MRCA is found deeper, closer to the root. The branching order, or topology, is the only factor that defines these relationships, not the physical closeness of the tips.
A clade, also known as a monophyletic group, is a fundamental unit of interpretation that includes a common ancestor and all of its descendants. You can identify a clade by selecting any node and mentally drawing a circle around that ancestor and every branch that stems from it.
The two lineages that diverge directly from a single common node are termed sister taxa or sister groups. Sister taxa are each other’s closest relatives on the tree because they share a unique common ancestor not shared by any other group. For example, if taxa A and B split from a node, and that node later splits from a branch leading to C, then A and B are sisters and are more closely related to each other than either is to C.
The physical arrangement of the branches can be misleading because the tree can be rotated around any node without altering the evolutionary information it conveys. This means that the order of the species listed at the tips can be flipped without changing the underlying relationships. The rule of rotation emphasizes that only the branching pattern matters for determining relatedness.
Understanding Branch Length and Tree Types
Phylogenetic trees are classified into different types based on whether the length of the branches carries meaning. A cladogram is a type of tree where the branch lengths are arbitrary and do not convey information about time or the amount of evolutionary change. In a cladogram, the focus is solely on the order of branching, which represents the sequence of evolutionary divergence.
In contrast, a phylogram is a scaled tree where the branch lengths are meaningful and proportional to some measure of evolutionary distance. This distance typically represents the amount of genetic change, such as the number of DNA mutations, that has occurred along that lineage. A longer branch in a phylogram therefore indicates a greater amount of change since the last common ancestor.
A specialized type of phylogram is a chronogram, where the branch lengths have been scaled specifically to represent geological time. In a chronogram, all of the modern-day tips typically align at the same vertical or horizontal point, representing the present day. These scaled trees often include a scale bar, which allows a reader to measure the actual time represented by a given branch length.
Avoiding Common Reading Errors
A frequent misinterpretation is the linear progression fallacy, which incorrectly views evolution as a ladder-like process where some organisms are “more evolved” than others. All modern species represented at the tips have evolved for the exact same amount of time since their shared common ancestor. Therefore, a species at the far right of a tree is not inherently “higher” or more advanced than a species at the far left.
Another common error is reading across the tips, where a reader mistakenly assumes that groups positioned next to each other are the most closely related. As demonstrated by the rule of rotation, proximity at the tips is arbitrary; the only way to determine relatedness is by examining the shared nodes. Taxa that appear adjacent may actually be distantly related if their shared common ancestor is deep within the tree.
Readers also sometimes incorrectly assume that a modern species at a tip is descended from another modern species on the tree. The tips represent contemporary or extinct species, while the nodes represent hypothetical common ancestors. Modern species are not ancestors of one another; rather, they share a common ancestor that existed at a divergence point in the past.